SOIL SCIENCE
SOIL SCIENCE
LEARNING OBJECTIVES
• The learner will gain a basic understanding of how soils form.
• The learner will understand the soil’s characteristics and how each soil
characteristic can affect crop growth.
• The learner will gain a basic understanding of soil nutrients.
• The learner will learn the purpose of taking soil samples, how to take soil
samples and what to do with the results after having the soil samples
tested.
• The learner will learn how to create and maintain a healthy ecosystem in
the soil for optimal soil fertility, which is used in crop production.
SOIL SCIENCE
The understanding of basic soil science is important for many different
reasons. It offers long-term farm security and viability through proper
management. It also plays a role in the health of a watershed or
regional eco-system by combating erosion or nutrient pollution in our
streams and rivers. Growers who study and pay close attention to
their soil’s structure and content will see better results in their
cultivation and yield. These growers will also be better prepared to
address many negative issues that they encounter while cultivating
their crops.
SOIL SCIENCE
SOIL PEDOGENESIS
Soil pedogenesis is a term that describes the process of soil development.[1]
The scientific study of pedogenesis examines how soil changes it’s
composition and it also studies where soil comes from. In the late 19th
century two scientists, Hilgard and Dukuchaev, independently found through
studying the soil that the composition of the soil that they were studying was
principally determined by both climate and vegetation. Later scientists,
including Hans Jenny, developed models for soil pedogenesis that
incorporated five interrelated factors.
These factors include: climate,
living organisms, parent
material, topography, and
time.[1]
SOIL SCIENCE
SOIL PEDOGENESIS
Climate as a factor in soil pedogenesis
The two most influential climatic factors in soil pedogenesis are
temperature and moisture.[1] Moisture in the form of water, along with
temperature, are used by nature to break down rocks in a process called
weathering. During the weathering process, water seeps into the cracks that
are in rocks. When the temperature drops below freezing, the water freezes
and expands, acting like a wedge to break the rocks apart.[2]
Temperature will also heat and
cool rocks which cause them to
expand and contract. The
change in temperature, year
after year, will cause the rocks to
crumble.[2]
SOIL SCIENCE
SOIL PEDOGENESIS
Climate as a factor in soil pedogenesis
Water plays a role in pedogenesis by absorbing into and swelling clay beds
that, in turn, break apart surrounding rock beds.[2] Another role of water is
to carry minerals from one area to another, including salt. When saltwater is
exposed to rocks or a rock bed, the saltwater will get into the cracks of the
rock and then evaporate, leaving behind the salt. With repeated exposure to
saltwater, the salt crystals in the cracks of the rock create formations that
expand and break apart the
rock.[2] Another climatic factor
that we should consider in
pedogenesis is acidic erosion.
This occurs when acids are
carried in water. These acids,
like carbonic acid, break down
limestone formations by
dissolving them.[2]
SOIL SCIENCE
SOIL PEDOGENESIS
Living organisms as a factor in soil pedogenesis
Living organisms work in a variety of ways to create and change soil in
pedogenesis. Tree roots and mosses break down rocks and rock beds by
splitting rocks into smaller pieces.[2] Living organisms have a large role in
changing the nitrogen and carbon composition in the soil.[1] When plants die
and decompose, or are eaten and defecated by animals, the plant material
is
] consumed by decomposers.
These decomposers like fungi,
bacteria or insects further
break down the plant material
into compounds rich in nitrates
and carbon.[3] These
compounds are beneficial for
plant growth.
SOIL SCIENCE
SOIL PEDOGENESIS
Parent material as a factor in soil pedogenesis
Parent material refers to the original material that the soil develops from.[1]
Parent material can be either minerals from eroded rock, like sand or silt. It
can also be organic material, like muck or peat.[4][5] Parent materials that
come from eroded rock are called parent rock sediments. These sediments
take a long time to erode from the rocks that they came from. In flatland
areas, parent material forms in place, from the bedrock beneath it.[1]
In mountainous or hilly areas,
parent material will wash down
hillsides and form a layer of
sediment. In the continental
United States, the sedimentary
parent material is most often
eroded granite. On most
islands, the sedimentary parent
material is eroded basalt.[4]
SOIL SCIENCE
SOIL PEDOGENESIS
Parent material as a factor in soil pedogenesis
Some parent material can be organic, rather than sedimentary rock. These
organic parent materials are called cumulate parent materials. Cumulate
parent material can be found in low-lying areas. These low-lying areas often
hold water and the resulting parent material can be either peat or muck.[5]
Places that we can find cumulate parent material are in marshes, swamps
and otherwise low-lying areas.
