i
The content of this handouts is a compilation of work coming from the following authors
who were acknowledge using proper citation to wit:
Brady, N.C., and R.R. Weil. 2001. The nature and properties of soils (13th ed.) Prentice
Hall, Upper Saddle River, NJ.
Carating, R.1997. The Survey and Classification of Andisols. SRDC Technical Information
Series No. 6. Soils Research and Development Center-JICA, Diliman, Quezon City,
Philippines.
Castillo, RL. 2007. Lecture Handouts in Soil Science for LEA Review.
Daquiado, N. n.d. Lecture Handouts in Soil Science for LEA Review, CMU, Bukidnon
Fanning, D.S., and M.C.B. Fanning. 1989. Soil morphology, genesis, and classification.
John Wiley and Sons, New York.
Hodges, S. 2011. Soil Fertility Basics. Soil Science Extension. North Carolina State
University.
Kenny, A. 2013. Recent Advances in Microbiology.
Lasquites, J. n.d. Lecture Handouts in Soil Science for LEA Review, USeP, Tagum City
LEA Reviewer in Soil Science. 2003. Visayas State University, Baybay City Leyte.
Lynn, I. et.al. 2013. Land Use Capability Survey Handbook.
Salibay, MA. 2010. Lecture Handouts in Soil Science for LEA Review, USeP, Tagum City
UPLB, 2007. Lecture Review Materials for LEA. College of Agriculture, UPLB, Laguna,
Philippines
University Code
Student’s Handbook
This material is used for instructional purpose only and is not intended for sale.
Should it be distributed to person other than those enrolled in this subject or should it be
dispensed for commercial purpose, the ESSU and the faculty who compiled this material will
not be responsible for any claims of the original authors.
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OVERVIEW OF THE HANDOUTS
You are provided with handouts in Soils 1 (Principles of Soil Science) which cover
subjects/topics for the entire semester. This is good for 90 hours or 36 regular sessions at
2.5 hours per meeting. However, in the absence of face-to-face interactions, this self-paced
learning material will be distributed by your professor so you can have advanced reading and
perform the expected tasks for the entire semester. Just remember, that any problem that
you will meet, meaning any part of this handouts that you can hardly understand, you can ask
the same to your professor using messenger or any other forms of social media which will be
created for this subject. He is very much willing to answer your queries to facilitate the
teaching and learning processes.
This handouts contain six (6) units which cover the following topics: 1) definition and
composition of soils, 2) soil formation and development, 3) physical properties of soils, 4)
chemical properties of soils, 5) biological properties of soils and organic matter, 6) nature,
properties and management of soils
COURSE GUIDE
Course: Soils 1
Semester: 2nd
Class Schedule:
Course Description: Principles of Soil Science
School Year: 2020-2021
Nature, Properties and Management of Soils
SCHEDULE
Week 1-3
Week 4-6
TOPICS
VMGO of the University and the College of Agriculture and Allied Sciences,
Topical Outline, Course Policies, Requirements and Quality Policy
I.
Definition and Composition of Soil
A. Soil defined, edaphological and pedological
B. Field of specialization in soil science
C. Composition of the Soil
1. Air
2. Water
3. Organic matter
4. Mineral matter
D. Composition of the mineral matter
1. The three major fractions: sand, silt and clay
2. The clay fraction: crystalline and non-crystalline
components
E. Elemental composition of the Earth’s crust
F. The essential nutrient elements
1. Macro and micronutrients
2. Criteria of Essentiality
3. Ionic forms of nutrients
II.
Soil Formation and Development
A. Soil forming rocks and minerals
1. Rocks and minerals: definition
2. Classes of rocks: igneous, sedimentary, metamorphic
3. Mode of formation of igneous rock: intrusive, extrusive
4. Other points of differences of igneous rock: texture,
color, acidity
5. Examples of sedimentary, metamorphic and igneous
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SCHEDULE
Week 7-10
TOPIC
III.
rocks
6. Primary vs secondary minerals and examples of each
class
B. Weathering
1. Definition
2. Physical weathering: exfoliation and cracking due to
temperature changes, crystal growth, abrasion,
movement of earth’s crust, prying action of roots
3. Chemical weathering: hydrolysis, hydration, oxidation,
carbonation, solution
C. Soil formation
1. Factors of soil formation: climate, organisms, relief,
parent material, time
D. Soil development
1. Pedogenic processes: addition, losses, translocation in
the soil body
E. The soil profile
Physical Properties of Soils
A. Definition of physical properties
1. Texture, structure, bulk density, particle density,
porosity, water holding capacity, hydraulic conductivity,
consistency and color
B. Soil texture
1. The twelve textural grades
2. Properties of sand, silt and clay: size ranges, shapes,
chemical composition, specific-surface area
3. Significance of soil texture on soil fertility, crop
suitability, porosity/aeration, water relation, tillage
4. Soil texture determination: feel method, hydrometer
method, pipette method
C. Soil structure
1. Types: platy, prismatic and columnar, blocky, and subangular blocky, granular and crumby, structureless
2. Cementing agents: OM, lime, microbial gums, fine clay
3. Soil management related to soil structure
D. Soil densities
1. Definition and formulas for B.D., P.D. and % porosity
2. Factors affecting B.D.
3. Sample problems
E. Soil water
1. Importance of water
2. Properties of water
3. Forces affecting soil water retention and movement:
cohesion, adhesion, osmotic pressure, capillary forces
4. Soil moisture tension concepts, unit of expression
5. Soil moisture availability: field capacity, hygroscopic
coefficient, permanent wilting point
6. Soil moisture measurements
F. Soil consistency and color
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SCHEDULE
Week 11-14
IV.
Week 15-16
V.
TOPIC
Chemical Properties of Soils
A. Soil Colloids
1. Definition
2. Classification and examples: organic and inorganic
B. Silicate Clays
1. Basic structural units: tetrahedron
2. Properties of silicate clays: expansion and contraction
(type), bonding, crystal size, specific surface area,
shape
3. Sources of negative charges
C. Organic colloids
1. Sources of negative charges
D. Factors affecting strength of absorption of ions in soil
colloids
E. Cation exchange capacity
F. Base saturation and exchangeable sodium percentage
G. Soil pH
1. Definition and significance
2. Sources of soil acidity
3. Kinds of acidity: active and reserve acidity
4. Buffering capacity
5. Effects of nutrients availability
H. Liming
1. Definition and examples of lime
2. Relative Neutralizing Value (RNV)
3. Sample problems
I. Soil salinity and sodicity
Soil Organisms and Organic Matter
A. Kinds of organisms: microorganisms and macro organisms
B. Bacterial: Characteristics and classification
1. Oxygen requirements-aerobic, anaerobic, facultative
Energy
and
carbon
requirements-antotrophic
(hototrophic and chemoautotrophic) and heterotrophic
2. Temperature adaptation-psychrophyllic, mesophyllic,
thermophyllic
C. Actinomycetes: characteristics and similarity and difference
from bacteria and fungi
D. Fungi: characteristics and unique adaptation to soil
conditions
E. Other organisms: viruses, protozoa, algal, worms, insects,
rodents
F. Beneficial activities of soil organisms
1. Decomposition of organic matter
2. Transformation of soil nutrients
3. Promoting soil aggregation through by-products of
their activities
4. Nitrogen
fixation
(rhizobia)
and
phosphorus
solubilization (mycorrhiza)
G. Composition of organic matter: carbohydrates, proteins,
lignins, fats, waxes, tannins
H. Organic matter decomposition and end products
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SCHEDULE
Week 17-18
VI.
TOPIC
1. Aerobic (oxidative) decomposition
2. Anaerobic (fermentation) decomposition
I. Transformation of nitrogen
1. Mineralization,
ammonification,
nitrification,
denitrification, immobilization, ammonia volatilization,
leaching
l. Effects of inorganic matter on soil properties
Nature, Properties and Management of Soils
A. Definition
B. Essential Nutrient
1. Criteria of Essentiality
2. Available forms & functions of nutrients in plants
3. Methods of assessing soil fertility status
4. Fertilizer nutrients: their properties and usage
5. Fertilizer computation
6. Causes of decline in soil fertility
Course Requirements:
Term Examinations
Other Requirements
Quizzes
Sets of Exercises
40%
60%
30%
30%
Course Learning Outcomes
At the end of the semester, the students must
be able to:
1. Appreciate the VMGO, Core Values,
Quality Policy and Grading System of
the University.
2. Present a unified view of the soil as a
medium for plant growth and as a
natural resource;
3. Develop skills in problem solving
requiring numerical data obtained from
physical,
chemical
and
biological
experiments involving the soil;
4. Recognize and diagnose soil problems
associated with plant growth;
5. Apply the principles of soil management
in the control of soil fertility, soil pH and
soil erosion; and
6. Interpret soil survey report.
Required Output
Laboratory Exercises (individual)
Series of Worksheets/Exercises
Vegetable Garden (at home)
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Unit 1 –DEFINITION AND COMPOSITION
OF SOIL
Introduction
A thin portion of the earth’s crust which is a mixture of organic and inorganic materials
is known as soils. To a great degree, soil quality determines the nature of plant ecosystems
and the capacity of land to support animal life and society. However, as human society
becomes increasingly urbanized, fewer people have intimate contact with the soil, and
individuals tend to lose sight of the many ways in which they depend upon soils for their
prosperity and survival.
Indeed, the earth is very unique as it is covered with life sustaining air and water.
These two resources are becoming scarce, as the population increases without increasing
the land area. Recent land-use is focusing now on commercial and residential leaving
agricultural prime lands converted to other uses; hence, food production is not sustained. A
lot of human activities and calamities are also threats to these resources; hence great care is
needed to preserve its quality if people want to live.
Learning Outcomes
At the end of this unit, the students should be able to:
1. Describe soils
2. Enumerate various field of specializations in soil science
3. Explain the function of soils in our ecosystem
4. Illustrate the four components of soils
5. List down the elemental composition of the earth’s crust
6. Classify the nutrient elements and its ionic forms absorbed by the plants
Activities
1. Examine the pictures below, and then write an essay focused on the importance
of soil to crop production and man’s survival.
2. Franklin D. Roosevelt once said “A Nation that destroys its soil destroys itself”.
What does it mean?
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Analysis
1. What do soils contribute to your family and the entire ecosystem?
2. What do you think soils are composed of?
3. Why plants are growing luxuriantly in some areas but stunted or dying in other
places?
4. How did you utilize the vacant lots in your front yard, backyard or in your
community? What will you suggest?
Abstraction
In any ecosystem, whether your backyard, a farm, a forest, or a watershed, soils
perform significant ecological functions. They act as the principal medium for plant growth,
regulate water supplies, recycle raw materials and waste products, and serve as a major
engineering medium for human-built structures. They are also home to many kinds of living
organisms. Soil is thus a major ecosystem in its own right. The soil of the world is extremely
diverse, each type of soil being characterized by a unique set of soil horizons. A typical
surface soil in good condition for plant growth consists about half solid material (mostly
mineral, but with a crucial organic component too) and half pore spaces filled with varying
proportions of water and air. These components interact to influence a myriad of complex
soil functions, a good understanding of which is essential for wise management of our
terrestrial ecosystem.
A. SOIL DEFINED: EDAPHOLOGICAL AND PEDOLOGICAL
Soil – a dynamic natural body composed of mineral and organic materials and living
forms in which plants grow. It refers to the collection of natural bodies occupying parts of the
earth’s surface that supports plants and have properties due to the integrated effect of
climate and living matter acting upon parent material, as conditioned by relief, over periods of
time.
Two approaches in studying soils:
a. Pedological approach - (Greek: pedon, soil or earth) is the study of soils as they
occur in nature with principal interest on the characterization and differentiation of
their properties and with only minor emphasis on their practical use. In short, the
study for their taxonomic classification is a pedological approach.
b. Edaphological approach – (Greek: edaphos, soil or ground) is the study of soil
with emphasis on their practical use, particularly the relationship of soil properties
to plant growth. The study of soil fertility is edaphological approach.
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B. FIELD OF SPECIALIZATIONS IN SOIL SCIENCE
1. Soil fertility – this refers to the study of quality of soils to provide optimum level of
nutrients for plant growth.
2. Soil physics – this study the characteristics, processes, or reactions of a soil caused
by physical forces
3. Soil chemistry – is the study of interaction of solid, liquid, and gaseous phases or
components of soil
4. Soil microbiology – this specializes on the study of soil biochemical reaction carried
out primarily by microorganisms
5. Soil mineralogy – this has something to do with the structural chemistry of the solid
components of soil
6. Soil genesis, morphology and classification – deals with the structural characteristics,
mode of origin, and systematic arrangement of soils.
7. Land Use – deals with the utilization or allocation of lands for general or broad
purposes such as agriculture, forestry, settlement and military reservations.
C. FUNCTIONS OF SOILS
1. Medium for plant growth – it is clear that the soil mass provides physical support,
anchoring the root system so that the plant does not fall over. It provides ventilation
allowing CO2 to escape and O2 to enter the root zone via the network of soil pores. It
absorbs water and holds it where it can be used by plant roots. It moderates
temperature fluctuations, allowing roots to function normally. Soil supply plants with
inorganic, mineral nutrients in the form of dissolved ions. The plant takes these
elements out of the soil solution and incorporates most of them into the thousands of
different organic compounds that constitute plant tissue. A fundamental role of soil in
supporting plant growth is to provide a continuing supply of dissolved mineral
nutrients in amounts and in relative proportions appropriate for plant growth.
2. Regulator of water supplies – we must recognize that nearly every drop of water in
our rivers, lakes, estuaries, and aquifers has either traveled through the soil or flowed
over its surface. If the soil allows the rain to soak in, some of the water may be stored
in the soil and used by the trees and other plants while some may seep slowly down
through the soil layers to the groundwater, eventually entering the river over a period
of months or years as base flows. If the water is contaminated, as it soaks through
the upper layers of soil it is purified and cleansed by soil processes that remove many
impurities and kill potential disease organisms. Clearly, the nature and management
of soils in a watershed will have a major influence on the purity and amount of water
findings its way to aquatic system.
3. Recycler of raw materials – what a world be like without the recycling functions
performed by soils? Without reuse of nutrients, plants and animals would have run
out of nourishment long ago. The world would be covered with a layer, possibly
hundreds of meters high, of plant and animal wastes and corpses. Obviously,
recycling must be a vital process in ecosystems. Whether forests, farms, or cities.
The soil system plays a pivotal role in the major geochemical cycle. Soils have the
capacity to assimilate great quantities of organic wastes to form, turning it into
beneficial humus, converting the mineral nutrients in the wastes to forms that can be
utilized by plants and animals, and returning the carbon to the atmosphere as carbon
dioxide, where it becomes part again of living organisms through plant
photosynthesis. Some soils can accumulate large amounts of carbon as soil organic
matter, thus having major impact on such global changes as the much-discussed
greenhouse effect.
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4. Habitat for soil organisms – soil is not a mere pile of broken rock and dead debris. A
handful of soil is a home to billions of organisms, belonging to thousands of species.
Even with this small quantity of soil, there are likely exist predators, prey, producers,
consumers and parasites. How is it possible for such diversity of organisms to live
and interact in such a small space? One explanation is the tremendous range of
niches and habitats in even a uniform-appearing soil. Some pores of the soil will be
filled with water where organisms such as roundworms, diatoms and rotifers swim.
Tiny insects and mites may be only millimeters from areas of anoxic conditions.
Different areas may be enriched with organic materials; some places may be highly
acidic, some more basic. Temperature, too, may vary widely. Hidden from view in
the world’s soil are communities of living organisms every bit as complex and
intrinsically valuable as their counterparts that roam the savannas, forests, and
oceans of the earth. Soils harbor much of the earth’s genetic diversity. Soils like air
and water are important components of the larger ecosystem. Yet only now is soil
quality taking its place, with air quality and water quality, in discussion of
environmental protection.
5. Engineering medium – “Terra firma, solid ground” we usually think of the soil as being
firm and solid, a good base on which to build roads and all kinds of structures.
Indeed, most structures rest on the soil, and many construction projects require
excavation into the soil. However, designs for roadbeds or building foundation that
work well in one location on one type of soil may be inadequate for another location
with different soils.
D. COMPOSITION OF SOILS
The four (4) major components of soils are air, water, mineral matter and organic
matter. The relative proportions of these four components greatly influence the behavior and
productivity of soils. It should be noted that about half of the soil volume consists of solid
material (mineral and organic), while the other half consists of pore spaces filled with air or
water. Of the solid material, typically most is mineral matter derived from rocks of the earth’s
crust. Only about 5% of the volume in this ideal soil consists of organic matter.
1. Air (25%) – the gases are found in the pores and is composed of nitrogen
(78%), oxygen (20%), carbon dioxide and other gases (2%). It provides
oxygen for respiration of plant roots, thus, there must be enough aeration
at the root zone for easy exchange of CO 2 and O2 between the soil pores
and the aboveground atmosphere. Otherwise, CO 2 could build to high
level which can be toxic to plant roots.
2. Water (25%) – is also in the pores that contain dissolved nutrients. It is
sometimes called soil water, soil moisture or soil solution. It serves as
solvent of nutrient compounds and at the same time carries nutrients to the
proximity of roots where they can be absorbed. The soil water also serves
as weathering agent of minerals to constantly renew the nutrient supply in
the soil. Soil water is also needed by plants in large amounts for their
metabolic functions. The dissolved CO 2 in soil water forms carbonic acid
which is a solvent that can release nutrients from minerals.
3. Organic matter (5%) – consists of decayed plant and animal bodies which
is the chief source of nitrogen, and is, in fact, an indication of the nitrogen
status of the soils. It also contributes P, S and micronutrients although in
small quantities compared to mineral sources. OM enables the soil to store
4
cations. The most pronounced effect of organic matter on soil is the
formation and stabilization of aggregates which, in turn give the soil
greater permeability and porosity.
4. Mineral matter (45%) - these are inorganic materials derived from rocks;
distinct minerals found in nature include quartz and feldspars. It is the
major source of P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, B, Mo, and Cl. It is
composed of sand, silt and clay fractions which determine several other
physical properties of soil such as texture, porosity, water-holding capacity,
permeability to water or hydraulic conductivity and ease of tillage or
workability.
Air, 25%
Water, 25%
Mineral Matter,
45%
Organic Matter, 5%
The ideal composition of soils is indicated in the illustration above. However, the
proportion of these components varies with the kind of soil:
a. In paddy soils, the pore spaces are nearly completely filled with water
b. In organic soils, the organic matter exceeds 20%
c. Most soils are classified as mineral soils, that is, they contain less than 20%
organic matter
d. The CO2 in soil air is typically higher in concentration than that above ground
because of the accumulation from CO 2 evolution from organic matter
decomposition, plant root respiration, and reaction products of carbonate
materials.
E. COMPOSITION OF THE MINERAL MATTER
Mineral matter has three major fractions.
a. Sand
b. Silt
c. Clay
The clay fraction consists of:
a. Crystalline components
b. Non-crystalline components
5
F. ELEMENTAL COMPOSITION OF THE EARTH’S CRUST
Soil has an innate elemental composition depending on the parent rocks from the
earth’s crust mantle. Earth’s crust is about 16 km depth. It has the following elemental
composition and its oxide forms:
Element
% by weight
Oxide
% by weight
O
46.60
SiO2
59.07
Si
27.70
Al2O3
15.22
Al
8.10
Fe2O3
3.10
Fe
5.00
FeO
3.10
Ca
3.60
Cao
5.10
Mg
2.10
MgO
3.45
Na
2.80
Na2O
3.71
K
2.60
K2O
3.11
Ti
0.50
TiO2
1.03
P2O5
0.30
H
0.14
MnO
0.11
P
0.11
Mn
0.09
S
0.03
G. THE ESSENTIAL NUTRIENT ELEMENTS
The following are the essential nutrient elements that must be present in the soil since
these are needed by the plants for its growth and development. Macronutrients are nutrient
elements needed by the plants in large quantities while micronutrients are nutrient elements
needed in small quantities. Ionic forms of the elements are the ones absorbed by the roots of
the plants.
IONIC FORMS OF THE ESSENTIAL ELEMENS TAKEN UP FROM THE SOIL
Nutrient Elements
Chemical Symbol
Ionic Forms Absorbed by
Plants
MACRONUTRIENTS
Nitrogen
N
NO3-, NH4+
Phosphorus
P
H2PO4-, HPO42Potassium
K
K+
Calcium
Ca
Ca2+
Magnesium
Mg
Mg2+
Sulfur
S
SO42MICRONUTRIENTS
Manganese
Iron
Boron
Zinc
Copper
Molybdenum
Chloride
Mn
Fe
B
Zn
Cu
Mo
Cl
Mn2+
Fe2+
BO32Zn2+
Cu2+
MoO42Cl6
The above nutrient elements were considered essentials considering the following
criteria:
1.) A plant is unable to complete its life cycle in the absence of the mineral
element.
2.) The function of the element is not replaceable by another mineral element.
3.) The element is directly involved in plant metabolism.
Application
Written
1. As a society, is our dependence on soils likely to increase or decrease in the decades
ahead? Explain.
2. Discuss how a soil as a natural body differs from soils used as material in building a
roadbed?
3. What are the five main roles of soil in an ecosystem? For each of these ecological
roles, take sample pictures within your community.
4. Think back over your activities during the past week. List as many incidents as you
can in which you came into direct or indirect contact with the soil.
5. What happens to the four (4) components of soil in a highly compact soil?
6. Explain in your own words how the soil’s nutrient supply is held in different forms,
much the way that a person’s financial assets might be held in different forms.
7. List the essential nutrient elements that plants derive mainly from the soil.
8. Are all elements contained in plant essential nutrients? Explain
Practical
a. Establish a VEGETABLE GARDEN in your frontyard or backyard. If no
vacant lot is available, do container gardening.
b. For checking by your professor (home visit after the midterm exam as your
major requirement)
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Unit 2 –SOIL FORMATION AND DEVELOPMENT
Introduction
On Earth, the rock at the surface, coming in contact with water, air, and living things,
was transformed into something new, into many different kinds of soils. The parent materials
from which soils develop vary widely around the world and from one location to another only
a few meters apart. Knowledge of these materials, their sources or origins, mechanisms for
their weathering and means of transport and deposition are essential to understanding soil
genesis.
Learning Outcomes
1.
2.
3.
4.
5.
6.
After studying this unit, the students are expected to:
Trace the origin of soils
Classify the different types of rocks
Differentiate physical and chemical weathering
Discuss the factors that affect soil formation
Describe the four basic processes of soil formation
Illustrate the soil profile and its horizons
Activity
1. From the illustrations below, tell something how soils are developed?
a. From Rocks to Soils
b. From Decayed Plant Residues to Soils
8
Analysis
1. How soils are formed out of rocks? What are the processes involved?
