Chapter 4
SOILS
Recycling of elements occurs in the soil.
Vegetation influences the development of soil, its chemical and physical properties, and its organic
matter.
DEFINITION OF SOIL
Soil is the product of weathered rock by the action of climate and living organisms. It is made of mineral
and organic matter and is capable supporting plant growth.
Soil is characterized by horizons or layers. It is different from the parent material by the addition and
transformation of organic matter.
Soils have five components: inorganic matter derived from parent rock, organic matter added by
organisms, water, air, and organisms.
The pedon is the basic unit used to study soils. It usually ranges from 1 to 100 m 2, and is threedimensional to include all horizons.
THE SOIL PROFILE
A soil profile is a vertical cut through a pedon or body of soil. The layers are called horizons.
Each horizons has its own characteristic features, e.g. color, texture, porosity, thickness, and chemical
composition
In general soils have six horizons:
1. O: loose leaves, organic debris and partially decomposed organic matter.
2. A: dark-colored, mineral material mixed with organic matter
3. E: is the area of greatest leaching; it has a granular, platelike or crumlike structure. Sometimes is
missing from some soils.
4. B: the zone where leached material accumulates; clay material, iron, aluminum silicates and
humus; below the B horizon, claypans (clay) and fragipans (silt and sand) form and interfere with
root and water penetration.
5. C: weathered material similar or not from that from which the soil formed
6. R: the unweathered bedrock.
PROPERTIES OF SOILS
PHYSICAL PROPERTIES
Color:
 It has little influence on the function of a soil.
 Is used in the classification of soil.
 Dark brown or black color usually indicates a lot of organic matter; volcanic soils are black due to
their origin from basalt, the parent material.
 Red and yellow soils contain iron oxides.
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Quartz, carbonates of Mg and Ca, gypsum and some compounds of iron give a whitish or grayish
color to the soil.
The standardized Munsell color charts are used to determine color of soils.
Texture:
 The texture of soil is determined by the size of the particles that make it.
 The particles are derived from the parent material or may be the result of soil-forming processes.
 Rock fragments are >2.0 mm in diameter.
 Sand ranges from 0.05 to 2.0 mm in diameter.
 Silt ranges from 0.002 to 0.05 mm.
 Clay is less than 0.002 mm.
 Clay controls the plasticity of the soil and exchange of ions.
 In ideal soils, particles make 50% of the soil, and the other 50% is pore space.
 Soil particles are held together in clusters or shapes of various sizes called aggregates or peds.
Depth:
 The depth of soils is variable; it depends on the slope, parent material, vegetation, and other
factors.
 Soils at the bottom of slopes tend to be deep, and thin on ridges.
Moisture:
 The transition between wet surface soil and dry soil is harp unless the soil clay.
 Depending on the amount of water, the downward flow of water stops after two or three days.
 The water stays in the pores.
 Field capacity (FC) is the maximum amount of water a soil can hold.
 Wilting point (WP) of soil is reached when plants cannot extract any more water from the soil.
 The difference in the amount of water between FC and WP is called the available water
capacity (AWC).
 Soil texture influences both FC and WT of soils, e.g. sandy soils have little FC capacity.
 The topography of the soil affects the soil moisture, e.g. ridges tend to be dry.
 There are seven standard drainage classes.
 Hydric soils develop where ponding and flooding occurs frequently, and form the wetland
ecosystems.
CHEMICAL PROPERTIES
Chemical elements in soil are found in solution, as constituents of organic matter and adsorbed of soil
particles.
Chemical elements travel from soil to plants, then to animals and continue in the biochemical cycle of
nature.
Clay controls the chemical properties of soils. This is due to its unique chemical structure.
The three basic elements in clay are aluminum (Al), silicon (Si) and oxygen (O).
The basic clay mineral is silica made of one atom of Si and four of O forming a tetrahedron.
These tetrahedrons are bound to each other by oxygen atoms and form a structural cell, (Si2O5)n.
These cells form sheets that become part of a more complex structure.
Aluminum forms with Oxygen octahedrons and form sheets similar to those of silica.
Shared bonds hold sheets of silica and aluminum together. These are the basic structure of clay colloids
or micelles.
Units of clay:
1. Kaolinite, a 1:1 clay, is made of one sheet of silica and one of aluminum held together by
hydrogen bonds. The oxygen atoms on the surface of the silicate layer face the hydroxyl atoms of
the aluminum layer. These hydrogen bonds hold the layers tightly together so water and cations
cannot enter the space in between the micelles.
2. Montmorillonite, a 2:1 clay, consists of one sheet of silica sandwiched between two sheets of
aluminum. These layers have twice as many Si as Al, twice as many O and half the OH. The O
atoms face each other the bond between the layers is weak and water and cations can enter the
interlayer space.