Here is a video that talks about
accumulated organic parent
material.
SOIL SCIENCE
SOIL PEDOGENESIS
Topography as a factor in soil pedogenesis
Topography is a term that describes the shape of the land, like the steepness
of a slope or how flat an area of land is.[4] Topography is an important factor
in soil pedogenesis because the topography of the land determines where
water will drain to. Water carries sedimentary and organic material from
one area to another and these materials will be parent materials for soil at
the water’s destination area.
The topography of an area will
also determine how fast an
area of land will erode. Steep
slopes like hillsides will erode
from wind and rain faster than
low-lying areas like valleys or
marshes.[4][5]
SOIL SCIENCE
SOIL PEDOGENESIS
Time as a factor in soil pedogenesis
Time is the factor in pedogenesis that all of the other factors rely on to
change the natural environment, like rocks and trees, into the soil that we
see today. Over time, sedimentary parent material breaks down into smaller
pieces and organic parent materials break down further through
decomposition. Low-lying areas tend to fill in with parent material over time
all while hillsides and mountain slopes erode from wind and rain.
Another thing that is occurring
over time is soil layering. As
low-lying areas fill in with
parent material, or flat areas
continue to age, the soil creates
layers that we can see. These
layers are called horizons.
SOIL SCIENCE
SOIL HORIZENS
Over time, the soil creates layers due to
weathering and rainfall. The soil’s composition
and color is different from one layer to the next.
The color difference between the soil layers
indicate how much organic matter is in the soil.
Darker colors indicate a higher carbon content,
and lighter colors indicate a higher mineral
content.[6]
Here is a video about soil horizons.
SOIL SCIENCE
SOIL HORIZENS
The O horizon is often a very thin layer of organic
matter that makes up the surface of the soil. The
organic matter that can be found at the O
horizon is at various stages of decomposition.[6]
It is usually comprised of plant material like
leaves, pine needles and other organic debris.
The decomposing organic materials add
nutrients to the soil while also enhancing the
soil’s ability to retain moisture.[7]
SOIL SCIENCE
SOIL HORIZENS
The A horizon lies beneath the O horizon. It is
comprised mainly of mineral particles and it is
usually dark in color. The dark color is the result
of mixing soils between the decomposing
organic material from the above layer, and the
minerals from the layer below. [6][7] The A horizon
is what we consider topsoil, or the soil that
plants need to grow. Another characteristic of
the A horizon is that a process called eluviation
takes place here. Eluviation is the downward
movement of dissolved and fine material within
the soil when rainfall exceeds evaporation.[6][7] In
other words, the rain washes smaller dirt
particles father into the ground, which go into
the E and B horizons.
SOIL SCIENCE
SOIL HORIZENS
The E horizon is strongly influenced by the layers
of soil above it. This layer is often light colored
as small particles of decomposed organic matter
wash through it, into the soil layer below it. The
E horizon commonly contains some rust from
iron or aluminum, which can give it different
colors from one area to another area.[7]
SOIL SCIENCE
SOIL HORIZENS
The B horizon lies beneath the E horizon and it is
the most dense of all the soil layers because of
all the fine soil particles that accumulate in this
layer.[7] The B horizon is considered subsoil and it
is very nutrient rich due to the clay that most soil
will contain in this horizon. In areas where there
is a lot of rainfall, the B horizon will be found
deeper down than areas that are arid and dry.[6]
Most living organisms, like earthworms, bacteria
and fungi, live in the horizons O, A, E and B.
Some burrowing animals will dig into the C
horizon, as will most tree roots.[7] The burrowing
of animals and the penetrating tree roots help to
aerate and distribute the soil between horizons.
SOIL SCIENCE
SOIL HORIZENS
The C horizon contains mostly parent material
that is weathered. [6][7] The parent material is
either made on site, from the bedrock beneath
it, or it can be transported through rainwater
runoff, from mountain slopes or hillsides. In
newer soil, rainwater transports the parent
material to it’s location and the soil horizons
above the parent material are built on top it
through time. In older soil, the parent material is
created from the weathering of the bedrock
underneath the parent soil. [6][7] The layer
underneath the C horizon is sometimes called
the R horizon. It is simply bedrock that the
parent material comes from. This layer is not
weathered and it is often found very deep down.