2. How decayed plant and animal residues are turned into soil (humus)?
Abstraction
Soil formation is stimulated by climate and living organisms acting on parent materials
over periods of time and under the modifying influence of topography. These five major
factors of soil formation determine the kinds of soil that will develop at a given site. When all
of these factors are the same at two locations, the kind of soil at these locations should be
the same.
Soil genesis starts when layers or horizons not present in the parent material begin to
appear in the soil profile. Organic matter accumulation in the upper horizons, the downward
movement of soluble ions, the synthesis and downward movement of clays, and the
development of specific soil particle groupings in both the upper and lower horizons are signs
that the process of soil formation is under way.
The four general processes of soil formation and the five major factors influencing
these processes provide us with an invaluable logical framework in site selection and in
predicting the nature of soil bodies likely to be found in particular site. Conversely, analysis
of the horizon properties of a soil profile can tell us much about the nature of the climate,
biological, and geological conditions (past and present) at the site.
A. SOIL FORMING ROCKS AND MINERALS
1. Rocks - is an aggregate of one or more minerals.
The rock exposed at the earth’s surface has crumbled and decayed to
produce a layer of unconsolidated debris overlying the hard unweathered rock.
Unconsolidated layer is called regolith (A,B and C horizons).
Saprolite is the underlying rock that has weathered in place to the degree that
is loose enough to be dug with a spade.
Solum (true soil) – consist of A and B horizons
2. Classes and Examples of Rocks
a. Igneous Rocks–formed from molten magma, a hot fluid mass or rock melt.
Examples of igneous rocks and its mineral compositions:
:
Granite (quartz, K-feldspar, biotite)
Diorite (plagioclase, amphibole, quartz)
Rhyolite (K-feldspar, quartz, biotite)
Gabbro (pyroxene, plagioclase, olivine)
Andesite (plagioclase, amphibole, quartz)
Basalt (pyroxene, plagioclase, olivine)
Obsidian (volcanic glass)
Volcanic tuff (fragmented volcanic rocks)
9
b. Sedimentary Rocks – this is formed when weathering products from old
rocks are compacted or cemented
Examples:
Sandstone (with sand sediments)
Siltstone (with silt sediments)
Shale or mudstones (with clay sediments)
Conglomerate (with rounded fragments)
Limestone (with lime sediments)
Gypsum
Chert
c. Metamorphic Rocks –formed by the metamorphism or change in form of
igneous or sedimentary rocks by heat or pressure.
Examples:
(Igneous or sedimentary) (metamorphic)
Granite
Gneiss
Basalt
Schists
Sandstone
Quartzite
Limestone
Marble
Shale
Slate
Conglomerate
Meta-conglomerate
3. Some Pictures of Sedimentary, Metamorphic and Igneous Rocks
a. Igneous rocks
10
b. Sedimentary rocks
Conglomerate
Limestone
Sandstone
Gypsum
c. Metamorphic rocks
Schist
Quartzite
11
Slate
Marble
Gneiss
4. Mode of Formation of Igneous Rock: Intrusive and Extrusive
Igneous rocks (the original rocks) – are formed from the molten magma as a result
of cooling and solidification.
12
5. Different Types of Igneous Rock
Types of Igneous Rock
1. Intrusive – form at depth in the earth crust, then intruded to the surface, it is
cooled slowly, thus forming coarse grained minerals. This is sometimes
referred to as plutonic rocks.
2. Extrusive – spewed out over the earth surface and cooled fast like in volcanic
eruption thus forming fine-grained minerals. This is sometimes known as
effusive rocks. This kind of igneous rock is very common in the Philippines.
Pyroclastic rocks. Extrusive rocks texture is referred to as pyroclastic. This
texture results from a very explosive eruption, which sends not only lava flying
through the air, but also fragment of the volcano itself. All airborne volcanic
fragments are referred to as pyroclasts.
Based on size and shape, pyroclasts are categorized into: ash the smallest
particle, lapilli slightly larger than ash, and blocks or bombs the biggest ones.
6. Primary vs. Secondary Minerals and Examples of each Class
Minerals – is a naturally occurring inorganic substance with fairly definite chemical
composition and specific physical properties.
Important physical properties
1. Hardness- resistance to scratching measured using Moh’s scale
Example: talc =1; diamond = 10
2. Cleavage – smooth surface of breakage
3. Fracture – rough surface of breakage
4. Specific gravity – ratio of weight substance to that an equal volume of water
5. Color – wavelength of light absorbed by the mineral
6. Streak – color of finely powdered minerals
7. Luster – reflection of ordinary light (e.g.gold)
8. Tenacity - resistance to breakage or bending
Two groups of Minerals:
1. Primary Minerals – formed at temperatures and/or pressure higher than normally
encountered at the earth’s surface (one atmosphere and <100 oC). Usually, these are
components of igneous and metamorphic rocks.
Examples:
a. Quartz (SiO2) – very resistant to weathering and it constitute 50%
to 90% of the sand and silt fraction of the soil. It is colorless when
pure, hardness of 7.0 (Moh’s scale) and a density of 2.65 g/cm 3.
13
b. Feldspar – aluminosilicates, hardness of 6.0 and density of 2.63
g/cm3
i. Orthoclase – potassium bearing feldspar
ii. Albite – sodium bearing feldspar
iii. Anorthite –calcium bearing feldspar
c. Micas – these minerals have excellent cleavage. These are very
soft minerals with hardness of 2.0 to 2.5
i. Muscovite - colorless flakes
ii. Biotite – dark colored, flake-like
14
d. Pyroxenes and Amphiboles – these are ferromagnesian with
hardness of 5.0 to 6.0 and with distinct cleavage.
i. Hornblende
ii. Augite
e. Apatite – calcium phosphate minerals with hardness of 5.0
i. Carbonatoapatite
ii. Sulfatoapatite
iii. Hydroxyapatite
iv. Chloroapatite
v. Flouroapatite
f.
Carbonate group – carbonates of Ca and Mg with hardness of 3.0
to 4.0 and a density of 2.72 to 2.85 g/cm3
i. Calcite
CaCO3
ii. Dolomite Ca.Mg(CO3)2
2. Secondary Minerals – formed under conditions of temperature and pressure found
at the earth’s surface by the weathering of preexisting minerals.
Examples:
a. Iron group – oxides of Fe in several states of hydration. It imparts red color
to the soils and largely found in highly weathered soils. This is common in
old/infertile and acidic soils.
i. Hematite
Fe2O3
ii. Limonite
Fe2O3.2H2O
b. Aluminum group – oxides of Al in several states of hydration. Also found
in highly weathered, old/infertile and acidic soils.
i. Corundum
Al2O3
ii. Boehmite
AlOOH
iii. Gibbsite
Al2O3.3H2O
c. Gypsum – source of Ca and S. It can be used as soil ameliorant or as a
retarder in cement. It has a hardness of 2.0 and has a molecular formula
of CaSO4.2H2O.
d. Clay group – hydrated aluminosilicates, usually come from micas and
feldspars. This is colloidal in nature.
15
i. Kaolin group – kaolinite, dikite, nacrite, anauxite
ii. Montmorillonite group – montmorillonite, beidellite, vermiculite,
nontronite
iii. Hydrous mica group -illite
B. WEATHERING
1. Definition
Weathering is the process by which all rocks at the earth's surface get broken
down. Weathering occurs by chemical (decomposition) and mechanical processes
(disintegration). This combines the process of destruction and synthesis. Original
rocks and minerals are destroyed by both physical disintegration and chemical
decomposition.
Physical disintegration breaks down rocks into smaller rocks and eventually
into sand and silt particles that are commonly made up of individual minerals. The
minerals decomposed chemically, release soluble materials and synthesize new
minerals, some of which are resistant end products.
New minerals are formed either from minor chemical alterations or by
complete chemical breakdown of the original ones. During the chemical changes,
particle size continues to decrease, and constituents continue to dissolve in the
aqueous weathering solution.
2. Types of Weathering
a. Physical weathering- rocks get broken into pieces but its chemical
composition remains unchanged.
Processes of Physical Weathering
1) Freeze / thaw weathering - occurs when temperature freezes at
night and rises during the day. Water expands when frozen which forces rocks
to open.
2) Biological weathering – this happens when roots of plants grow into
cracks of rocks eventually at time goes by it forces cracks to open and
disintegrate.
3) Exfoliation or Unloading – this is the process by which rock at the
earth's surface is worn away. After a rock that has formed deep in the earth is
exposed at the surface, it expands and gradually breaks into sheets.
b. Chemical weathering - the chemical composition of the rocks has been
altered or changed. Water always plays a part of the process
Carbon dioxide dissolves in rain water forming carbonic acid which
dissolves limestone rock which is carried away in solution as calcium
hydrogen carbonate.
Chemical weathering is faster for limestone than sandstone and is
speeded up by heat.
16
Processes of Chemical Weathering
1. Hydration – this is the process by which intact water molecules combine with a
mineral
5Fe2O3
Hematite
+
9H2O
water
Fe10O15(9H2O)
Ferrihydrite
2. Hydrolysis – a process by which water molecule splits into hydrogen and a
hydroxyl and the hydrogen replaces a cation from the mineral structure
KAlSi3O8
+
H2O
HAlSi3O8 + K+ + OH-
3. Dissolution – this is aided by small amounts of acid in the water; or soluble ions
are retained in the underground water supply
CaCO3 + 2[H+(H2)O]
CaSO4(2H2O) + 2H2O
Ca2+ + CO2 + 3H2O
Ca2+ + SO42- + 4H2O
OR
4. Carbonation – in this process, weathering is accelerated by the presence of weak
acids
CO2 + H2O
H2CO3 + CaCO3
H2CO3 (Carbonic Acid)
Ca2+ + 2HCO3-
Then
5. Oxidation-reduction – refers to any chemical reaction in which a compound or
radical loses electrons; this is important in decomposing ferromagnesian minerals
4Fe + 3O2
4Fe(2-)O + O2 + 2H2O
2Fe2O3
OR
4Fe(3-)OOH (Goethite)
Some Points to Remember
 Chemical weathering occurs fastest at the sharp edges of rocks as they have a large
surface and less volume so the chemical reactions are faster.
Gradually the sharp edges become rounded
 Chemical weathering produces clays on which vegetation can grow.
 A mixture of dead vegetation, clay, rock fragments of sand and silt size particles
produces soil.
 Common chemical weathering processes are: hydrolysis, dissolution, and oxidation.
 Chemical weathering tends to weaken rock, thereby making it easier to break.
 Physical and chemical weathering occurs together.
 Physical weathering breaks rocks into pieces so more surface is exposed to chemical
weathering which breaks it down further.
 Weathering is controlled largely by climate. The more water available, the more likely
that chemical processes can proceed.
 Additionally, in warm temperatures chemical weathering can proceed even faster.
 In arid climates, weathering processes occur very slowly because of the lack of water.
 Mechanical weathering will be the dominant process in arid climates;
however, because physical weathering relies on chemical weathering, it will also be
quite slow
17
C. SOIL FORMATION
Factors of Soil Formation
1. Climate – this is a function of rainfall and temperature
Effective Precipitation – depends on timing, topography, and soil type. Higher
rainfall increases the rate of weathering and soil development and there will be
greater leaching. On the contrary, lower rainfall decreases the rate of weathering and
soil development.
Temperature –weathering of rocks and minerals and biochemical reaction
increases with increasing temperature.
Climate also affects vegetation
2. Living Organisms – this refers to the biological portion of the soil and its
environment. This includes vegetation, soil organisms, human that are vital to the
cycle of life on earth. They incorporate plant and animal residues into the soil, digest
them and return CO2 to the atmosphere where it can be recycled through the higher
plants. Organisms affect soil development by their mixing activities and addition of
organic matter.
Animals like earthworms, crawfish, scorpions, gophers through their mixing
activities result in the destruction of horizons. Vegetation mainly functions in the
addition of organic matter. Grasslands added OM to upper 2 ft. of soil due to fibrous
root system of grass plants. Forest added OM to upper 4 inches due to yearly leaf fall
to surface of soil.
18
3. Parent Materials – this refers to inorganic (rocks and minerals) and organic material
where soils may originate
Types of Parent Materials:
a. Residual – parent materials (bedrocks that are formed in place (in situ).
b. Transported – loose materials like sand silt and clay deposits where soils
developed (Colluvium, Alluvium, Floodplain, Glacial, Marine Sediments,
Loess, Organic Bogs)
Alluvium – parent materials transported by rivers or stream
Aeolian - sand transported by wind (dune)
silt transported by wind (loess)
Colluvium – transported by gravity
Lacustrine – transported by lakes
Glacial Drift/glacier - all materials transported by ice or as a result of
glacial activity
Beaches (marine) – materials transported by oceans and seas
c. Organic – materials deposited by accumulated organic materials such as
peat soils and bogs
Organic bogs/peat soils – materials originating from organic materials
4. Topography/Relief – this refers to the soil’s position in the landscape
19
Summit will have minimum erosion and maximum soil development (greatest
horizonation).
Backslope will be similar to summit unless slope is > 20%. Here soil develops
slower because rainfall will runoff this slope position faster and there will be more soil
erosion.
Shoulder will have the greatest erosion, least water infiltration, greatest runoff
and minimal soil development.
Footslope will have the deposition of materials from upslope, may be near
water table and may have greatest leaching due to water from upslope and rainfall.
5. Time- vegetation and climate act on the parent material and topography over time.
The age of a soil is determined by its development and not the actual number
of years it has been developing. As to how long it takes for a soil to become old
depends on the intensity of the soil forming processes or intensity of the other 4 soil
forming factors. All soil forming processes occur over a very long period of time. The
time it takes to develop a soil is relative, dependent upon climate, vegetation and
human interaction.
What happens to a soil with time?
• Loss of nutrients (bases) that results in lower pH, hence soil becomes acidic
• Increase in concentration of iron, thus, soil becomes redder
• Increase in clay content or old soils have more clay
• Deeper weathering into the parent material
Other Important Terms
Catenas. A group of soils are developed from the same parent material but
differ on the basis of drainage due to variations in relief.
Chronosequence. A sequence of related soils that differ in certain properties
primarily as a result of time which acted as a factor of soil-forming processes.
Lithosequence. A group of related soils that differ as a result of differences in
parent material.
20
Climosequence. A sequence of soil that differ as a result of changes in
climatic regimes (temperature and precipitation)
Biosequence. A group of related soils that differ primarily due to variation in
kinds and number of plants and soil organisms.
D. FOUR BASIC PROCESSES OF SOIL FORMATION
Soils are formed mainly due to four important processes, namely:
transformation, translocation, addition and losses.
1.) Transformation - When soil constituents are chemically or physically modified or
destroyed and others are synthesized from precursor materials.
2.) Translocation – This refers to the movement of organic and inorganic materials
horizontally or vertically across a pedon. Movement of OM, clay, water, iron, and nutrients in
colloidal size (very small particles) are evidence of this.
3.) Additions - Inputs of materials (water, organic matter, air, soil particles, salt) from
outside sources (i.e. plant litter).
4.) Losses – A process whereby materials (water, organic matter, CO2, nutrients) are
removed from the soil profile by leaching, erosion or plant removal.
21
E. SOIL PROFILE and its HORIZON
Soil profile is the vertical section exposing a set of horizons in the wall in a soil pit is
termed as soil profile.
Soil horizon is a layer of soil approximately parallel to the soil’s surface, differing in
properties and characteristics from adjacent layers below or above it.
A soil horizon is a layer of soil, revealed in a soil profile, lying approximately parallel to
the earth's surface, and possessing relatively homogeneous physical, chemical, and
biological properties.
Three pedogenic horizons are A, B and C horizons.
Regolith is the A, B and C horizon
Solum is the A and B horizon which is also the true soil
Parent material is the C horizon, the weathered rock or unconsolidated sediments
Soil Horizon (Master Horizons)
1. O Horizons – Organic horizon at the soil surface, usually unconsolidated organic material
(leaf litter, roots, leaves, etc.), not saturated with water.
Types of O Horizon
a. Oi Horizon (i) - Fibric material - Recognizable plant and animal part
b. Oe Horizon (e) - Hemic materials - Finely fragmented residues intermediately
decomposed
c. Oa Horizon (a) - Sapric materials - Highly decomposed, smooth, and amorphous
residues
22
2. A Horizons
Mineral horizon formed at or near the surface where humified organic matter is
associated with mineral materials. This is usually darker in color because of the organic
matter. The organically enriched A horizon at the soil surface is sometimes referred to as
top soil. When a soil is plowed and cultivated, the natural state of the upper 12 to 25 cm is
modified. In this case, the topsoil may also be called the plow layer or the furrow slice in a
situation where a moldboard plow has turned or “sliced” the upper part of the soil. In
cultivated soil, the majority of plant roots can be found in the topsoil. The topsoil contains a
large part of the nutrients and water supplies needed by plants.
3. E Horizons
Mineral horizon just below the soil surface that has lost its silicate clay, organic
matter, aluminum, or iron by downward movement, leaving a concentration of resistant sand
and silt particles, and is usually lighter than the above or lower horizons.
"E" stands for "eluvial horizon," a soil layer formed by the removal of constituents
such as clay or iron. Eluviation describes the process whereby constituents of soil are
removed in suspension.
4. B Horizons. A subsurface mineral horizon resulting from (1) the change in situ of soil
material, i.e., the obliteration of the original rock structure, or (2) the washing in of material
from overlying horizons, i.e., the accumulation of silicate clay, organic matter, aluminum, or
iron. Illuviation describes the process of accumulation of materials from overlying horizons.
5. C Horizons. Unconsolidated or weakly consolidated mineral horizon that retains evidence
of rock structure, but lacks diagnostic properties of the overlying A, E, and B horizons. This
horizon is little affected by pedogenic (i.e. soil forming) processes. Examples include beach
sand, windblown silt (or loess), alluvium deposited by rivers, and glacial till deposited by
glacial ice.
6. R Horizons continuous (consolidated) hard or very hard bedrock.
Subscripts - all B horizons have a subscript, most transition horizons do not.
used subscripts and its description are indicated below.
Commonly
a - sapric - organic soils - well decomposed
b - buried soil horizon
d - dense - geogenic soil material (compacted by glacier)
e - hemic - moderately decomposed organic soil
f - frozen soil - permanently frozen, permafrost
g - gleyed soil - gray color due to low O2 - reduction of Fe
h - accumulation of humus - O.M. other than in the A or O horizons
i - fibric - organic - non-decomposed
k - accumulation of calcium carbonate (CaCO3)
m - cementation - hard – indurated
n - sodium accumulation
p - plowing - only used with A
q - silica accumulation - very weathered or old soil
r - soft rock - used with C or Cr
s - sesquioxides (1.502) (Fe2O3) accumulation of Fe and Al - red color
ss – slickensides present –shiny surface on ped face caused from soil rubbing against
soil
t - clay accumulation - clay films
w - color or structure development (Bw)
23
x - fragipan - hard, dense layer that developed with time
y - gypsum accumulation (CaSO4)
z - salts more soluble than gypsum (KCL - NaCl - NaSO4)
Factors that retard soil profile development
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
low rainfall
high lime content
high clay content
steep slopes
cold temperature
severe erosion
low humidity
high quartz
hard rock
high water table
constant deposition
mixing by animals
climate (low rainfall, low humidity, cold temperature)
biota (mixing activities of animals or man)
Applications
1. What do you think are the common types of rocks present:
a. in beach resorts?
b. near the riverbanks?
c. near the volcano?
2. What are minerals? How do primary minerals differ from secondary minerals? Give
examples of each type.
3. What is meant by the statement, “weathering combines the process of destruction
and synthesis”?
4. Give examples of physical and chemical weathering.
5. How is water involved in the main types of chemical weathering reactions?
6. Name the five factors affecting soil formation. If one of these factors is missing, do
you think weathering will still takes place?
7. How do colluvium, glacial till, and alluvium differ in terms of agency of transport?
8. What is loess, and what are some of its properties as a parent material?
9. Give two examples for each of the four broad processes of soil formation.
10. If both O and A horizons are absent in the soil profile, do you think it is still suited for
agricultural production? Why?
11. If you were to dig a 1 meter pit in your backyard, what can you say about its soil
profile? Take picture if necessary.
.
24
Unit III. PHYSICAL PROPERTIES OF
SOILS
Introduction
Physical properties exert a marked influence on the behavior of soils with regard to
plant growth, hydrology, environmental management, and engineering uses. The nature and
properties of the individual particles, their size distribution, and their arrangement in soils
determine the total volume of nonsolid pore space, as well as the pore sizes, thereby
impacting on water and air relationships.
These properties of individual particles and their proportionate distribution (soil
texture) are subject to little human control in field soils. However, it is possible to exert some
control over the arrangement of these particles into aggregates (soil structure) and on the
stability of these aggregates. Tillage must be carefully controlled to avoid undue damage to
soil tilth, especially when soils are wet. Generally, nature takes good care of soil structure,
and humans can learn much about soil management by studying natural systems. Vigorous
and diverse plant growth, generous return of organic residues, and minimal physical
disturbance are attributes to natural systems worthy of emulations.
Particle size, moisture content, and plasticity of the colloidal fraction all help
determine the stability of soil in response to leading force from tillage or building foundations.
Learning Outcomes
At the end of this unit, the students should be able to:
1. Describe the following physical properties of soils
a. Soil texture
b. Soil structure
c. Soil density
d. Soil porosity
e. Soil water
f. Soil aeration
g. Soil consistency
h. Soil color
i. Soil temperature
2. Explain the importance of these properties on soil fertility and crop suitability
3. Discuss how these properties are being determined or estimated.
Activities
1. Using the table below, try to characterize the following soil samples.
Areas
Area 1
Characteristics
Area 2
Area 3
25
Area 1
Area 2
Area 3
Analysis
1. Why do soils differ in characteristics in our ecosystem?
2. What do you think is the effect of these variations to soil management and crop
production?
3. Are the soils in your community suited for crop production? Why?
Abstraction
Soils have innate characteristics that are observable by man. These observable
characteristics are known as the physical properties of soils. Some of these properties can
be seen by our naked eye such as color, structure or arrangement of soil aggregates and
compactness of the soil. To some extent by rubbing soil samples in between the thumb and
forefingers, one can tell the soil texture and consistency. However, these properties are
scientifically established using appropriate laboratory facilities
All these are important considerations in crop production because it has significant
bearing to soil fertility, proper plant species selection, crop rotation, and management of
chemical and biological factors which can help assure maintenance of soil physical quality.
A. SOIL TEXTURE
Soil texture is the relative proportion of sand, silt and clay and the single most
important physical property of the soil. Knowing the soil texture alone will provide
information about: 1) water flow potential, 2) water holding capacity, 3) fertility potential,
4) suitability for many urban uses like bearing capacity
o
Critical for understanding soil behavior and management
26
o
o
Soil texture is not subject to change in the field but can be changed in potting mixes
Soil separates include: sand, silt and clay
Sand
0.05 – 2.0 mm
Visible without microscope, Rounded or angular in shape
Sand grains usually quartz if sand looks white or many minerals if sand looks brown,
Some sands in soil will be brown, yellow, or red because of Fe and/or Al oxide
coatings
 Feels gritty and non-cohesive – does not stick together in a mass unless it is very wet.
 Low specific surface area (Sandy Loam – 10-40 m 2/gram)