3. Illitite is made of one aluminum sheet with silica sheets on either side with Mg or K ions holding
together the adjacent micelles.
In 2:1 clays, one atom can be replaced by another, e.g. Si 4+ by Al3+, Al3+ by Mg2+, Fe3+ by Fe2+, and vice
versa. These replacements do not change the structure of the micelles. This is called isomorphous
substitution.
These structures and substitutions result in a net negative charge on the surface of the micelles that is
balanced by positive ions, cations.
Exchangeable cations are loosely held on the surface of the micelles and can be replaced by others.
The total number of negatively charged exchange sites on clay and humus particles that attract cations is
called the cation exchange capacity, CEC.
The negative charges of the clay particles prevent the leaching of cations.
Mg2+, Ca2+, K+, Na+, NH+ and H+ are some of the cations that cling to the surface of the clay particles.
Al3+ and H+ are more strongly held than other cations. These two cations can replace the more loosely
held Mg2+, Ca2+, K+, Na+, and NH+.
The percentage of sites occupied by basic ions (Mg2+, Ca2+, K+, Na+), is called percent base saturation.
Acidic soils have a low % base saturation because there are many H + available.
Soils with high CEC are potentially fertile because they can hold many cations needed by plants.
Soils with high CEC and % base saturation are potentially fertile unless they are too saline or contain
toxic cations, e.g. Cd2+.
Cations held in the exchange sites are dynamic equilibrium with those in solution.
Cations in solution are continuously being replaced by or exchanged with those in the exchangeable
sites.
The removal of cations by the roots of plants lowers their concentration in the soil solution and enhances
the releasing of more cations by the micelles.
Hydrogen is added to the soil by rain, humus and the roots of plants. The soil slowly becomes more
acidic.
As the acidity of the soil increases, the solubility of Al3+ increase and that of Na+, Ca2+ and other cations
decreases. This process decreases the fertility of the soil.
THE LIVING SOIL
Soil is relatively stable structurally and chemically and offers an underground environment that varies
little.
Soil is a living system that contains a wide array of organisms: bacteria, fungi, protozoans, vertebrates,
algae, etc.
Examples of density:
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Bacteria: 108 to 109 per gram of soil.
Protozoans: 104 to 105 per gram of soil.
Nematodes: 10 to 102 per gram of soil.
Mites (Acarina) and springtails (Collembola) are the most common soil animals. Together they make
about 80% of the soil fauna. They feed on fungi or other animals.
Earthworms (Lumbricidae) ingest soil and fresh litter, and then defecate these materials mixed with
intestinal juices producing aggregates in the soil.
These aggregates bind the soil particles together and improve aeration and capillarity of the soil.
SOIL FORMATION
FACTORS IN SOIL FORMATION
1. Parent Material
The parent material is an unconsolidated mass of rock, sand, clay or silt.
It may be derived from the bedrock on which it is found or transported by water, glaciers, wind, or gravity.
The parent material comes from igneous, sedimentary or metamorphic rocks.
1. Igneous rocks are formed when magma cools on the surface (extrusive) or below the surface of
the Earth (intrusive).
2. Sedimentary rocks are formed from material that accumulates at the bottom of lakes or oceans,
and then converted to rock by the pressure of sediments that accumulate above. These rocks
may be exposed by uplifting of the Earth’s surface or by the erosion on mountains.
3. Metamorphic rocks are formed when igneous or sedimentary rocks are buried deep in the Earth
and subjected to great pressure and high temperatures. The original minerals melt and form new
minerals.
2. Climate: Radian energy and water.
Heating and cooling depends on the climate and determines the intensity of weathering.
Temperature affects the rates of biochemical reactions.
Radiant energy influences the evaporation and the dryness of the soil.
Water is the carrier of acids that affect biochemical processes.
As water moves through the soil, it leaves behind suspended materials and carries away minerals in
solution, a process called leaching.
3. Topography
Topography affects the intensity of the radiant energy hitting the soil and the amount of water that enters
the soil.
Water that drains from slopes enters the soil on low land.
Steep slopes are subject to erosion and landslides and soil creep.
Slopes usually have poorly developed soils because erosion removes the soil as soon as it is formed.
4. Biota
Vegetation is responsible for the nutrient content of the soil.
Organic acids produced by plants speed up the weathering process.
5. Time
Well-developed soils in balance with erosion, weathering and biological action may require 2000 to
20,000 years to form.
Soil differentiation from parent material may be as fast as 30 years.
Soil develops more slowly in dry regions than in humid ones.
Lowlands constantly accumulate soil from the highlands around. These soils are usually more fertile than
old ones because they have not been exposed to leaching as long as the old soils.