SOIL SCIENCE
SOIL COMPONENTS
The soil that we consider topsoil,
found in the A horizon, is made of
four components.[8] If the soil has
not been disturbed by human
activity, like compacting the soil or
digging, the soil will contain
approximately: 45% minerals often
in the form of sand, 25% water,
25% air and about 5% organic
matter.[8]
When the soil gets compacted from human activity, like driving a tractor over
the soil, the air and water are pressed out of the soil, while leaching some of
the organic matter to the layers of soil below it. Plants and other living
organisms need the air in soil that is not compacted in order to thrive.[9]
SOIL SCIENCE
SOIL TILTH
The term soil tilth refers to the soil’s general ability to support healthy
root growth and plant development.[10] A soil with good tilth has enough
pore space for air pockets and the soil allows for excess water drainage.
Another component of having good tilth is that the soil will have the
adequate nutrients for plant development. Altogether, tilth is a function
of soil texture, fertility, organic content and the living organisms that live
in the soil.[10]
Here is a video that talks about creating tilth in clay soil.
SOIL SCIENCE
SOIL TILTH
Soil texture
Soil texture is a term that refers to the size of the particles that make up the
soil.[10] The terms sand, silt, and clay refer to relative sizes of the individual
soil particles.[10] The size of the particles in the soil can greatly affect the
soil’s ability to retain water, while also providing air to the roots.[11]
SOIL SCIENCE
SOIL TILTH
Soil texture
A common way to depict the type
of soil a person has is based on the
percentage of clay, silt and sand in
a given area.[10] This triangle
represents the 12 classes of soil
that are based on the percentage
of the three particle sizes being;
clay, silt and sand. Each class of soil
will behave differently in it’s
nutrient availability to the plant’s
root system and each class will be
different in it’s water retaining
ability.[10][11]
Here is a video that
talks about
creating tilth in clay
soil.
SOIL SCIENCE
SOIL TILTH
Soil properties of clay
Clay particles are the smallest of the three types of
soil particles.[10] They are flat, plate-shaped particles
that have a lot of surface area, when compared to
silt and sand.[11] The flat, plate-shape particles form
layers. The layers of clay particles make it very
difficult for water to drain through them. Another
property of clay soil is that clay is rich in nutrients,
because the soil is dense and it has a larger surface
area for soil-to-root contact.[10]
Clay soils have poor tilth and compact easy. When
compacted, the air pockets are displaced by water
and more clay particles. Growers should be careful
not to walk on clay soil that is used for growing
crops.[12] When clay is wet, it can mould easy to
form a ball in your hand. When it is dry, it becomes
very brittle and can form cracks in the soil.[12]
SOIL SCIENCE
SOIL TILTH
Soil properties of silt
Soil with a high silt content is considered to be
among the most fertile of soils.[13] Silt is often found
in river estuaries, because the fine particles are
washed downstream and then deposited. Soil that
has a lot of silt in it is soft and smooth. Like clay, silt
holds a lot of water. Silt, however, has slightly larger
particles than clay and that makes it a little better
at draining water than clay.
Silt has a flour texture when it is dry and it is runny
when wet.[10] Silt is almost always found with other
soil particles like clay or sand, in the form of
loam.[11]
SOIL SCIENCE
SOIL TILTH
Soil properties of sand
Sand is the largest soil particle when compared to silt
or clay. Sand can be either coarse, medium or fine, in
the particle size. Fine sand adds little to the soil
characteristic because it does not increase the pore
spaces for air pockets that are used in draining
water.[13] An example of fine sand is the bagged sand
sold for children’s sandboxes.
Soil that has the characteristic of sandy soil has a
composition of greater than 50-60% medium to
coarse size sand particles (greater than
0.05mm).[10] Sandy soils have good drainage and
aeration, but low water and nutrient holding capacity
because of the large particle size.[11] There is much
less soil-to-root contact in course sand than in clay.