 Sand has less nutrients for plants than smaller particles
 Voids between sand particles promote free drainage and entry of air
 Holds little water and prone to drought
Silt
0.002 – 0.05 mm
Not visible without microscope
Quartz often dominant mineral in silt since other minerals have weathered away.
Does not feel gritty, Floury feel –smooth like silly putty
Wet silt does not exhibit stickiness / plasticity / malleability
Smaller size allows rapid weathering of non-quartz minerals
Smaller particles – retains more water for plants and have slower drainage than sand.
Easily washed away by flowing water – highly erosive.
Holds more plant nutrients than sand.
Silt is responsible for silting over gravel beds in rivers that are needed by fish for
spawning.
 Responsible for the murky color of the water in the river or streams










27
Clay













< 0.002 mm
Flat plates or tiny flakes
Small clay particles are colloids; If suspended in water will not settle
Large surface area (150- 2000 m 2/gram)
Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod.
Easily formed into long ribbons
Shrink swell – none to considerable depending on the kind of clay.
Pores spaces are very small and convoluted
Movement of water and air very slow
Water holding capacity is high
Tremendous capacity to adsorb water- not all available for plants.
Soil strength- shrink/swell affects buildings, roads and walls.
Chemical adsorption is large
28
General Characteristics Associated with the Soil Separates
Characteristics
Sand
Low
Ratings
Silt
Medium to
high
Clay
High
2. Aeration (exchange of gases)
Good
Moderate
Poor
3. Drainage rate (removal of excess
of H2O)
High
Slow to
medium
Very slow
4. Compactness
compaction)
to
Low
Medium
High
5. Ability to store plant nutrients
(CEC)
Poor
Medium to
high
High
6. Resistance to pH
(buffering capacity
Low
Medium
High
Low
Medium
High
1. Water holding capacity (ability to
hold water)
(susceptible
changes
7. Total surface area (TSA)
Characteristics and Feel of Soil Separates
Soil Separate
Diameter range (mm)
USDA
ISSS
Sand
2 - 0.05
2 – 0.05
Silt
0.05 – 0.002
0.02-0.002
Clay
<0.002
<0.002
Characteristics and Feed
Coarse, gritty, mostly primary minerals
(quartz and feldspars), cubic to
spherical in shape
Smooth, powder, mostly primary
minerals (quartz, feldspars), cubit to
spherical in shape
Sticky and plastic when moist, mostly
secondary “clay” minerals very high
specific surface area hence the most
reactive component of the soil, platelike, or flake-like and are tubular in
shape.
Changing Soil Texture






Soil texture can be changed only by mixing with another soil with a different textural
class in small quantities
Adding sand to a clay soil creates a cement like substance
Find soil with native textural properties you desired– don’t try to make the texture you
want.
Adding peat or compost to a mineral soil is not considered changing the texture –
since it only adds organic matter not sand, silt or clay
Over long periods (1000s yrs) pedologic processes alter soil horizon textures.
As soils get older sand weathers to silt and silt weathers to clay….therefore old soils
have more clay.
29


Clay also translocates down in the soil profile so subsoils generally have more clay
than topsoils.
Thus ‘Argillic’ horizons form and are zones of clay accumulation
Significance of Soil Texture