WEATHERING
Weathering is the physical disintegration and the chemical decomposition of the parent material.
1. Physical weathering
Physical weathering is due to the action of water, wind and temperature changes.
Cracking and flaking of the soil occurs; freezing in crevices expands and cracks the rocks; erosion carries
away and deposits loose materials in other locations and exposes new surfaces to weathering.
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Pressure Release
Rocks below the earth surface support the weight of the overlying column of rock. Erosion strips away
this overlying rock and decreases pressure on buried rocks. All rocks are slightly elastic, so the buried
rocks respond to the reduction of pressure by expanding upwards. This results in the formation of
pressure release fractures (cracks) that form parallel to the surface. With continued erosion, these
rocks are exposed on the surface and slabs of rock break off along the pressure release fractures.
This weathering creates bare rock surfaces that may be more resistant than surrounding rocks.
These features are termed exfoliation domes; the slabs of rock that break off are termed exfoliation
sheets.
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2. Chemical weathering
Several processes are involved:
Oxidation is the loss of electrons. It occurs in cases where the oxygen supply is high and the biological
oxygen demand is low.
Reduction is the gain of electrons. It occurs where the material is water saturated, the oxygen supply is
low, and biological oxygen demand is high.
Hydrolysis involves H+ that attack silicates. Aluminosilicates and feldspar are transformed into clay
kaolinite and montmorillonite, which may weather into other clays.
The free hydrogen ions may alter mineral composition by replacing other ions in a mineral’s
atomic structure; this reaction is termed hydrolysis. Hydrolysis occurs when minerals react with
water to form other products. Feldspar, the most common mineral in rocks on the earth's surface,
reacts with water to form a secondary mineral such as kaolinite (a type of clay) and additional
ions that are dissolved in water. The weaker clay is readily worn away by physical weathering.
Feldspar + hydrogen ions + water à clay + dissolved ions
4KAlSi3O8 + 4H+ + 2H2O à Al4Si4O10(OH)8 + 4K+ + 8SiO2
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David McConnell
Iron is reactive with water and oxygen and iron bearing materials are prone to rapid decomposition.
Water and carbon dioxide react to form carbonic acid, which reacts with hydroxides of potassium, sodium,
magnesium and calcium to produce carbonates and bicarbonates. These materials accumulate deeper in
the soil or are leached away.
Leaching is greater in regions of high rainfall and high temperatures.
ROLE OF THE BIOTA
Lichens colonize bare rocks and produce acids that attack silicates. This activity encourages the growth
of mosses that covers the rock.
Plant roots penetrate and further break down the parent material. Roots remove nutrients from deep in
the soil and bring them to the surface.
Plants convert the sun’s energy into organic carbon that is added to the soil. This energy source provides
food for bacteria, fungi, and other invertebrates that colonize the soil.
Higher invertebrates (centipedes, millipedes, mites, earthworms, etc.) eat fresh material and leave
partially decompose products in their excreta.
Microorganisms convert these materials to lignin, waxes, proteins, carbohydrates, ash and other
substances that are eventually converted to inorganic products.
Humus is the organic matter that remains after having been processed by organisms and weather. It is a
non-cellular, dark colored, chemically complex organic material.
Its characteristic component is humin, a complex substance of organic acids containing humic and fulvic
acids among others.
Soil organisms are involved in the formation of the A and O horizons of the soil.
Three types of humus formation exist in the soil in temperate regions:
1. Mor: characteristic of moist or dry heathland and coniferous forests; well-defined, unincorporated,
matted or compacted plant material; remains unmixed with the mineral soil; fungi consume mostly
the vascular cells of leaves; proteins precipitate in a complex reaction making them resistant to
decomposition.
2. Mull: characteristic of mixed and deciduous woods on fresh and moist soils with a reasonable
supply of calcium; all organic matter is converted to humic substances; animal activity is high and
these products are absorbed into the mineral soil below; bacteria are the chief decomposers;
there is a great variety of organisms that process the organic matter in different ways and mixed
with the mineral particles; mineral and organic parts are inseparably bound together.
3. Moder: organic matter is converted to droppings of small arthropods, particularly Collembolans
and mites; it contains high amount of organic matter, nitrification is limited; the droppings form a
dense matted material with little mineral parts.
SOIL DEVELOPMENT PROCESSES
Four processes are involved in soil formation:
1. Additions of organic or inorganic material.
2. Losses of material through erosion and leaching.
3. Translocation vertically and laterally within the soil.
4. Transformation of mineral and organic substances into peds.
See table 4.2. “Processes of Soil Formation.”
SOIL CLASSIFICATION, LOWER SOIL CATEGORIES AND MAPPING SOILS.
Read this sections, pages 69 to 76.