SOIL SCIENCE
NUTRIENT CYCLE
The nutrient cycle is a term that
refers to the creation and removal
of compounds in the soil from a
biological process.[14] These
compounds, or nutrients, are
needed by plants for healthy
development.[14][15] Chemical and
physical processes interplay in the
nutrient cycle of most compounds
through weathering and washing
away nutrients from the root zone,
which is the area below the
plant.[14] There are at least 16
nutrients that we consider
essential for healthy plant
development.[14][15]
SOIL SCIENCE
SOIL NUTRIENTS
Macronutrients
There are six essential nutrients that
plants need in larger amounts, when
compared to other nutrients.[14][15]
These nutrients are called
macronutrients. Of these six
macronutrients, they can be further
subdivided into primary
macronutrients and secondary
macronutrients. Most crops will use 50
to 150 pounds per acre of primary
macronutrients, while using 10 to 50
pounds per acre of secondary
macronutrients.[14]
Primary macronutrients are: nitrogen, phosphorus and potassium. Secondary
macronutrients are: calcium, magnesium and sulfur.[14]
SOIL SCIENCE
SOIL NUTRIENTS
Primary macronutrients
Nitrogen is a primary macronutrient
that made available to plants in a
variety of ways. The nitrogen cycle is
a term that describes some of the
ways that nitrogen is added to, and
used from the soil. In the nitrogen
cycle, nitrogen is converted into
various compounds through
biological and chemical processes.[14]
Here is a video about green
manure, also called covercropping. Growing a green
manure crop between growing
seasons is a farming strategy that
uses plants to fertilize the soil
with nitrogen.
SOIL SCIENCE
SOIL NUTRIENTS
Primary macronutrients
There are two big reasons why growers need to
pay close attention to the nitrogen availability in
their soil. Nitrogen deficiencies will stunt the
growth of most crops and nitrogen compounds,
in the form of nitrates, are water soluble and
they are easily washed out of the soil with
excessive rain water and poor drainage.[14]
Nitrogen deficiencies in the soil are evident on
the plant in a condition called chlorosis, which is
the yellowing of the leaves. Another sign of
nitrogen deficiencies are in seen in stunted
produce, like shortened ears of corn.[15]
Here is a video about how crops, like corn, use
nitrogen.
SOIL SCIENCE
SOIL NUTRIENTS
Primary macronutrients
Phosphorus is a primary macronutrient that is used by the plant in
photosynthesis, and it is needed for plant maturity, healthy roots,
and energy transfers within the plants themselves.[15] Phosphorus
is not very water-soluble and phosphorus will only move about
1/10 of an inch per growing season.[14]
Phosphorus deficiencies can be seen when
plants develop purple or red in the stem and
leaves.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Primary macronutrients
Potassium is a water-soluble nutrient that is used by plants in
creating sugar, and distributing that sugar throughout the plant.
It is used in photosynthesis by allowing the plant to make more
chlorophyll and it allows for the plant to use other nutrients
more effectively. Another function of potassium in the plant is to
open and close the pores, called stomata, in the plant leaves.[15]
Potassium deficiencies can be seen when
plants develop spots on the leaves.
Another sign is yellow and green
streaking on the the leaves and stems.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Secondary macronutrients
Secondary macronutrients include calcium, magnesium
and sulfur.[14] Plants need smaller amounts of secondary
macronutrients, yet they are essential for the plant’s
growth.
Calcium is essential for building the cell walls of plant
cells, neutralizing harmful acids, regulating the
availability of other nutrients, building plant proteins,
and preventing magnesium toxicity.[15]
Calcium deficiencies can be seen in the yellowing of the
upper part of the plant. Often, the leaves of calcium
deficient plants will develop red spots in the center area
of the leaf, that move outward toward the edges as the
plant matures. The stems of calcium deficient plants can
feel soft and the plants will often suffer blossom-end
rot.[15]
Tomatoes with blossom-end rot
SOIL SCIENCE
SOIL NUTRIENTS
Secondary macronutrients
Magnesium is necessary for photosynthesis to occur.
Without magnesium, plants cannot make the energy
that they need in order to be healthy and productive.
Magnesium also aids in a plant's use of other nutrients
like nitrogen, phosphorus, and sulfur. It is easy to
overlook a magnesium deficiency in plants because
the magnesium deficiency can look like a nitrogen or
phosphorus deficiency, since the plants have a hard
time using these nutrients.[15]
Magnesium deficiencies, most often, look like both
nitrogen and phosphorus deficiencies. The yellowing
usually starts from the edges of the leaves and will
continue inward toward the stem.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Secondary macronutrients
Sulfur is essential for plants to produce proteins and enzymes. Sulfur is not held
very well by other soil particles. Excessive rain water causes sulfur to leach past
the root zone where it becomes unavailable to the plant roots. Sandy soils that
have high porosity and low organic content can intensify the problem.[14][15]
When looking for a sulfur deficiency, look at
lettuce and legume crops first as they will be
affected first. Lettuce and legume crops use more
sulfur than most other vegetable crops.[15]
Chlorosis is the primary result of sulfur deficiencies.