Affects water retention, permeability and movement (infiltration, percolation,
drainability, etc.) and soil aeration (sandy soils are more porous)
Determines crop suitability (Root crops best planted on loam, while rice paddy is best
planted in clay loam to clay)
May indicate inherent soil fertility (clay soils are generally more fertile than sandy
soils, which are usually K-deficient)
Method of Soil Texture Determination
1. Roll Method – making ribbon out of the soil; the length of ribbon indicates clay
content
2. Feel Method – rubbing moist soil in between fingers; Grit indicates sand,
smoothness indicates silt
3. Mechanical Method – involves particle size analysis. This entails the use of a nest
of sieves with different size openings.
4. Sedimentation method – makes use of the principle of Stoke’s Law which states
the “the settling velocity of spherical particles in viscous medium is directly proportional to the
size of the particles.” Bigger particles settle more quickly.
i. Hydrometer Method
ii. Pipette method
How to determine soil textural classes?
After the analysis of soil samples, you can determine soil textural classes using the
soil triangle. In essence, there are 12 textural classes which can be determined using the
relative proportions of sand, silt and clay. This is simply done by locating the point of
intersections based from the percentage of each soil separate.
The 12 Textural Classes
Coarse:
Sand – 85% or more sand content
Loamy sand – 75 -85% sand
Sandy loam – 43-52% sand
Medium:
Loam – <52% sand, 28-50% silt, 7-27% clay
Silt Loam - >50% silt
Silt – 80% silt
Clay Loam
Sandy clay loam
Silty Clay Loam
Fine:
Sandy Clay
Silty Clay
Clay
30
USDA Textural Triangle
Examples:
1.
A soil sample with 58% sand, 32% silt, 10% clay is considered SANDY
LOAM (please see red arrows to show the process).
31
2. A soil sample with 48% sand, 8% silt, and 44% clay is SANDY CLAY (please
see intersection of three arrows as illustrated below)
B. SOIL STRUCTURE
Soil Structure is the arrangement of primary soil particles into peds. These are
formed in the soil by wetting, drying, freezing and thawing and are held by clay and organic
matter
The structure influence




Water movement
Heat transfer
Aeration
Porosity
Types of Soil Structure
1. Spheroidal
Granular – porous
2. Platy
3. Blocky
Angular
Sub-angular
4. Prismatic
Columnar
Prismatic
Crumb – non-porous
32
33
Formation of Soil Structure
1. Binding Agents : soil organic matter, products of microbial decomposition (gums,
polysaccharides, etc), Lime, colloidal clays
2. Effect of adsorbed cations
Calcium – encourages flocculation of aggregation of soil
Sodium – encourages dispersion of soil
Factors that Affect Soil Structure:






Kind of clay
Amount of organic matter
Freezing and thawing
Wetting and drying
Action of burrowing organisms
Growth of root systems of plants
Aggregate Stability - ability of the soil aggregates to resist breakage upon wetting
 OM is mainly responsible
 Soils with high Kaolinite and hydrous oxide clays have high stability
 Mechanisms of Na and Ca effects (dispersion and flocculation)
Soil Management related to soil structure
 OM for sandy soils – Sandy soils have good drainage and aeration but easily become
dry and droughty; OM increases water holding capacity (WHC)
 Proper tillage or cultivation of clay soils – If plowed when too wet, the soil loses its
structure and becomes puddled
 Cropping system – continuous corn – less water stable aggregates; corn in rotation
with grass cover – larger aggregates
 Mulching or Cover Cropping – protects structure from rain
Pore space is very important for providing aeration and paths for water to flow and be stored.
 Macropores – Pores larger than 0.08 mm. Allow the movement of air, water, and
plant roots.
 Micropores – Pores smaller than 0.08 mm. Usually water filled in field conditions.
C. SOIL DENSITY (Bulk and Particle Density)
a. Soil Bulk Density - Ratio of the mass of dry soil to the total volume of the soil
Determined by dividing the weight of oven-dry soil in grams by its volume in cubic
centimeters


The variation in bulk density is due largely to the difference in total pore space
Bulk density affects engineering properties, water movement, rooting depth of plants
Example :
Dry weight of soil (ODW) = 160 gram
Volume of soil (Vt) = 100 cm 3
Volume of soil solids (Vss) = 60 cm 3
Bulk Density (Pb) = ODW = 160 gram = 1.6 gram/cm 3
Vt
100 cm3
34
b. Soil Particle Density - Particle density is the mass of the soil particles divided by the
volume of the soil solid particles alone . Particle density normally is 2.65 g/cm 3
Particle Density (Pp) = ODW = 160 gram = 2.67 gram/cm3
Vss
60 cm3
Range of Values:
Pb = 1.0-1.6 (clay, clay loam, silt loam)
1.2– 1.8 (sand, sandy loam)
Pp = 2.5 – 2.75 (average = 2.65 g/cm 3)
Factors Affecting Bulk Density




Organic Matter content – the higher the OM, the lower the Bulk Density
Porosity of the soil - the higher the porosity, the lower the Bulk Density
Soil depth - deeper horizons has higher bulk density due to lower OM, less
aggregation, less root penetration, and compaction due to weight of overlying
horizons
Soil Texture – sandy soils have higher bulk density because particles lie close
together. Fine textured soils (silt loam, clay loam, clay) due to their granulation,
because of generally high OM have large pore spaces and hence low bulk density
Factors Affecting Particle Density


Mineralogy
Organic Matter content – the higher the OM, the lower the Bulk Density
D. SOIL POROSITY
- the volume of the pores divided by the bulk soil volume. It influences air and water
movement in the soil.
Porosity (E) = {1 – (BD ÷ PD) } x 100
={1 – (1.6 g/cm3 ÷ 2.67 g/cm3) } x 100
= 40.07%
Factors Affecting Porosity


Bulk Density and Soil Texture
Organic Matter content – the higher the OM, the lower the Bulk Density
Bearing Capacity - determined by Soil Texture







Ability of the soil to withstand a load
Or the average load per unit area that will cause failure by rupture of a supporting soil
mass.
Soil stabilization - any method that prevents a soil system from moving under a load.
Compaction - increase the density - thereby increasing stability - apply large pressure
to soil at optimum moisture
soils with more than 30% clay have a high Shrink Swell > clay more S.S. Potential (if
clay is 2:1)
to avoid this problem soil must be compacted and water must be kept out.
without the above - Shrink/swell will crack foundations and pavements or move
telephone poles.
35
Chemical Soil Stabilization


Lime, cement, and pozzolan (high silica volcanic ash) can be used as chemical
additives.
Lime is most effective on clay soils, and can be used in combination with cement and
pozzolan.
The use of PENETROMETER - easy and reliable method for determining in-place soil
strength.
 Great for checking building pads, excavations, or potential building sites.
Simply measure the penetration and look up soil strength on the strength chart included with
the unit.
E. SOIL WATER – relationship of water to soil; universal solvent and the agent of
translocating solutes to plants and carry nutrients
Properties of Water




A simple compound with H and O tied by covalent bonds (each H + proton sharing
one electron of O).
The side of the H atoms tend to be electropositive while the opposite side is
electronegative. Thus, water is dipolar (polarity) and the (+) end attracts the (-) of the
other water molecules resulting in polymer-like grouping
Electronegativity causes water to be attracted to Na +, Ca ++ , K + and make them
hydrated
Polarity causes dissolution of salts in water because the ionic components have
greater affinity for water molecules than for each other (NaCl) as well as it also
causes water molecules to be attracted to negatively charged clay surfaces
Forces acting on Water
a.) Adhesion – force of attraction of water to solid surfaces (soil water to clay
particles). Held by strong electrical forces - low energy, little movement- held tight by soil,
exists as a film, unavailable to plants, removed from soil by drying in an oven
b.) Cohesion – force of attraction of water molecules to water molecules.
Held by hydrogen bonding (–when + & - of water molecules are close together), liquid state in
water film, major source of water for plants, greater energy than adhesion water
Cohesion and Adhesion cause capillary movement of water on soil pores.
Capillary Action – The water molecules are attracted to the sides of the solid and
spread out in response to that attraction.
Capillarity is due to:
 adhesion of water on the walls of channels
 surface tension (cohesion) which results in any form of water except that a flat
plane at the air liquid interface
 height is inversely proportional to the radius which means that the capillarity is
greater in fine-textured soils than in coarse-textured soils
Soil Water Potential – the difference in energy levels between pure water in the reference
state and soil water.
• The difference in energy levels of water from one site or condition to another
determines the direction and rate of water movement.
36
Soil water potential - water is held in soil by “tension” or attraction of water molecules to solid
surfaces and to other water molecules

Soil water potential = amount of work that must be done per unit quantity of water in
order to transport a quantity of water from a pool of pure water to the soil water



Total soil water potential = Matric potential + gravitational potential + Osmotic (salts)
As the soil dries the matric potential decreases or a larger negative number
Tension = - pressure
o Gravitational Potential – The force of gravity acting on soil water attracts the
water towards the center of the Earth
o Submergence Potential – The positive hydrostatic pressure associated with
the weight of water in a saturated soil
o Matric Potential – The negative pressure due to the attractive force between
water and the soil matrix
o Osmotic Potential – The potential energy of water as it is reduced by the
presence of solutes
The tenacity with which water is held in the soils is an expression of soil water potential.
Tensiometers – Measure the attraction of water in the soil column by adhesion and
cohesion.
Preferential Flow – Water travels quickly down large soil pathways, such as
connected macropores. This leads to the quick disbursal of pesticides and other toxins into
the soil and finally the groundwater.
Infiltration – The process by which water enters the soil pore spaces and becomes
soil water.
Percolation – Once the water is in the soil, it moves through the soil column by
percolation.
Wetting Front – A sharp boundary demarcating the transport of water through the
column
Soil Water Energy Concepts:
•
•
•
•
•
•
Water in the soil is attracted to the surface of solids and to soil solutes and by gravity,
thus plant roots must exert force or work to draw this water
The greater the moisture content (the more wet) is the soil, the lower the pressure or
force needed to move the water or vis-a vis
The pressure is called the Soil Moisture Tension or SMT
Water moves from the region of low moisture tension to a region of high moisture
tension
SMT is high in dry soil as it needs more force to draw the water
Water is held more strongly in fine-textured soils (clay loam, clay) than in coarse
textured soils (sandy loam, sand) as there are more micropores in fine textured soils
Units of Soil Moisture Tension
•
•
•
•
Cm of height of water
Atmosphere, atm (14.7 lbs/inch)
bar = atm
millibar (1/1000 bar)
37
Soil Water Classification - a way to quantitatively describe the water in the soil.
1. Gravitational Water - exists in macro –pores, has greatest energy (true liquid), moves
freely due to gravitational forces
•
•
•
•
Water between saturation point and field capacity
Not retained by the soil or is converted to capillary water when it drains to unsaturated
soil layers
Water in the macropores after heavy rain or irrigation drains
Not available to plants
2. Field Capacity – the water content in the soil after the gravitational water has drained out (
water left around 2-3 days after heavy rain when soil becomes saturated)
•
•
•
Air instead of water occupies the macropores. Micropores and capillary pores are still
filled with water (Pore space is half air filler and half water filled)
SMT at - 1/3 bar or - 0.33 atmosphere
Maximum amount of the upper limit of available water
3. Permanent Wilting Point - The time at which plants have removed all of the water from
the capillary pores and no more available water remains in the soil.
•
•
•
SMT at -15 atmosphere
Plants start to wilt due to less available water for absorption and it will not recover
even if you apply irrigation
Lowest limit of available water
4. Hygroscopic (Coefficient) Water water held at SMT > - 31 atmosphere; water is in
vapor form and not available for plant use
5. Available water- water held in the soil (by medium - sized pore spaces) between field
capacity and permanent wilting point (AW= FC-PWP).
Comprises the soil solution and is a major source of water for plant growth
6. Unavailable water- water content when SMT is greater than -15 atmosphere or PWP
Hydrologic Cycle is driven by the energy from the sun through Evaporation and
Transpiration
•
•
•
•
Water is heated by the sun
Surface molecules become sufficiently energized to break free of the attractive force
binding them together
Water molecules evaporate and rise as invisible vapor into the atmosphere
Water vapor emitted from plant leaves
World Total Water
97.2 % Ocean
2.8 % Fresh
2.15 % glaciers
0.65 % ground water
0.0001 % streams
0.009 % lakes
38
0.008 % seas
0.005 % soil
0.001 % atmosphere


Actively growing plants transpire 5 to 10 times as much water as they can hold at
once
These water particles then collect and form clouds
Considerations in Water Management
a) Soil Infiltration – nature of pores and water content of soil are major determinants of
soil profile Characteristics
b) Tillage gives a rough soil surface which controls run-off and it also loosens soil and
increases total porosity and thickness of plow layer for greater water storage
c) Placement of residue mulches at the end of rainy season conserves water by
controlling run-off, increasing infiltration, reducing weed growth and decreasing
evaporation
F. SOIL AERATION
Oxygen availability is controlled by
a) Soil macroporosity (as affected by texture and structure)
b) Soil water content (as it affects water/air ratio)
c) Oxygen consumption by respiration (Including plant roots and microorganisms)
Poor aeration occurs when 80% to 90% of the soil pore space is filled with water.
a)
b)
c)
d)
Results in little air storage in the soil
Mainly cuts off pathways of gas exchange with the atmosphere
Compaction can also result in a loss of gas exchange with the atmosphere
When the soil has an extreme amount of excess water it is water saturated or
waterlogged.
e) Hydrophytes are plants that have ways of obtaining oxygen in waterlogged soils
f) Some grasses use aerenchyma tissues to transport oxygen down the stem to the
roots
g) Some trees such as mangrove produce aerial roots to obtain oxygen
Oxygen in Soil
Atmospheric concentrations
78% Nitrogen (N 2)
21% Oxygen (O 2)
0.035% Carbon Dioxide (CO 2)
• Soil oxygen varies from about 20% near the surface to less than 5% at depth. •
During respiration in soils, oxygen is replaced with carbon dioxide, which could approach
toxic levels
• When soil oxygen becomes low it results in anaerobic conditions
Other Gases in Soil
• CO2 and O2 have an inverse relationship in the soils
• Water vapor (H2O) is usually close to the saturation level in soils
• When soils are water logged, methane (CH 4) and hydrogen sulfide (H2S) can build
up
39
• Under anaerobic conditions ethylene (C2H4) is produced and can be very toxic to
plant roots
G. SOIL CONSISTENCY – this is the physical state of the soil at various moisture levels
when subjected to mechanical stress such as plowing. Consistency is affected by
OM, texture and nature of clay. A soil behaves differently at different soil moisture
content. It is hard when it is dry, friable when it is moist, sticky and plastic when it is
wet and viscous (it flows like liquid) when super saturated.
Implications of Soil Consistency
a.) At liquid consistency the soil is easily puddled (good for paddy rice culture). But
puddling completely destroy aggregation and, hence, destruction of soil structure
(very bad if growing upland crops like corn and vegetables)
b.) At plastic consistency the soil is plastic, sticky and compact (good for pottery but it
has bad effects if soil is cultivated since it is sticky and easily compacted resulting to
poor soil structure)
c.) At friable consistency is the best for cultivation since it is soft, friable, mellow and soil
structure is rejuvenated.
d.) At harsh consistency the soil is very hard, which requires very high amount of energy
to pull the plow, resulting to cloddy seed bed.
H. SOIL COLOR - this represents the color of the spectrum that is absorbed by the soil.
Soil color is measured using the Munsell Soil Color Chart.
Aspects of Soil Color.
a. Hue – this represents the dominant spectral color. Most common soil hues are in
the red-to-yellow range, getting their color from iron oxide minerals coating soil
particles.
b. Value – this refers to the degree of lightness/darkness of a color with a value of 0
being black.
c. Chroma – this refers to the strength of purity of dominant spectral color with 0
being neutral gray.
40
Significance of Soil Color
a) It is an Indicator of soil productivity. Dark soils are high in organic matter content
b) Has no direct effect on plant growth but has an indirect effect on temperature and
moisture
c) Indicator of climatic condition under which a soil is developed or its parent materials
d) Three factors have the greatest influence on soil color which includes organic content,
water content and presence and oxidation states of iron and manganese
e) Red color indicates unhydrated Fe ions, good drainage and aeration. It is also an
indicator of old and infertile soils.
f) Yellow color is due to iron oxides
g) Gray and whitish color are caused by quartz, kaolin, CaCO 3 and MgCO3, gypsum and
other salts.
h) Grayish color indicates permanently saturated horizons
i) Poorly drained soils are nearly always mottled with various shades of gray, brown and
yellow
I. SOIL TEMPERATURE
a) The temperature of soil affects physical, biological, and chemical processes
b) Cold temperatures slow chemical reactions and biological decomposition eventually
slows down the production of nitrogen, phosphorous, sulfur, and calcium
c) High temperatures can also inhibit biological activity or kill off microorganisms
d) Warm soil temperatures generally induce plant germination and growth
e) Seeds of plants that are adapted to open gaps react to warm soil temperatures
caused by direct solar radiation
f) Seeds of prairie grasses require a cold period to enable them to germinate. This is
called vernalization .
g) Some plants, such as tulips, require chilling in early winter to develop buds
h) Soil microbial activity and organic matter decomposition cease below 5ºC which is
known as biological zero
i) Clear plastic can be used to raise soil temperatures reducing fungal diseases and
control pests, called soil solarization.
When soil is heated by fire:
 Fire breaks down the organic layer and recycles the nutrients into the soil
 Some seeds with hard coatings need to be heated above 70ºC before they will
germinate (some pines)
 The heat from fire can kill some weed seeds
 Fire can cause the development of a hydrophobic layer
 When soil is heated by solar radiation
 On average only 50% of the solar radiation actually reaches the soil
 The majority of that energy goes to evaporating water or is reflected back into the
atmosphere because of a high albedo
 Only about 10% of the solar radiation actually heats the soil
Applications
1. Is it possible to change the soil texture of your farm? How?
2. You are considering to purchase a farmland in your locality with variable soil textures.
The soils in one farm are mostly sandy loams and loamy sands, while those on a
second farm are mostly clay loams and clays. List the potential advantages and
disadvantages of each farm as suggested by the texture of the soils.
41
3. If the soil samples has 20% sand, 50% silt and 30% clay, what is the soil texture
using the soil triangle?
4. As a home gardener, what would be the three best things that you could do to
manage the soil structure in your home garden?
5. Is it true that soil with high particle density is associated with poor soil porosity? Why?
6. Using the term adhesion, cohesion, surface tension, atmospheric pressure, and
hydrophilic surface, write a brief essay to explain why water rises up from water table
in mineral soil.
7. Will you plant pechay in soils with hard consistency? Why?
8. If you were investigating a site for proposed agricultural projects, how could you use
soil colors to help predict appropriate nutrient management practices?
42
Unit IV. Chemical Properties of Soils
Introduction
Soil Chemistry is the study of the interaction of solid and liquid components. The
solid component indicates many and varied substances dominated by silicate (inorganic) and
carbon (organic) systems. Liquid component refers to the soil solution which consists of
water and dissolved ions. Colloidal materials control most of the chemical and physical
properties of soils. These materials include five major classes of crystalline silicate clays,
other aluminosilicates such as allophane, the oxides of iron and aluminum, and the organic
colloids (humus). Due to their extreme small particle size and platelike structures, those
colloids posses enormous surface areas, much of which are internal surfaces.
Learning Outcomes
After completing this unit, the students should be able to:
Define colloids
Differentiate organic and inorganic colloids
Illustrate the basic structural units of silicate clays
Explain the influence of Cation Exchange Capacity (CEC) and Base Saturation (BS)
on nutrient holding capacity of the soils
5. Describe the relationship of soil pH and nutrient availability
6. Discuss the effects of liming on neutralizing soil acidity
1.
2.
3.
4.
Activity
1.
A picture below depicts the dynamics of nutrients exchange in the soil solution.
Describe the role of clay, organic matter and roots of the plants in the movement of
nutrient elements.
Analysis
1. Why do nutrient elements are adsorbed in the clay and humus particles?
2. Why roots of plants exude considerable amount of H +?
3. How is ion exchange made possible?
43
Abstraction
The mineralogical and chemical constitutions of these colloids result in electrostatic
charges on or near the surface of the colloidal particles. Negative charges predominate on
most silicate clays. Positive charges characterize some colloids, such as the oxides of Fe
and Al, especially if the pH is low. The charges on soil colloids attract ions and other
substances with opposite chares: cations being attracted to negatively charged sites and
anion to the positively charged ones.
Cation exchange joins photosynthesis as a fundamental life-supporting process.
Without this soil property, terrestrial ecosystems would not be able to retain sufficient
nutrients to support natural or introduced vegetation, especially in the event of such
disturbances as time harvest, fire, or cultivation.
A. SOIL COLLOIDS
1. Definition
The chemical properties of the soils include mineral solubility, nutrient availability, soil
reaction (pH), cation exchange and buffering capacity.
The seat of the chemical reactions in soil is the soil colloid. It is either organic or
inorganic with very small particle size (< 0.001 mm or 1 µm), has large surface area and
presence of electrical charges (+/-)
The electrical charges of the soil affect the attraction and repulsion of the particles
towards each other, thereby affecting the physical and chemical properties of the soil.
2. Classification and examples: organic and inorganic
a. Organic Colloids – this is represented by humus from decayed plant and animal
residues.
b. Inorganic Colloids – this refers to clay colloids of various kinds such as layer silicate
clays (kaolinite, illite, montmorillonite, vermiculite, smectite), oxides of iron and
aluminium (Goethite, Hematite, Gibbsite) and amorphous clays (allophane and
imogolite)
3. Important Characteristics of Soil Colloids Related to Chemical Reactivity:
a. Fineness and large specific surface area
b. Presence of electric charges
4. Main Types of Negative Charges:
a. Permanent Charge – imperfection in their crystal structure, isomorphous
substitution
b. pH – dependent charge – neutralization of weakly acidic OH- groups
B. SILICATE CLAYS
1. Basic structural units of layer silicate clays
44
a. Silica tetrahedron (4O2 and 1 Si)
b. Alumina Octahedron (6 OH- and 1 Al3+ or Mg2+)
2. Properties of silicate clays
a.
b.
c.
d.
e.
Particles are made up of layers
Crystalline and have an ordered internal arrangement
Expose a large amount of external surface because of their small size
Particles carry negative charges
Basic structural units are silica tetrahedral and aluminum octahedral sheets.
45
3. Sources of Negative Charges
a. Isomorphous substitution - substitution of one ion for another ion of similar
size within crystal lattice
In 2:1 layer silicates
Al3+ for Si4+ in the tetrahedral sheet
Mg2+, Fe2+ for Al3+ in the octahedral sheet
b. Exposed –ON at broken edges of crystals
At high pH, the hydrogen of these exposed hydroxyls, dissociates at
the surface of the clay and is left with the negative charge of the oxygen ions
This type is called a pH – dependent charge or variable charge
46
4. Types of Silicate Clay Minerals
1:1 Type Silicate Clay Minerals – Kaolinite
 Consist of one layer of silica tetrahedron and one layer of Alumina
Octahedron
 The layers are held together tightly by H-bond to restricts the expansion
and limits the reactive area to external surface
 Two sheets are held together by O atoms that are mutually shared by Si
and Al
 Has little isomorphous substitution
 Low colloidal property
 Highly impermeable – water cannot permeate the layers to cause
expansion or contraction with wetting and drying
 Low CEC (6 – 15 meq/100 g soils)
2:1 (Expanding Type) – Montmorillonite and Vermiculite
 Consist of one layer of aluminium octahedral sheet sandwiched in
between by two silica tetrahedral sheets
 Layers are held loosely by weak O to O bond and cation – O linkages
 Exchangeable cations and water molecules are attracted between layers
causing the expansion of the layers or lattice when they are wet
 Flake-like crystals and much smaller than kaolinite
 Has larger specific surface area due to contribution of internal surfaces
 Soils with high montmorillonite cracked markedly upon drying and tend to
be impermeable when wet
 Montmorrilonite can hold more water than its volume
 It has more negative charges
 Montmorillonite derives its negative charges from the isomorphous
substitution of Mg 2+ for Al3+ in the octahedral sheet
 Vermiculite derives its negative charges from the isomorphous substitution
of Al3+ for Si4+ in the tetrahedral sheet
47
Water molecules along with other ions act as bridges holding
the unit together rather than apart
- Vermiculite swells less than montmorillonite
 High CEC (80-100 meq/100 g soils)
-
2:1 (Non - Expanding Type) – illite
 Crystals occur as irregular flakes
 Unit layers are held together by K ions, O-K-O bond is created hence no
expansion upon wetting
 If all or most of K is removed, illite becomes vermiculite
2:1:1 or 2:2 (Non - Expanding Type) – Chlorite


2:1 layers alternate with a hydroxide sheet commonly dominated by a
magnesium
No water adsorption between the unit layers hence it does not expand
in nature
C. OXIDE CLAYS – mainly Iron and Aluminum Oxides







Common in highly weathered soils (Ultisol and Oxisol)
Oxides of Iron includes Goethite, Limonite, Hematite and Magnetite
Oxides of Aluminum includes Gibbsite, Boehmite and Alumina
Has low CEC and pH dependent
High phosphate fixing capacity
Has stable aggregates and exhibit low plasticity
Infiltration of water is rapid
48
D. AMORPHOUS CLAYS – Allophane and Imogolite
These type of clays are common in soils developed from volcanic ash
(Andisol)
E. ORGANIC COLLOIDS
Source of Negative Charges
Functional groups of organic colloids such as carboxyl (-COOH), phenolic (OH), alcohols – OH
Source of Positive Charges
Protonation of exposed hydroxyl groups and NH2 groups in the case of
organic colloids
Al-OH – H+
= AlOH2+
Remarks:
Generally, agriculturally important soils possess net negative charge
Negatively charged colloids attract cations which become adsorbed on the
surface. In humid regions, (H+, Al3+) > Ca2+ > Mg2+ > K+ > Na+ while in arid regions,
Ca2+, Mg2+ > Na+ , K+ > H+
In general, the order of adsorbability or replacing power of cations commonly
found in soils is: (H+, Al3+) > Ca2+ > Mg2+ > K+ = NH4+ > Na+
F. FACTORS AFFECTING THE STRENGTH OF ADSORPTION
1. Charge (Valence) : The higher the valence, the stronger the adsorption
2. Ionic sizes (Radius) : The smaller the size, the stronger the adsorption
3. Hydration of Ion : The smaller the hydrated size, the stronger the adsorption
49
4. Concentration of Solution : The higher the concentration, the stronger the
adsorption
G. ION EXCHANGE
This is a reversible process by which ions are exchanged between solid and liquid
phases and between solid phases if in close contact with each other which occurs due to the
presence of electrical charges in the soil. A soil with high clay content has higher CEC than
sandy soils.
Two Types of Ion Exchange
a. Cation Exchange – the attraction of cations (positively charged ions) on the surface of
colloids and exchanged for ions in the soil solution (NH 4+, Ca2+,Mg2+, Na+, H+, K+)
b. Anion Exchange – the attraction of anions (negatively charged ions) on the surface of
colloids and exchanged for ions in the soil solution (NO 3-, PO4-, SO4-)
Ion Exchange Reaction is instantaneous, reversible, stoichiometric (1
milliequivalent (meq) of one cation is replaced by 1 meq of another cation
Concept of Milliequivalent
Weight of 1 meq = Atomic Weight
Valence x 1000
Examples :
Weight of 1 meq of Ca2+ = 40 g___
2 x 1000
= 0.02 g/meq
50
Ion
Valence
Molecular weight (g)
Weight of 1 meq (g)
2+
1+
2+
1+
3+
1+
40
39
24
23
27
1
0.02
0.039
0.012
0.023
0.009
0.001
Ca
K
Mg
Na
Al
H
Sample Problem:
Calculate the weight (g) of Ca2+ needed to replace 1 g of H+?
Answer:
x g Ca2+
0.02 g Ca
0.001 g H
=
1gH
x = 20 g Ca
2+
Cation Exchange Capacity – is the capacity of a soil to adsorb and exchange
cations and it is the sum of all exchangeable cations adsorb in a soil expressed in meq/100 g
soil or in centimole/kg soil (cmol/kg)
Sample Problem: Calculate the CEC of the given soil with the following analysis:
Exchangeable Ions
K+
Mg2+
Ca2+
Na+
Al3+
NH4+
H+
CEC
meq/100 g soil
4.5
8.7
12.54
0.89
2.10
1.5
3.5
33.73 meq/100 g soil
CEC indicates the fertility of the soil as it reflects the capacity of the soil to retain
nutrients.
CEC Determination: Ammonium Acetate (NH4OAc) Method
The amount of NH4+ is a measure of the CEC
51
Sample Problem: A 100 g soil adsorbed 0.27 g of NH 4+. Calculate its CEC.
Solution:
0.27 g NH4+
CEC
=
15 meq
1 meq
x
100 g soil
0.018 g NH4+
=
100 g soil
Since : Atomic Weight of NH4+ = 18 g ( N = 14, H = 1)
Weight of 1 meq of NH4+ = 18/1000 = 0.018 g/meq
Clay Minerals
Humus
Vermiculite
Montmorillonite
Hydrous Mica
Kaolinite
Cation Exchange Capacity (meq/100 g soil)
100 – 300
80 – 150
60 – 100
25 – 40
3 – 15
Factors Affecting CEC:
a.
b.
c.
d.
Soil Texture : the finer the texture, the higher the CEC
Amount and type of clay: higher amount of clay would mean higher CEC; more
montmorillonitic type of clay is also associated with higher CEC
Amount of OM: higher OM results in higher CEC
pH: higher pH, higher CEC
H. BASE SATURATION AND EXCHANGEABLE SODIUM PERCENTAGE
Base Saturation – is the percentage of CEC that is satisfied by the
exchangeable bases (Ca2+,Mg2+, Na+, K+).
BS (%)
=
Meq of exchangeable
bases
CEC
x
100
In the previous problem on CEC, the base saturation of that soil is :
BS (%) = (4.5+ 8.7+ 12.54 + 0.89) / 33.73 = 26.63/33.73 (100%) = 78.95%





NH4+, H+ and Al 3+ are not base but acids
Acidity or alkalinity is governed by the relative concentration of H + and OHBases tend to form hydroxides – KOH-, NaOH, Ca(OH)2, Mg(OH)2
Al tends to contribute H+ through hydrolysis:
Al3+
+ HOH =
Al (OH) 2+ + H+
2+
Al(OH)
+ HOH =
Al (OH) 2+ + H+
+
Al(OH)2
+ HOH =
Al (OH) 3 + H+
Generally, as BS increases, pH increases
Exchangeable Sodium Percentage (ESP %) - the proportion of the exchange sites occupied
by sodium (Na)
Meq of Na+
ESP (%)
=
CEC
x
100
52
In the previous problem on CEC, the ESP of that soil is :
ESP (%) = 0.89/33.73 (100%) = 2.64%