The yellowing of the leaf is slightly different that of
a nitrogen deficiency because the yellow leaf does
not become brittle, as it often does with nitrogen
deficiencies.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Micronutrients, sometimes called trace elements,[15] are needed in smaller
amounts that are generally less than 1 lb/acre.[14] Some micronutrients come
from the air pockets in the soil or from rainwater that washes through the soil.
These include hydrogen, oxygen and carbon.[14][15] Other micronutrients like
sodium, silicon, and nickel are essential elements for some crops, but not they
are not necessarily required by other crops. They will, however, have positive or
beneficial effects on the growth of any crop.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Zinc is a micronutrient that is used by plants in a variety chemical reactions,
including the creation of amino acids that are used in building and maintaining
cells. Plants that have a zinc deficiency are often stunted with leaves that show
dead spots and yellowing. Zinc deficient plants will often lose their color early.[15]
Zinc deficiency in some plants, like corn, will show a striping effect in the leaves.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Boron is a micronutrient that is used by plants in over 16 functions, including cell
division and cell development. Plants that have a boron deficiency exhibit a
variety of slow, irregular growth habits such as dwarfing and bushiness. This
often happens because the growth at the top of the plant stops and is replaced
by lower branches.[15]
Boron deficiencies in root
crops, like beets and
turnips, are evident in the
presence of blackheart
disease.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Manganese is a micronutrient that, when deficient, can cause several disorders
that look like other nutrient deficiencies. Chlorosis, or the yellowing of the
leaves, is a common symptom of a manganese deficiency in plants.[15]
Manganese deficiencies are most commonly caused by the soil being too
alkaline. Adding organic matter to the soil, like composted manure, will provide
the soil with sufficient manganese. The soil, however, needs to be acidic in order
for the plant to use the manganese in the soil.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Iron plays a major role in the production of chlorophyll.
Iron also plays a role in nitrogen-fixation by converting
nitrates into ammonia, which the plants can use.[15]
There are a lot of symptoms of iron deficiencies, most look
like other nutrient deficiencies. The yellowing of the plant’s
leaves is the most common indicator of an iron deficiency.
Another symptom is tissue death on the ends of new
shoots. Fruit tree leaves can show an iron deficiency
through the yellowing of the leaves that develop brown
areas, and the fruit will have less flavor. Since iron
deficiencies look like other nutrient deficiencies, a soil test
is highly recommended if an iron deficiency is suspected.[15]
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Copper is a micronutrient that acts as a catalyst, making
other nutrients available to the plant in a form that
they can use. Copper is also used by the plant in the
process of respiration.[15]
Plants lacking copper develop a condition known as
withertip. With withertip, the leaves on the tips of
stems will show wilting that watering doesn't revive.[15]
Some plants, like tomatoes, will not grow healthy root
systems. Above-ground evidence of the unhealthy root
system can be seen in awkward and stunted growth in
the plant’s vertical development. Leafy plants, like
lettuce, will whiten in the leaves and the whitening will
often encompass the entire plant. Leaf bunching and
discoloration can also be a result of a copper
deficiency.[15]
Leaf bunching from
inadequate copper
Withertip in wheat
SOIL SCIENCE
SOIL NUTRIENTS
Micronutrients
Molybdenum is a micronutrient that is responsible for nitrogen fixation by
rhizobium bacteria on legume roots. These bacteria convert nitrogen compounds
into compounds that have nitrogen in a form that plants can use.[14][15] Plants
that are not legumes use molybdenum in a similar way, by converting soil nitrates
to ammonia, which the plants can use.[15]
Molybdenum deficiencies look like nitrogen deficiencies, because molybdenum
helps in nitrogen availability to plant roots. Another sign of a molybdenum
deficiency is spindly plants because of the insufficient nitrogen.