I.
Soils with high ESP (> 15%) are considered sodic and need to be reclaimed
through the addition of gypsum (CaSO4.2H20)
SOIL pH or SOIL REACTION (potential of Hydrogen/potential H +)
1.
Definition and Significance
Soil pH- refers to the acidity or alkalinity of the soil due to the relative
concentration of H+ and OH- ions. This is one of the most important chemical
properties of the soil which affects the availability of nutrient elements in the soil as
well as the microbial activities associated with soil fertility. It is the negative logarithm
of the hydrogen ion concentration in the soil.
Soil pH is defined by the equation:
Soil pH = - log H+
where: H+] = concentration of H+ in moles/liter
The pH scale covers a range from 0-14. A pH value of 7.0 is neutral (pure
water). Values below 7.0 are acid and above 7.0 are basic. Each unit change in pH
is associated with tenfold change in the H + concentration. As pH increases, the H+
concentration decreases. pH scale is indicated below.
53
Determination of pH:
a.) Colorimetric Method – this entails using litmus paper or organic dyes, however
this is less accurate.
Ex. blue to red = acid
b.) Electrometric Method – this utilizes the pH meter and is considered more
accurate method of determining soil pH.
pH
Acidity
Category
Extremely Acid
Very strongly Acid
Strongly Acid
Moderately Acid
Slightly Acid
NEUTRAL
Mildly alkaline
Moderately alkaline
Strongly alkaline
Very strongly alkaline
Neutrality
Alkalinity
pH value
< 4.5
4.5 – 5.0
5.1 – 5.5
5.6 - 6.0
6.1 – 6.5
6.6 – 7.3
7.4 – 7.8
7.9 – 8.4
8.5 – 9.0
> 9.1
2. Sources of soil acidity
a) H+ and Al3+ ions ( Hydrolysis of Al3+ indirectly contributes to soil acidity)
b) Carbonic acid (H2CO3) dissociation
CO2 + H20 = H2CO3 = 2 H+ + CO32c) Organic Acids from OM decomposition
i. Fulvic, humic and other inorganic acids are formed during organic
matter decomposition
ii. Production of CO2 during organic matter decomposition is responsible
for the lowering of pH of calcareous soils in submerged soils
d) Mineral weathering
e) Acid rain
2 SO2 + O2
2 NO + O2
2 SO3;
2 NO2;
SO3 + H2O
2 NO2 + H2O
H2SO4 ( Sulfuric Acid)
HNO3 + HNO2
Heavy cropping removes (crop removal) basic cations and replaced by H + ions from
roots
g) Long-term use of acidifying fertilizers (NH 4+ containing fertilizers) due to nitrification
process ( conversion of NH4+ to NO3- and release of H+ in the soil)
f)
Nitrosomonas sp.
NH4+
NO2- + 2H+ + H2O
+ 3/2 O2
Nitrobacter sp.
NO2- + ½ O2
(NH4)2 SO4 + 4 O2
NO32 NO3- + 2H2O + 4H+ + SO42-
J. Kinds of acidity: active and reserve acidity
a. Active Acidity – due to H+ in the soil solution, determined as pH
b. Reserve Acidity – H+ and Al ions held on the soil colloids; measure of buffering
capacity of the soil
54
K. Buffering capacity – this refers to the resistance of a soil to changes in soil pH. As
such, this is an important property of soil because it tends to ensure stability in the soil
pH, preventing drastic fluctuations that might be detrimental to plants and soil
microorganisms. It likewise influences the amount of amendments, such as lime or sulfur,
required to effect a desired change in soil pH.
L. Effects of nutrients availability
Soil pH is associated to availability of plant nutrients as illustrated below.
55
N
Ca and Mg
P
K
S
Mo
B
Fe, Mn and Zn





decreases at pH < 5.5
decreases at pH < 6.0
decreases at pH < 6.0 and pH > 7.0
(maximum availability occurs at narrow pH range)
decreases at pH < 6.0
decreases at pH < 5.5
decreases at pH < 6.5
decreases at pH < 5.0 and pH > 7.0
decreases at pH > 5.0
(at pH < 5.0, their concentrations are usually toxic)
At pH < 5.5, exchangeable Al becomes available
At pH 4.0, soil is largely saturated with exchangeable Al
Amount of exchangeable bases decreases as acidity increases due to
their displacement from exchange sites by H + and subsequent losses
through leaching due to the increase in solubility of compounds bearing
Fe, Mn, Zn, Cu and Co
The optimum or ideal pH is 6.5
P fixations occurs in extreme soil acidity or soil alkalinity
Soil Acidity
Fe3+ + 2 H2O + H2PO4(soluble or available )
=
2H+ + Fe(OH)2H2PO4
(strengite; insoluble)
Al3+ + 2 H2O + H2PO4(soluble or available )
=
2H+ + Al(OH)2H2PO4
(variscite; insoluble)
Soil Alkalinity
H2PO4- combines with Ca2+ or Ca-bearing minerals to form insoluble
calcium phosphates like dicalcium phosphate dehydrate (CaHPO 4.2H2O),
tricalcium phosphate (Ca3(PO4)2)
Microbial Activity Related to OM decomposition
Fungi – unaffected by pH level
Bacteria and Actinomycetes – inhibited at pH 5.5
M. LIMING
1. Definition and examples of lime
Lime - any Ca and/or Mg containing compound used to neutralize soil acidity or
oxide, hydroxide or carbonates of Ca or Mg,
Carbonate Forms
CaCO3 - calcitic limestone or calcic lime
CaMg(CO3)2 – dolomite or dolomitic lime
Oxides and Hydroxide Forms
CaO – burned lime
Ca(OH)2 – hydrated lime
56
2. Relative Neutralizing Value (RNV)
Calcium Carbonate Equivalent (CCE) or Relative Neutralizing Value (RNV) –
capacity of the material to neutralize soil acidity relative to that of pure CaCO 3
Lime Material
CCE/RNV
2-
(CaCO3)
100
2–
CaMg(CO3)
109
Ca(OH)2
136
CaO
179
CCE or RNV = (MW CaCO3/ MW material) x 100
Factors affecting the effectivity of lime:
a.) Form and kind of lime : Reactivity is in the order :
CaO> Ca(OH)2>CaMg(CO3)2> CaCO3
b.) Size particles : the smaller the particles, the more reactive is the lime
Lime requirement – the amount of liming material required to raise the pH to a
desired level and is determined by required change in pH and texture. The finer the
texture, the higher the CEC and greater buffering capacity of the soil
Benefits of Lime in the Soil:
a.
Physical – increased granulation as Calcium favors flocculation but the effect
is largely indirect due to favourable effect on organisms which decompose OM
b. Chemical – neutralization of H+ ions; increased availability of P, Mo, Ca, K;
reduced solubility and toxicity of Fe, Al and Mn
c. Biological – promotes activity of microorganisms such as those that
decompose OM, mineralize nutrients and fix N symbiotically, but it may
encourage potato scab
Sources of Alkalinity:
1.) Base- forming cations
 As the basic cations such as Ca, Mg, K and Na saturates the soil’s exchange
complex, the H+ ion concentrations in the soil solution will decrease and the
concentration of OH- increases.
 Alkaline reactions results from the hydrolysis of colloids saturated with basic
cations
Na + H2O
H+ + Na+ + OH2.) Carbonates (CO32-) and bicarbonates (HCO3-)
N. SOIL SALINITY AND SODICITY
1.) Sodic soils have > 15% of their cation exchange sites occupied by Na+ ions
 pH > 8.5
 Soil is highly dispersed (Sodium favors dispersion)
 Reclamation :
57
 Replacement of excess Na on the exchange sites by Ca and
leaching of Na out of the root zone using Gypsum (cheapest
and commonly used)
Na
Ca
Na
+ CaSO4 =
+ Na2 SO4



S is usually used when the soil contains free lime in addition to too much Na
Addition of OM is also beneficial
Deep plowing
2.) Saline soils are soils which concentration of soluble salts is high enough to hamper plant
growth
 Salts are mostly chlorides and sulfates of Na, Ca and Mg
 pH usually > 8.5; sometimes referred to as white alkali
 Specific toxicities due to high concentration of Na, Cl, etc.
 Plant growth is reduced due to osmotic effect
 Reclamation :
 Leaching of excess salts out of the root zone (internal and surface
drainage and salt disposal dump areas)
 Retardation of evaporation using surface mulch
 Use of salt-tolerant crops
c.


Saline – sodic soils have high levels of soluble salts and exchangeable Na
Soil amendment must first be applied before leaching
Leaching without the amendment will only convert saline-sodic into sodic soils
Technical Description:
Electrical Conductivity
(EC) (ds/cm) of
saturation extract
Saline
Sodic
Saline- Sodic
>4
<4
>4
Exchangeable
Sodium
Percentage
(ESP)
< 15
> 15
> 15
pH
Sodium
Adsoprtion
Ratio (SAR)
< 8.5
> 8.5
< 8.5
< 13
> 13
> 13
Applications
1. Describe the colloidal complex, indicate its various components, and explain how it tends
to serve as a “bank” for plant nutrients.
2. Contrast the difference in crystalline structure among kaolinite, smectites, fine-grained
micas, vermiculites, and chlorites.
3. If you wanted to find a soil high in kaolinite, where would you go?
4. Which of the silicate clay minerals would be most and least desired if one were interested
in (1) good foundation for a building, (2) a high cation exchange capacity, (3) an adequate
source of potassium, and (4) a soil on which hard clods form after plowing?
5. Soil pH gives a measure of the concentration of H+ ions in the soil solutions. What does it
tell you about the concentration of OH- ions? Explain.
6. Describe the role of aluminium and its associated ions in enhancing soil acidity.
7. What is meant by buffering? Why is it important in soils? What are the mechanisms by
which it occurs?
8. What is acid rain, and why does it seem to have greater impact on forests than on
commercial agriculture?
9. Discuss the significance of soil pH in determining specific nutrients availabilities and
toxicities, as well as species composition of natural vegetation in an area.
58
Unit V. BIOLOGICAL PROPERTIES OF
SOILS AND ORGANIC MATTER
Introduction
The soil is a complex ecosystem with a diverse community of organisms. Soil
organisms are vital to the cycle of life on earth. They incorporate plant and animal residues
into the soil and digest them, returning carbon dioxide to the atmosphere, where it can be
recycled through higher plants. Simultaneously, they create humus, the organic constituent
so vital to good physical and chemical soil conditions. During digestion of organic substrates,
they release essential plant nutrients in inorganic forms that can be absorbed by the plant
roots or be leached from the soil. They also mediate the redox reactions that influence soil
color and nutrient cycling.
The organisms of the soil must have energy and nutrients if they are to function
efficiently. To obtain these, soil organisms break down organic matter, aid in the production
of humus, and leave behind compounds that are useful to higher plants.
Learning Outcomes
At the end of this unit, the students are expected to:
1. Characterize the different types of soil organisms
a) Bacteria
b) Actinomycetes
c) Fungi
d) Virus
e) Protozoa
f) Algae
g) Worms
h) Insects
i) Rodents
2. Describe the beneficial activities of soil organisms
3. Discuss the process of organic matter decomposition
4. Explain the effects of organic matter on soil properties and plant growth
5. Illustrate the transformation of nitrogen and other elements in the soil.
Activity
1. List down the functions of these organisms in the soil.
59
Analysis
1.
2.
3.
What do you think will happen to our environment if soil organisms are absent in our
ecosystem?
Why do peanut, beans and other leguminous plants can effectively utilize
atmospheric nitrogen? How does Rhizobia (bacteria) facilitate the process?
How does mycorrhiza (fungi) assist the plants to absorb phosphorus in acidic soils?
Abstraction
Most of the work of the soil community is carried out by creatures whose “jaws” are
chemical enzymes that eat away all forms of organic substances left in the soil by their
coinhabitants. The diversity of substrates and environmental conditions found in every
handful of soil spawns a diversity of adapted organisms that staggers the imagination. The
collective vitality, diversity, and balance among these organisms engender a healthy
ecosystem and make possible the function of a high-quality soil.
A. KINDS OF ORGANISMS: MICROORGANISMS AND MACRO ORGANISMS
Soil Microbiology – is the study of soil organisms and their processes. Soil organisms
are creatures that spend all or part of their life in the soil environment. These include:
 Macrofauna – earthworms, millipedes, termits
 Mesofauna – nematodes to single-celled protozoans
 Plants (flora) – include the roots of higher plants, macroscopic algae and diatoms
60
Major Roles of Soil Microorganisms



Responsible for biochemical changes
Agents in the decomposition of plant and animal residues
Improve soil structure through aggregation
Relative Number and Biomass of Fauna and Flora
Micro flora
Bacteria
Fungi
Actinomycetes
Algae
Fauna
Protozoa
Nematodes
Earthworms, Collembola
Mites, other Fauna
Major Groups of Soil Microorganisms





Bacteria
Actinomycetes
Fungi
Algae
Protozoa
B. BACTERIA: CHARACTERISTICS AND CLASSIFICATION
Bacteria are very small, single-celled prokaryotic organisms without nucleus. They
range in size from 0.5 to 5 mm. Bacteria are either autotrophic or heterotrophic. Most
common genera include: Pseudomonas, Rhizobium, Bacillus, Clostridium and
Arthrobacter. Bacteria grow best under neutral to slightly basic conditions
61
 Shape of bacteria
 Coccus – round
 Bacillus – rodlike
 Spirillum – spiral
 Temperature Adaptation
 Psychrophilic – capable of growing at 0oC
 Mesophilic – optimum growth is at 25oC to 39oC
 Thermophilic – microorganism that thrive only at high temperature
o
(50 C to 72oC)
 Energy and Carbon Requirements
 Photoautotrophs – obtain their energy from sunlight and carbon
from carbon dioxide. Ex.
- Photospirillum – purple bacteria
- Cyanobacteria – blue green algae
 Chemoautotroph – energy is obtain from the oxidation of inorganic
constituents as ammonium, sulfur and iron and carbon from carbon
dioxide or dissolved carbonates.
- Nitrosomonas
- Nitrobacter
- Thiobacillus
 Heterotrophs – energy and carbon sources are preformed organic
carbon from organic matter. Ex.
- Heterotrophic bacteria – Bacillus, pseudomonas
- Fungi – Rhizopus, Aspergillus
 Oxygen Requirements
 Aerobic – use O2 as the electron acceptor in their metabolism. It
can grow only in the presence of oxygen.
 Anaerobic – use substances other than O2 as electron acceptor in
their metabolism. Can live in the complete absence of oxygen.
 Facultative – can use either aerobic or anaerobic metabolism. Can
live in the presence or absence of oxygen.
Importance of Bacteria
a)
b)
c)
d)
Help in the remediation of polluted soils
Breakdown of hydrocarbon compounds such as gasoline and diesel fuel
Responsible for the biological oxidation of inorganic compounds
Microbial action controls toxicity and deficiency of some nutrients such as
iron and manganese
e) Organic matter decomposition is the most significant contribution of the
soil fauna and flora to higher plants
f) Plant residue decomposition is the process wherein dead leaves, roots
and other plant tissues are broken down converting organically held
nutrients into mineral forms available for plant uptake and recycle the
elements.
62
g) By-products of organic matter decomposition, are utilize by microorganism
to synthesize new compounds to stabilize soil structure and contribute to
humus formation.
Factors affecting the growth of soil bacteria and other soil microorganism
a)
b)
c)
d)
e)
Oxygen requirement
Moisture
Temperature
Amount of organic matter
pH
C. ACTINOMYCETES: CHARACTERISTICS, SIMILARITY AND DIFFERENCE FROM
BACTERIA AND FUNGI
Actinomycetes – are unicellular aerobic microorganism, which form branched
mycelium and reproduced by fragmentation or asexual spore formation. Generally,
actinomycetes are aerobic and heterotrophic living in decaying organic matter in the soil.
Common genera are Nocardia, Streptomyces, Micromonospora and Thermoactinomyces.
They played important role in the decomposition of resistant compounds such as
cellulose, chitin and phospholipids to simpler forms.
Characteristics:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Branched mycelial structures
Intermediate between bacteria and fungi
Very fine hyphae (< 1 micron diameter)
Heterotrophic
Aerobic : some microaerophilic
Major Antibiotic Producer – Streptomycin, Erythromycin)
Acid Sensitive (Critical pH = 5.5)
Population – 105 – 106 CFU/g soil
Biomass:  4,000 kg/HFS
D. FUNGI: CHARACTERISTICS AND UNIQUE ADAPTATION TO SOIL CONDITIONS
Fungi – nucleated, spore-bearing, achloropholous organisms which generally
reproduce sexually and asexually, and whose usually filamentous, branched somatic
structures are typically surrounded by cell walls containing cellulose or chitin or both.
They decompose cellulose, starch, gums and lignin as well as the easily metabolized
proteins and sugars. Common genera are Penicillium, Aspergillus, Trichoderma, Mucor
and Fusarium
 Nutrition and Growth
 Parasites – fungi obtain their food either by infecting living organisms.
 Saprophyte – attack of dead organic matter.
Characteristics of Fungi
a)
b)
c)
d)
Complex morphology (multicellular; highly branched)
Heterotrophic
Aerobic
Acid-loving (efficient OM decomposers under acidic conditions)
63
e)
f)
Population – 104 – 105 CFU/g soil
Biomass:  8,000 kg/HFS
 Mycorrhiza – association between fungal hyphae and the roots of higher plants
E. OTHER ORGANISMS:
RODENTS
VIRUSES,
PROTOZOA,
ALGAE,
WORMS,
INSECTS,
PROTOZOA
Characteristics of Protozoa
a)
Single-celled animals (20-50 microns in diameter)
b)
Aerobic
c)
Ingest food through oral openings
d)
Reproduction : Binary fission; budding
e)
Population – 103 – 105 CFU/g soil
f)
Biomass:  100 kg/HFS
ALGAE
 Algae – most important photosynthetic microorganisms in the soil. The blue green
algae are prokaryotic and contain chlorophyll, carotenoid and phycocyanin pigments
which are diffused throughout the cytoplasm.
Characteristics of Algae
a)
b)
c)
d)
e)
f)
g)
h)
Principally aquatic, love moist habitat
Both single – celled and multicellular species are present in the soil
Aerobic
Photoautotrophs
Blue-Green Algae are capable of N2 fixation
Excellent host for bacteria due to oxygenating capacity
Population – 103 – 105 CFU/g soil
Biomass:  250 kg/HFS
EARHWORMS
a)
b)
c)
Eat detritus, soil organic matter and microorganisms found on these materials
Probably, the most significant macroorganism in humid temperate region soils
7000 species worldwide
Epigeic – live in the litter layer (Compost worm – Eisenia foetida)
Endogeic – live in the top 10-30 cm of soil (Pale-pink-red worm –
Allolobophora caliginosa)
Anecic – live in vertical burrow up to 1 meter. (Introduced Night Crawler Lumbricus terrestris)
F. BENEFICIAL ACTIVITIES OF SOIL ORGANISMS
5. Decomposition of organic matter
6. Transformation of soil nutrients
7. Promoting soil aggregation through by-products of their activities
64
8. Nitrogen fixation (rhizobia) and phosphorus solubilization (mycorrhiza)
G. PRODUCTION of TOXINS
1.) Types of Toxins
 Organic ( antibiotics)
 Inorganic (heavy metals: Cd, Hg, Pb, Cu, Zn)
2.) Potential source of toxins
 Soil microorganisms
 Plant roots
 Decomposing organic materials
 Sewage sludge
3.) Ranking of soil microbial groups in relation to antibiotic production
 1st – actinomycetes
 2nd – fungi
 3rd – bacteria
4.) Mode of action of antibiotics
 Disruption of cell wall synthesis (Penicillin, Cephalosporin)
 Disruption of protein synthesis ( Streptomycin, Chloramphenicol)
 Inhibition of cell membrane function (Polymixin)
 Inhibition of DNA/RNA function (Rifampicin)
5.) Production of Growth Stimulating Substances
 Inorganic P solubilization
 Genera of bacteria capable of solubilizing Calcium phosphates
1.
Pseudomonas
2.
Mycobacterium
3.
Bacillus
4.
Micrococcus
 Genera of fungi capable of solubilizing Calcium phosphates
i.
Penicillium
ii.
Fusarium
iii.
Aspergillus
 Microbiological means by which inorganic P is solubilized
Production of organic acids
Nitric acid or sulphuric acid production
Flooding resulting in the reduction of Fe in insoluble
ferric phosphates
Mycorrhizal association (related to organic production)
a.) Types of Mycorrhiza
- Ectotrophic
- Endomycorrhiza
Ectotrophic:
 fungus forms a mantle around root exteriors
 hyphae enters into spaces between plant cells
 examples (pine, eucalyptus)
Endomycorrhiza:
 fungus penetrates the cells of the plants
 examples (orchids, coffee, fruit trees, rice and
corn)
65
G. COMPOSITION OF ORGANIC MATTER: CARBOHYDRATES, PROTEINS, LIGNINS,
FATS, WAXES, TANNINS
Soil Organic Matter (SOM)