When suspecting a deficiency of molybdenum, look at
lettuce and brassica crops first (broccoli, cauliflower,
brussels sprouts, kale, etc.), as these crops rely on
molybdenum for nitrogen more than other crops.[15]
Molybdenum deficiencies are commonly found in acidic
soil because molybdenum is chemically bound to other
elements in acidic conditions.[15]
SOIL SCIENCE
SOIL PH
Soil pH is a term that refers to how acidic or alkaline a sample of soil is. PH is
defined as the negative logarithm of the hydrogen ion concentration.[18] In
testing the pH in water, we are testing the positively charged ions of hydrogen
(H+ and H3O+aq ).[18] Ions can be either positively charged or negatively
charged. Positively charged ions are called cations, while negatively charged
ions are called anions. Most plant-available forms of essential plant nutrients
are ionic, meaning that they carry an electrical charge.[14]
We use a 14 point scale to measure pH, with 7 being neutral. Many gardening
books suggest keeping a pH rage in the soil between 6.0 and 7.2.[17]
SOIL SCIENCE
SOIL PH
Many living organisms in the
soil are beneficial to the
crops that we grow and they
can not survive in soil that is
too acidic or alkaline.[14]
Soil pH is an important factor
in the soil for our crops
because the soil pH will
determine how available
nutrients are for the crop to
use.[14][16][17][18]
SOIL SCIENCE
CATION EXCHANGE CAPACITY
Soils can be thought of as storehouses for nutrients. Many of the nutrients
that plants need are positively charged, making them cations. The soil’s ability
to hold and maintain cations is called the cation exchange capacity (CEC). Soils
are made from minerals and organic matter. The organic matter, much of
which is in the clay, has a negative charge. The ability of the negatively
charged organic matter to hold cations determines the cation exchange
capacity for a soil sample. Having a high CEC protects soluble cations from
leaching out of the plant root zone. Cation exchange is the major nutrient
reservoir for potassium, calcium, magnesium and nitrogen (in the form of
ammonium NH4+).[14][16]
Here is a video that explains cation exchange capacity.
SOIL SCIENCE
CATION EXCHANGE CAPACITY
Buffer capacity
Cations can be removed from a clay particle in several ways. The cation can be
used by a plant root, washed away by irrigation or rain or used in another
chemical reaction. Once a cation is removed, another cation will take it’s
place. The replaced cation will often come from another nearby clay
particle.[14][19] The soil’s ability to replace cations of the same kind is called the
soil’s buffer capacity. Generally, a soil that is rich in clay and organic material
will have a high buffer capacity.[16]
There is a relationship between soil pH and cation exchange. When cations
like calcium (CA++) and magnesium (MG++) decrease and can not be replaced
by similar ones, hydrogen (H+) and aluminum (AL+++) will replace the removed
cations and increase the soil acidity because of the excess hydrogen (H+).[16]
Soils rich in organic content, including clay, have additional protection against
becoming too acidic for most crop’s needs because of the soil’s high buffer
capacity.[14][16]
Here is a video that explains the soil’s buffer capacity.
SOIL SCIENCE
CATION EXCHANGE CAPACITY
Basic cation saturation ratio
One approach to maintaining a sufficient amount of cation nutrients in the
soil is to use a basic cation saturation ratio (BCSR), also called the Albrecht
system. These systems use the concept that maximum yields of a crop can
only be achieved by creating an ideal ratio of calcium, magnesium and
potassium in the soil. By supplying enough calcium, magnesium and
potassium; the plants will receive their needed nutrients and soil pH will not
change due to an increase of hydrogen cations.[19]
The normal values for the exchangeable cations are not in perfect agreement
between scientists. Many soil testing labs use the ranges of approximately 6585% for cation exchange between calcium, magnesium and potassium. Of that
65-85% exchange; approximately 60 to 80% should be calcium, 10 to 20%
magnesium, and 2 to 5% potassium.[19]
SOIL SCIENCE
SOIL TESTING
Soil testing is a way for growers to know what their soil condition is before
adding fertilizers. Soil testing is relatively cheap and practical for all growers.
Generally, soil testing should be done every three to five years.[20] It is
common for some growers to do more frequent testing for pH or nitrogen.
Many growers choose to do soil testing in the autumn, because it is more
practical and it gives the growers time to add the fertilizer or compost before
the next growing season.
If a growing area has never had the soil tested before, it is a good idea to draw
a diagram of the growing area that the soil samples were taken from. The
diagram should include details like individual garden beds and this diagram
should be kept with the field notes. By keeping field notes, soil sample results
and a diagram of where the soil samples were taken from; growers are much
more prepared to predict how the soil nutrients will change over the course of
the growing season in their growing area.[20]
Here is a video that explains soil testing.
SOIL SCIENCE
SOIL TESTING
Soil testing procedure
• Remove surface debris, such as plant residues or mulch from the soil before
inserting the trowel.
• Sample gardens and flower beds to a depth of 6-8 inches.
• Take samples from different areas of a particular growing area, being careful not
to mix these samples with other samples from another growing area. Each growing
area should have samples that are representative of that particular growing area.
This is done by taking several samples from a growing area, like different corners of
a garden bed, and mixing them.