Refers to the totality of all carbon-containing compounds in the soil derived from
either plants or animals
Organic constituents of plants:
 Water (75%)
 Cellulose (15 – 60%)
 Hemicellulose (10 – 30%)
 Lignin (5 – 30%)
 Water-soluble fractions – amino sugars, amino acids (5-30%)
 Proteins
 Fats, oils and waxes (2%)

H.
ORGANIC MATTER DECOMPOSITION AND END PRODUCTS
3.
Aerobic (oxidative) decomposition
This is characterized by the presence of oxygen gas, resulting in the
production of carbon dioxide, water and energy.
Enzyme Oxidation
R --- (C, 4H) + 2O2
CO2 + 2H2O + energy
Protein
CO2 + H2O + energy + glycine, cysteine +
NH4 + NO3- + SO42-
Lignin
Phenolic ringed compounds
(broken down mainly by fungi)
4.
Anaerobic (fermentation) decomposition
It is characterized by absence of oxygen gas. Here, the decomposition rate is
slow. Energy yield is low for the organism, hence, end products such as CH4 have high
energy.
bacteria
4C2H5COOH + 2H2O
4CH3COOH + CO2 + 3CH4
proportionate
acetate
methane
bacteria
CH3COOH
CO2 + CH4
bacteria
CO2 +4H2
2H2O + CH4
(broken down mainly by fungi)
a. Decomposition of SOM
i.Carried out by heterotrophs
ii.Reactants are compounds containing C, H and O 2
iii.Products are CO2, H2O and energy
66
b. Environmental factors affecting decomposition:
i.Soil pH
ii.Moisture
iii.Temperature
iv.Aeration
c. Physical factors affecting decomposition:
i.Location of residues (surface vs. incorporated)
ii.Particle size (chopped vs. unchopped)
d. Chemical factors affecting decomposition:
i.C/N ratio:
1. High C/N ratio; slow decomposition
2. Low C/N ratio; rapid decomposition
ii.Lignin and polyphenol
Compost and Composting
Composting – process of creating humus-like organic materials by piling,
mixing, and storing of organic materials under conditions favourable for aerobic
decomposition
Compost – finished product of composting and used as soil conditioner or
slow- release fertilizer


I.
After composting, the C/N ratio of organic materials is reduced to about 1420:1
Pathogenic organisms are destroyed during thermophilic stage (50-75ºC) but
heavy metals (inorganic contaminants) are not destroyed
TRANSFORMATION OF NITROGEN
a. Mineralization – the conversion of an element from an organic form to an inorganic
state as a result of microbial decomposition
b. Immobilization - the conversion of an element from the inorganic to the organic form
in microbial tissues or in plant tissues, thus rendering the element not readily
available to other organisms or to plants.
c. Ammonification –the conversion of NO3- to NH4+
d. Nitrification –is the biological formation of NO3- or NO2 from compounds containing
reduced nitrogen. The most common initial substrate is NH 4- and the final product is
NO3-. Two separate and distinct steps are distinguishable in nitrification: The first is
initial oxidation of ammonium to nitrite and the further oxidation of nitrate. The
micororganism responsible for nitrification are the Chemoautotrophic Nitrosomonas
and Nitrobacter. Both these species are gram-negative non spore forming.
Nitrosomonas, which are ellipsoildal or short rods is responsible for the oxidation of
NH4+ to NO2- . Nitrobacter, which are short rods, further oxidizes nitrite to nitrate.
Nitrosomonas
NH4+ + 1 ½ O2
2H+ + H2O + NO2
Nitrobacter
NO2- + 1 ½ O2
NO3- + 17.5 Kcal
67
e. Denitrification –The biochemical reduction of nitrate or nitrite to gaseous nitrous oxide
(N2O) or N2 and lost to the atmosphere. Denitrification is a process where nitrate is
used by the micro-organisms as the terminal hydrogen acceptor in either dissimilation
of organic compounds or in the oxidation of inorganic compounds resulting in the
production of higher reduced compounds such as NO2, NO, N2O, N2
that are
commonly lost to the atmosphere. The respiratory reduction by electron transport
phosphorylation can be summarized as follows:
NR
NiR
NOR
NOS
2NO3  2NO2  2NO  N2O  N2
Enzymes:
NR: Nitrate reductase
NiR: Nitrite reductase
NOR: Nitric oxide reductase
NOS: Nitrous oxide reductase.
f.
Volatilization - process which nitrogen fertilizer such as Ammonium
Nitrate or Ammonium. Sulfate when applied to the surface of alkaline or calcareous
soils, changes immediately to gaseous form which may cause the loss of N as NH 3
gas. Similar reaction can occur on recently limed soils. Volatilization losses can be
high under some high temperature and certain moisture conditions. Urea when
applied in the soil converts rapidly to NH3 or NH4+ with
adequate
moisture,
temperature and the presence of urease (enzyme). This NH3
can be lost to the
atmosphere through volatilization. To avoid losses, incorporate urea when
temperatures are low or apply when there is sufficient moisture in the soil.
g. Leaching - refers to the downward movement of free water (percolation) out of the
root zone carrying the other nutrients in the soil. Nitrates are highly mobile in the soil
and move freely with soil water. Most of these nitrates leach through the soil
profile…more on deep, sandy soils than on fine-textured soils with moderate drainage
and high rainfall. Little Phosphorus is lost by leaching, though it moves freely in sandy
than in clay soils, and part of Potassium can be leached in very sandy or organic
soils.
68
J. EFFECTS OF ORGANIC MATTER ON SOIL PROPERTIES
Effect of OM on Plant Growth
a.) As OM decays, growth promoting substances are released (vitamins, amino acids,
auxins and gibberellins) that stimulate growth of both higher plant and
microorganisms
b.) Allelophatic effect: decomposing crop residues left on the soil surface may inhibit the
germination and growth of the next crop; eg. Wheat residues inhibit the germination
and growth of sorghum
c.) Influence of OM on Soil Properties

Biological Properties:
 Provides food for the heterotrophic organisms
 Improves SOM accumulation
 Increase microbial activity such as N fixation, decomposition, and
nutrient transformation

Chemical Properties:
 Increases in CEC of soils
 Increases soil buffering capacity
 Increase micronutrient availability through chelation
 Reduces Al toxicity by binding Al3+ in non-toxic complexes
 Adsorb pollutants such as Pb, Cd and Cu
 Inactivates toxin and pesticides.

Physical Properties:
 Enhances soil aggregation and aggregate stability
 Gives surface horizons dark brown to black color
 Reduces bulk density and compaction
 Helps reduce plasticity, cohesion and stickiness in clayey soil
 Increase infiltration rate and water holding capacity
Application
1. Identify the organisms that play the roles of primary producers, primary consumers,
secondary consumers and tertiary consumers.
2. Describe some of the ways in which mesofauna play significant roles in soil
metabolism even though their biomass and respiratory activity is only a small fraction
of the total in the soil.
3. What are the four main types of metabolism carried out by soil organisms relative to
their sources of energy and carbon?
4. What role does O2 play in aerobic metabolism? What elements take its place under
anaerobic conditions?
5. In what ways is soil improved as a result of earthworm activity? Are there possible
detrimental effects as well?
6. Explain and compare the effects of tillage and manure application on the abundance
and diversity of soil organisms.
69
Unit VI. NATURE, PROPERTIES AND
MANAGEMENT OF SOILS
Introduction
As stewards of the land, soil managers must keep nutrient cycles in balance. By
doing so, they maintain the soil’s capacity to supply the nutritional needs of the plants.
These tools include method of nutrient recycling as well as sources of additional nutrients
that can be applied to soils or plants. It is important therefore, that one should learn how to
diagnose nutritional disorder of the plants and correct soil fertility problems.
Learning Outcomes
After completing this unit, the student should be able to:
1. Explain the criteria of the essentiality of soil nutrients
2. Outline the essential elements, its available forms and functions in plant metabolic
processes
3. Describe the methods of assessing soil fertility status
4. Demonstrate how to estimate or calculate the quantity of fertilizer to apply
5. Discuss the process of soil erosion
Activities
1. Write an essay describing the interplay of physical, chemical and biological properties of
soils on crop production?
70
Analysis
1.
2.
How do you determine that the nutrient elements are essentials to the plants?
How will you estimate the amount of fertilizers needed by the plants?
Abstraction
The continuous availability of plant nutrients is critical for the sustainability of most
ecosystem. The challenge of nutrient management is threefold: 1) to provide adequate
nutrient for plants in the system, 2) to simultaneously ensure that inputs are in balance with
plant utilization of nutrients, thereby conserving nutrient resources, and 2) to prevent
contamination of the environment with unutilized nutrients.
The use of fertilizer, both inorganic and organic, should not be done in a simply
habitual manner. Rather, soil testing and other diagnostic tools should be used to determine
the true need for added nutrients. Adding fertilizers whenever the nutrient-supplying power
of the soil is sufficient is likely to be damaging to the environment.
C.
DEFINITION
Plant Nutrition






D.
Plants use inorganic minerals for nutrition, whether grown in the field or in a
container
Complex interactions involving weathering of rock minerals, decaying organic
matter, animals, and microbes take place to form inorganic minerals in soil.
Roots absorb mineral nutrients as ions in soil water.
Many factors influence nutrient uptake for plants.
Ions can be readily available to roots or could be "tied up" by other elements or
the soil itself.
Soil too high in pH (alkaline) or too low (acid) makes minerals unavailable to
plants.
ESSENTIAL NUTRIENT
Essential Nutrients and their forms in the soil







Plants need 16 essential elements for their growth and development
Nine of the elements are termed as MACRONUTRIENTS (major nutrients), and
these are needed by plants in large amounts
C, H, O, N, P, K, Ca, Mg, S
Seven of the elements are needed by plants in smaller amounts and referred to
as MICRONUTRIENTS
B, Zn, Mn, Cu, Fe, Cl, Mo
Most of the essential nutrients except N are derived from minerals while three
of the elements are derived from air and water (C,H,O)
Elements exist in the soil in two combinations with organic compounds, in the
complex structure of minerals of in salts in the soil solution
When the organic and inorganic compounds decompose, and the solutes
dissociate into their component ions, the nutrients become available for
absorption by plants or adsorb on colloid surfaces
It is the ionic form of nutrients that are available for plant use.
71
Beneficial Elements