• Dry samples at room temperature. (Do not use artificial heat.)
• Break up any lumps and remove all stones, debris, etc.
• When dry, mix well and crush so all the soil is the size of wheat grains or
smaller, but do not pulverize.
• Remove 1 pint per representative sample and place it in a clean, labeled
container.
SOIL SCIENCE
SOIL TESTING
Soil testing data analysis
Individual soil testing laboratories can be different from each other in the way
that they test the soil samples. The nutrient compounds that they look for can
also be different.[20][21][22]
Plants can only use nutrients in certain forms, like ammonia for nitrogen.[14][15]
Since the nutrients in the soil exist in many forms, soil testing labs will test the
soil sample for it’s nutrient availability to plants. The results from a soil test are
called a “nutrient index” and they are a profile of the soil’s nutrient
potential.[21][22]
Most soil testing laboratories will base their results with what they think the
crop will need, and make recommendations on what fertilizers to use.[21][22] It is
common for growers to send soil samples to more than one laboratory to get a
well-rounded estimation of the soil’s nutrient potential.[21]
SOIL SCIENCE
SOIL TESTING
Soil testing data analysis
Even though soil testing results vary, there are generalities that make
interpreting the results easier. Most laboratories can test for pH, soluble salts,
organic matter, nitrate nitrogen, phosphorus, potassium, zinc, iron, copper,
manganese, lime and soil texture.[22]
• pH is given on a 14 point scale with 7 being neutral.
• Soluble salts are often measured by electrical conductivity and are reported
in mmhos/cm. They will often relate their findings to the specific crop that is
going to be grown. Typically, this measure will be below 10.[22]
0-2 mmhos/cm: Satisfactory for Crops
2-4 mmhos/cm: Affects sensitive Crops
4-8 mmhos/cm: High for many Crops
above 8 mmhos/cm: Very high for most Crops
SOIL SCIENCE
SOIL TESTING
Soil testing data analysis
• Organic matter (O.M.), is reported as percent of total soil. Organic matter
contains about 95 percent of all soil nitrogen. About 30 pounds N per acre will
be released (mineralized to nitrate) during the growing season from each 1
percent O.M. present.[22]
• Most soil testing laboratories test for nitrogen in the form of nitrate
nitrogen, reported in ppm NO3-N.[22] This nitrate is soluble and readily available
for plant uptake. To determine the approximate pounds of NO3-N/acre-foot (1
acre to a depth of 1 foot), multiply the soil test value (ppm) by 3.6. For
example, 10 ppm x 3.6 = 36 pounds NO3-N/acre to a depth of one foot. Crops
vary in their nitrogen use. Most crops need 50-150 pounds of nitrogen per
acre.[14]
SOIL SCIENCE
SOIL TESTING
Soil testing data analysis
• Phosphorus, potassium, zinc, iron, copper and manganese are not tested
directly. Instead, soil tests will test the soil’s availability of these nutrients for
the plant’s use. Test results for these nutrients are given in ppm. Fertilization
recommendations from the laboratory will be given based on the results.[22]
• Lime (CaCO3) is estimated as percentage of free lime. Test results are
reported as low (0 to 1 percent), medium (1 to 2 percent), and high (above 2
percent).[22]
• Texture is often estimated by the hand-feel method. Soil texture is
important to know when apply nitrogen fertilizer, because coarse soil texture
will leach nitrates faster. It is important on sands, loamy sands and sandy loams
that nitrogen applications be split to avoid mid- or late-season deficiency. It's
also recommended that high nitrogen rates be split for many crops. This can
help avoid ground-water contamination from soluble nitrates.[22]
SOIL SCIENCE
SOIL TESTING
Soil testing kits
Soil testing kits are less reliable[17] and they are limited in the nutrients they can
test for. However, kits are handy for seasonal use in testing soil pH and nitrates.
Here is a video that shows how to use a simple pH soil tester.
Here is a video that talks about testing soil texture without a testing kit.
SOIL SCIENCE
SOIL AMENDMENTS
Changing pH levels
Soil pH can change over time, especially in sandy soil that has little clay or
organic matter.[14]
If the pH is too acidic, a grower should ask themselves why the soil is too acidic
before making any changes.[18][14]
• Is rainwater leaching away basic ions (calcium, magnesium, potassium and
sodium)?
• Is carbon dioxide creating a high acidity from decomposing organic matter
and root respiration? Carbon dioxide creates a weak acid in water and soil.