7.
Elements that can compensate for toxic effects of other elements or may
replace mineral nutrients in some other less specific function such as the
maintenance of osmotic pressure.
The omission of beneficial nutrients in commercial production could mean that
plants are not being grown to their optimum genetic potential but are merely
produced at a subsistence level.
Cobalt is essential for nitrogen fixation in legumes. It may also inhibit ethylene
formation (Samimy, 1978) and extend the life of cut roses (Venkatarayappa et
al., 1980).
Silicon, deposited in cell walls, has been found to improve heat and drought
tolerance and increase resistance to insects and fungal infections.
Silicon, acting as a beneficial element, can help compensate for toxic levels of
manganese, iron, phosphorus and aluminum as well as zinc deficiency.
Criteria of Essentiality
Three (3) criteria must be met for an element to be considered essential:
1.) A plant must be unable to complete its life cycle in the absence of the
mineral element.
2.) The function of the element must not be replaceable by another mineral
element.
3.) The element must be directly involved in plant metabolism.
These criteria were set by plant nutritionists, and were arrived through series
of experiments in culture solutions as well as field studies. Cobalt, Vanadium,
Sodium and Silicon were added as they are essential for specific plants and they are
referred to as Beneficial elements
8.
Available forms & functions of nutrients in plants
The following elements are considered essentials for plant growth and development.
Its available forms and their corresponding function in the plants is summarized in the table
below.
Plant Nutrients, their form of uptake and functions
Nutrients
Nitrogen
Available Forms
NO3-, NH4+
Phosphorus
H2PO4 -, HPO4-2
Potassium
K+
Calcium
Ca 2+
Magnesium
Mg 2+
Sulfur
SO4 –2
Functions in Plants
Component of important cell compounds
ranging from proteins to chlorophyll and
genes
Constituent of genes, has a central role in
plant energy transfer and protein metabolism
Helps in osmotic and ionic regulation,
important for many enzymes functions in
carbohydrates and protein metabolism
Involved in cell division and plays a major role
in
the maintenance of membrane
integrity
Component of chlorophyll, and a factor in
many enzymatic reactions
Constituent of proteins amino acids and
vitamins necessary for production of plant oils
72
Nutrients
Iron
Available Forms
Fe 2+
Zinc
Zn 2+
Manganese
Mn 2+
Copper
Cu 2+
Boron
H3BO3
Molybdenum
MoO2-4
Chlorine
Cl-
Nickel
Ni2+
9.
Functions in Plants
Component of many enzymes (including
cytochromes, respiratory enzyme) and the
ferredoxins involved in functions such as N
fixation and photosynthesis
Necessary for the correct functioning of a
range of important enzyme systems for the
synthesis of nucleic acids and the metabolism
of auxin (a plant
hormone)
Component of several enzymes including
those involved in photosynthesis
Component of a range of important enzymes
necessary
for
proper
photosynthesis.
Involved in grain production
Directly involved in cell differentiation,
maturation, division and elongation. When B
is limiting, cell division rate is reduced.
Required for normal assimilation of N in
plants, for the reduction of NO3 to NH4+, Also
required for N fixation and for chlorophyll.
Essentials for photosynthesis, and for osmoregulations of plants growing on saline
soils
Constituent of the enzyme urease in legumes
Methods of assessing soil fertility status
The available nutrient supply of most soils is seldom adequate to support the
requirements of crops but it becomes depleted due to crop removal, leaching losses,
volatilization (N fertilizer), erosion of topsoil, fixation by clays and immobilization into organic
complexes, hence application of fertilizers must be done to supplement the soil of nutrients.
The amount of fertilizers that should be added may be determined by one or a
combination of soil analysis, field fertilizer experiments, plant tissue analysis, greenhouse
tests and evaluation of symptoms of nutrient deficiencies.
a) Soil Analysis
This is a relatively rapid method of determining the fertilizer need of the crops
by taking soil samples properly, chemical analysis and interpretation of analytical
results. This is a routine analysis includes the determination of pH, organic matter
content (Walkley and Black), available P (Bray2, Olsen and Troug Method),
exchangeable K (Ammonium Acetate Method) and lime requirement (Veitch Method).
Soil test results are compared with known values of deficiency or sufficiency
which are derived from previously calibrated data of correlations between the soil
tests and field fertilizer experiments
Procedure of Soil Sampling:
1.) The farm for soil fertility evaluation is first delineated on a rough map to group
similar areas in terms of visible soil properties and management (soil texture,
topography, productivity level, drainage condition, color of topsoil, previous
cropping and management and crops grown or to be grown
73
2.) Samples are taken randomly all over the sampling area using an auger,
shovel or spade. If shallow rooted plants are to be planted, 0-30 cm depth is
made, while 30-60 cm or more depth is made. Samples from several borings
(10-15 holes) are mixed in a container that will represent one composite
sample.
3.) Proper labelling must be made, and it should be air-dried and pulverized prior
to analysis in the lab.
b.) Field Fertilizer Experiments (Biological test)
This method is usually done to determine the optimum amount of fertilizer for
a particular crop consisting of treatment plots starting from zero and increasing at
regular increments
c.) Greenhouse Experiments
This is usually primarily exploratory or preliminary approach to determine what
nutrients are sufficient in specific soils. It cannot be used to estimate the amount of
fertilizer nutrient that should be applied because of highly artificial conditions under
which it is being conducted. Also, the volume of soil explored in a pot by plant roots
is limited by the size of the pot. Its advantages include that several kinds of soils can
be tested simultaneously under similar conditions and that it is cheaper to establish
and maintain.
d.) Plant Analysis
This measures the amount of nutrients that are absorbed by the plants and
this method integrates the effect of soil, plant, climate and management variables.
Sampling of the plant parts to be analyzed, time of sampling, and sample preparation
must be given due attention. Plant parts to be sampled for different crops vary and it
depends on the type and age of the crop tissue
e.) Observation of Nutrient Deficiency and Toxicity Symptoms
Plants exhibit symptoms of deficiency and toxicity and they can be used to
assess the need of plants for nutrients. Occurrence of nutrient deficiency occurs due
to insufficient amount and supply of soil nutrients, unavailability of forms of the
nutrients present, no proper balance among the different nutrients. This method is
usually complemented by plant analysis and/or by soil analysis.
f.) Soil Test Kit
A handy, less costly and rapid procedure of soil nutrient analysis that results
in qualitative interpretation of the relative amount of nutrients. This was developed in
Department of Soil Science in UPLB, it involves the use of specialized chemicals and
dyes wherein the intensity of color developed is referred to as correlated color chart
that interprets the relative condition of soil reaction and amounts of N, P,K and lime
requirement. It has acceptable precision results to the results of soil testing in the
laboratories
g.) Leaf Color Chart (LCC)
This was developed by IRRI and intended for rice. A handy plastic ruler with
strips of four shades of green to simulate the color of rice leaves under field
74
condition. It is cheap, fast and handy field instrument to help farmers visually assess
the nitrogen status of the rice plant
h.) Microbiological Methods
This is used for approximating the degree of deficiency of elements using the
growth of the test organism as an indicator. These methods include:
Azotobacter Plaque Method- the method is good for P, K and Ca. The
organism is grown in a culture medium in which all essential elements are provided,
except that being tested. The colony growth will increase with the increase in the
amount of
nutrients being tested. The growth of colonies in the soil being tested
is compared with standards to determine the degree of deficiency
Aspergillus Niger Method- the fungus produces black spores which can be
used to assay for potassium, magnesium, zinc and copper deficiency. The weight of
pad (mycelia) that it produces in a given soil can be related to the amount of nutrient
present by using standard soils (soils with known level of nutrient) for comparison
10. Fertilizer nutrients: their properties and usage
Fertilizers - are either organic or inorganic compounds that are added to the soil to
supply the plants with the nutrient elements that the soil is incapable of supplying.
Organic Fertilizer – fertilizers derived from plants and/or animals
Inorganic Fertilizer – fertilizers derived from mineral deposits, atmospheric gases,
water and other materials
Fertilizer Grade (or analysis). This refers to the minimum guarantee of the plant
nutrient content in terms of percentage total nitrogen (N), available phosphorus (P or P2O5)
and water-soluble potassium (K or K2O). If Ammonium Sulfate has an analysis of 21% N, it
means that for every 100 kg of Ammonium Sulfate, there is 21 kg of available N.
Conventional labelling of fertilizer products reports percentage N, P 2O5 and K2O
(oxide form). Thus, a fertilizer packages as 14-14-14 (14% N, 14% P2O5 and 14% K2O)
actually contains 14% N, 6% P and 12% K).
75
Fertilizer Recommendation.
This is the recommended rate of fertilizer
application/cropping or year. It is usually expressed as kg N, kg P 2O5, kg K2O per hectare.
Fertilizer*
% Nutrient
Analysis
(N-P205-K20)
Urea
% Conversion Factor (Material to Elemental
Nutrients)
N
P
K
Mg
(46-0-0)
0.45
Complete
(14-14-14)
0.14
Ammosul
(21-0-0)
0.21
Ammophos
(16-20-0)
0.16
0.09
DAP
(18-46-0)
0.18
0.20
Muriate of Potash (KCl)
(0-0-60)
0.50
Potassium Sulfate (K2SO4)
(0-0-53)
0.44
Epsom (MgSO4)
10% Mg
0.10
Kieserite (MgSO4)
17% Mg
0.17
0.06
B
Fe
Zn
0.12
Solubor
20% Boron
0.20
Inkabor
20% Boron
0.20
FeSO4
20%Fe
ZnSO4
20% Zn
0.20
0.19
Other Factors Affecting Fertilizer Recommendation
a. Season of the year. Generally, crops require higher rates of fertilizer during
dry season that wet season as there is greater solar energy available, more
vigorous plant metabolism and therefore high demand of nutrients during dry
season.
b. Economic value of the crop. High fertilizer rates and cost may be justified for
high value crops. For low value crops, the value of returns at high fertilizer
rate may not offset the cost. Thus, the value of crop yield must always be
pitted against the fertilizer cost. High yield of crops do not always mean high
economic returns.
c. Nutrient preference of particular type of crops. Different crops have different
demands for nutrients; Grain crops demand high for N, legumes for P, and
sugar, fiber, tuber and oil crops for K. These are considered in the
interpretation of soil tests. Sufficiency level of a nutrient for a certain crop may
not be a sufficient level for another crop.
d. Soil pH. It affects the behaviour and availability of applied nutrients. Acidic
soils have greater P fixing ability; therefore, P fertilizer application must be
consider the proportion of P that will be immobilized in the soil
e. Soil Moisture conditions.
Moisture does not only affect the solubility of
applied fertilizers but also the activity of the microorganisms. Under limited
availability of moisture, lower fertilizers must be used and since, microbial
76
activity particularly those relating to mineralization of nutrients is inhibited,
less native supply of nutrients is expected
Fertilizer Use. The use of fertilizers is based on basic principles of plant nutrition,
properties of the soil, environmental conditions, target yield and other considerations.
Method of Fertilizer Application
1. Broadcast – fertilizer is spread uniformly over the surface of the soil before
planting. This is suitable where crops are closely growing such that roots cover.
Broadcasting of fertilizers when leaves are wet is not recommended as it may cause
burning injury. In topdressing fertilizer is broadcasted on crop after emergence
2. Localized
Sidedressing – fertilizer is applied along the side of seed or plant
In the row – fertilizer is applied along the bottom of the furrow, slightly
covered with soil and then seeds are planted
Ring application – fertilizers are applied in band around trees in shallow
trench then covered lightly with soil. Distance from the base depends
on type of crop and age
Hole/Trench/Perforation application – fertilizers are dropped in holes around
trees
Seed pelleting – done by coating the seeds with the fertilizers by means of
adhesives
Seedling dippings – done for micronutrients and seedlings are dipped into
fertilizer solution to enhance the survival ability of seedlings and lessen
the effects of transplanting shock
5.
Foliar Application – involves dissolving of solid fertilizer materials in
water and applying it as spray to the plant or direct application of liquid fertilizers as
foliar sprays. Provides rapid utilization of nutrients by plants; and where soil has a
high fixing capacity for the nutrient. This is commonly done for commercial pineapple
and banana plantation. There is risk of leaf burning for N fertilizers when too high
concentrations are used. It is not suitable for grain crops and other crops with small
or narrow leaves. It is Suitable for applying micronutrients since the small dosage
needed can be applied uniformly with water serving as carrier. Also, most
micronutrients are easily fixed in the soils unless a suitable chelate is used (Zn,Mn,
Fe are more efficiently used by plants when applies as sprays). Fertilizer application
is made at more frequent intervals particularly for N since high solution
concentrations can burn the leaves. It is popular for ornamentals particularly orchids.
It is suitable when deficiency is detected early and if quick remedy is desired. Foliar
sprays can be mixed with insecticides spray, hence two operations can be
accomplished.
Timing of Application
-
depends on climate, soil, nutrient and crop
In sandy soils, N is necessarily split as well as K
For heavy clays, all of N is sometimes placed at planting.
P and K are usually applied at planting as they are less mobile, less subject to
leaching and less soluble
P is also needed at young age to accelerate root development
In alkaline soils, ammonium fertilizer is necessary deep placed to minimize
volatilization of ammonia
77
11. Fertilizer computation
Recommended Rate
Weight of Fertilizer Material =
Nutrient Content of the Material
Sample Calculation 1:
Determine how many bags of Ammonium Sulfate (21-0-0) will be needed to satisfy
the fertilizer recommendation of 90-0-0. Each bag weighs 50 kg.
Weight of AS = 90 kg N
0.21
= 428.57 kg AS
Bags of AS = 428.57 kg AS = 8.57 bags AS
50 kg/bag
Sample Calculation 2:
Determine how much Ammonium Sulfate (21-0-0) and Ammonium Phosphate (1620-0) that will be needed to satisfy the fertilizer recommendation of 90-30-0.
In this example, identify first the fertilizer that contains two elements. AP is a source of N and
P2O5, 16% and 20%, respectively. Consider first the lowest recommendation. Since, it
contains only 30 kg of P2O5, calculate first the P2O5 content.
Weight of AP = 30 kg P2O5
0.20
= 150 kg AP
Since AP contains both N (16%) and P2O5 (20%), the application of 150 kg AP, able
to supply: 24 kg N and 30 kg P2O5.
150 kg AP x 16% N = 24 kg N
150 kg AP x 20% P2O5 = 30 kg P2O5
With the application of 150 kg AP, the recommendation of 90-30-0 becomes:
90- 30-0
- 24 -30-0
66 – 0-0
The remaining 66 kg N, will be satisfied
\\ application of AS
by the
90- 30-0
24 -30-0
66 – 0 - 0
66 – 0- 0 (AS)
0 – 0- 0
Weight of AS = 66 kg N =314.29 kg AS
0.21
12. Causes of decline in soil fertility
a. Crop removal of nutrients. Different plants would remove different amounts of
nutrients or bases. If these bases are continually removed from the soil, then
the soil would become acidic.
b. Leaching of nutrients. The bases which have been replaced by the exchange
sites of soils are removed in the drainage water. This process removes the
78
metallic cations which compete with hydrogen and aluminum on the
exchange complex.
c. Gaseous losses of nutrients. Loss of nutrients in the form of gases would
also result to decline in soil fertility.
d. Soil erosion losses. Soil erosion is the wearing away of the land surface by
running water, wind, ice or other geological agents would result to decline in
soil fertility.
13. Soil Erosion Process
 Detachment and subsequent transport of soil materials (including rock fragments)
by an agent (water, wind or gravity) to an area of deposition
 Water is the most important agent of erosion in humid tropical areas, while wind for
arid and semi-arid areas
Two general types of erosion
a.) Geologic or natural erosion –type of erosion at natural rates, unaffected by human
activities
b.) Accelerated erosion – water or wind erosion at more rapid than normal, usually
associated with human activities like shifting cultivation, overgrazing
Mechanics of Soil Erosion Process:
1.) Soil detachment by rainfall – process by which rainfall splash soil sediments from the
soil surface into the run-off. It requires energy and this is supplied by the kinetic
energy of raindrops
2.) Entrainment or transportation of sediments from upslope to downhill direction
whether in rills, between rills and in sheet flow
3.) Sediment deposition – process by which sediment settles out under the action of
gravity and depends on particle size, being rapid for sand and slow for clay, it is a
selective process.
Types of Erosion
1. Raindrop erosion – occurs as soil particles are detached due to the impact of
raindrops and splashed at a longer distance in the down slope than in the upslope
direction
2. Sheet wash erosion – uniform removal of soil in thin layers from sloping lands,
resulting from sheet or overland flow occurring in thin layers
3. Rill Erosion – removal of soil by water from small but well-defined channels formed
when there is concentration of sheet wash flow
4. Gully Erosion – removal of soil by water from channels formed when rills combine
and develop to the extent that they cannot be eliminated by normal tillage operation
5. Stream Bank Erosion – happens when stream banks erode by run-off flowing the
side of the stream banks or by scouring and undercutting below the water surface
6. Scour erosion – influenced by the velocity and direction of flow, depth and width of
channel and soil texture
7. Wind Erosion – removal of soil due to wind specially when the soil is dry
79
Factors Affecting Soil Erosion
1. Rainfall – Rainfall intensity and not rainfall amount is positively correlated to soil
erosion.
Erosivity – potential ability of the rain to cause erosion
2. Soil erodibility – refers to the vulnerability or susceptibility or proneness of the soil to
erosion due to soil texture, soil structure.
3. Vegetation or Plant Cover – shorter plants growing closely to the ground surface are
more effective in dissipating the raindrops as hence reducing erosion
4. Relief (Slope) – slope steepness, slope length affect splash erosion and run-off
behaviour
5. Human Activities – shifting cultivation, improper cultivation, indiscriminate lumbering,
overgrazing, burning, road construction, urbanization, mining
Adverse Effect of Erosion
1. On-site Effect
a.) Particle Selectivity
b.) Surface Sealing and Hardening
c.) Nutrient Loss
d.) Loss of Soil depth
e.) Secondary changes
i. Water and nutrient availability
ii. Soil pH and lime requirement
iii. Soil temperature
iv. Soil tilt
2. Off-site Effects or Erosion
`
a.) Eroded materials may bury crops or lower the fertility of the adjacent bottom
lands
b.) Siltation – affected the drainage canals, irrigation canals, ponds, reservoirs
c.) Raise the level of river beds
d.) Raised level of the river bed introduces hazards to navigation which limit river
traffic
Soil and Water Conservation Measures
1.) Mechanical/engineering measures
i.
Terracing
ii.
Grassed waterways
iii.
Pond
iv.
Check dam
2.) Biological/ Vegetative Measures
i.
Mulching
ii.
Cover cropping
iii.
Strip Cropping
iv.
Crop Rotation
v.
Relay Cropping
vi.
Multiple Cropping
vii.
Alley Cropping
viii.
High density planting
ix.
Agroforestry
80
Application
1. When do we say a particular nutrient element is essential?
2. Tabulate the macro and micro elements needed by the plants, indicate its available
forms and functions in the plant.
3. During this pandemic where most of laboratory facilities are not available, what do
you think is the best approach to study the fertility status of the soil.
4. Calculate the quantity of fertilizer to apply when the fertilizer recommendations is 9060-30 and the available fertilizers are: Urea (45%), Ordinary superphosphate (0-20-0)
and muriate of potash (0-0-60)
5. Why does soil erosion degrades soil quality?
REFERENCES
University Code
Student’s Handbook
Brady, N.C., and R.R. Weil. 2001. The nature and properties of soils (13th ed.) Prentice Hall,
Upper Saddle River, NJ.
Carating, R.1997. The Survey and Classification of Andisols. SRDC Technical Information
Series No. 6. Soils Research and Development Center-JICA, Diliman, Quezon City,
Philippines.
Castillo, RL. 2007. Lecture Handouts in Soil Science for LEA Review.
Daquiado, N. n.d. Lecture Handouts in Soil Science for LEA Review, CMU, Bukidnon
Fanning, D.S., and M.C.B. Fanning. 1989. Soil morphology, genesis, and classification. John
Wiley and Sons, New York.
Hodges, S. 2011. Soil Fertility Basics. Soil Science Extension. North Carolina State
University.
Kenny, A. 2013. Recent Advances in Microbiology.
Lasquites, J. n.d. Lecture Handouts in Soil Science for LEA Review, USeP, Tagum City
LEA Reviewer in Soil Science. 2003. Visayas State University, Baybay City Leyte.
Lynn, I. et.al. 2013. Land Use Capability Survey Handbook.
Salibay, MA. 2010. Lecture Handouts in Soil Science for LEA Review, USeP, Tagum City
UPLB, 2007. Lecture Review Materials for LEA. College of Agriculture, UPLB, Laguna,
Philippines
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