• Is the acidity due to the formation of strong organic and inorganic acids like,
nitric and sulfuric acid? These acids can come from decaying organic matter
and the oxidation of ammonium and sulfur fertilizers.
SOIL SCIENCE
SOIL AMENDMENTS
Changing pH levels
Changing an acidic soil’s pH is usually done with a liming material. Some
common liming materials are: calcic limestone which is ground limestone (high
in calcium), dolomitic limestone from ground limestone (high in magnesium),
and miscellaneous sources such as wood ashes.[18] Each type of lime material
has different nutrients for the crop. The correct amount of lime to apply to a
particular soil acidity problem is affected by a number of factors, including soil
pH, soil texture, soil structure, and amount of organic matter.[18] If the soil test
was sent to a soil testing laboratory, recommendations for liming will be
included with the test results.[22] Liming materials are relatively inexpensive,
generally safe to handle and leave no objectionable residues in the soil.[18]
SOIL SCIENCE
Here is a video that
talks about
SOIL AMENDMENTS
growing plants in
Changing pH levels
alkaline soil.
The soil’s pH can change to become too alkaline for a particular crop’s needs.
Dry areas, or areas with a lot of limestone can have problems with the soil
being too alkaline. Alkaline soils greatly reduce the nutrient availability of iron,
manganese, copper, boron and zinc.[17]
Correcting alkaline soil can be done in several ways. Adding organic matter, like
compost, can increase the soil acidity while also increasing soil tilth and future
nitrogen availability.[14][18] Some crops, like blueberries, need to be in acidic soil
in order to thrive (4.0 to 5.5). Unprocessed elemental sulfur is commonly used
by growers to maintain an acidic soil.[14] Aluminum-sulfate should be avoided,
especially on low organic matter soils, because of the potential for aluminum
toxicity to plant roots. It is not allowed in organic crop production. Ammoniumsulfate is the most acidifying nitrogen fertilizer. It is not allowed for organic
crop production either. Do not use ammonium sulfate for large pH changes,
because that will result in excessive nitrate contamination in the ground
water.[14]
SOIL SCIENCE
SOIL AMENDMENTS
A soil amendment is any material that is added to soil in order to improve the
soil’s physical properties through adding nutrients while also creating tilth in
the soil.[23]
When soil changes in pH frequently, it creates salts in the soil. In choosing
amendments, it is important to keep in mind the salt content in the soil that is
to be treated. Some soil amendments contain salt or create salt in the
soil.[14][23] Other amendments can change the soil pH, and this should be
considered as well.[14][23]
Most amendments need to be mixed into the soil, not spread or buried. If it is
simply buried, the effectiveness is reduced and it will interfere with the water
and air movement in the soil.[23]
SOIL SCIENCE
SOIL AMENDMENTS
There are two broad categories of amendments, organic and inorganic. Organic
amendments come from something that was once alive. Inorganic
amendments are either mined or man-made.[23]
Organic amendments include sphagnum peat, wood chips, grass clippings,
straw, worm castings, compost, manure, bio-solids, sawdust and wood ash.[23]
Inorganic amendments include, vermiculite, perlite, pea gravel and coarse
sand.[23]
Here is a video about soil amendments.
SOIL SCIENCE
SOIL AMENDMENTS
Correcting soil texture
Soils that are high in sand are a challenge to grow crops on. Ideally, soils should
have 4-5% organic matter.[14] Soils that are high in sand often do not have
enough organic content in them because large particle size of the sand allows
decomposed organic material to wash through the topsoil.[10] However, the
goal in amending sandy soil is to increase the nutrient availability of the soil
while also increasing the ability of the soil retain water. The only way to do this
is by adding more organic matter to the soil.[10][14][23] When amending soil that
is very sandy, use organic material that is already decomposed like plant
composts, peat, or aged manures.[23] These amendments will increase the soil’s
ability to retain water.
SOIL SCIENCE
SOIL AMENDMENTS
Correcting soil texture
Soils that are high in clay tend to hold water and compact easy. Clay particles
are small and plate-like, which form layers. The physical properties of clay, with
the layers, makes a soil that is difficult for the root of plants to grow through.
Soil with a lot of clay in it will generally have very low porosity, meaning there
are few air pockets for the respiration of plant roots and other living organisms.
Keep in mind that an ideal soil will have 25% soil air and 25% soil water.[8][9]
Since clay soils are dense and have low porosity, choose an amendment like
peat, straw, composted wood chips or composted hardwood bark. These
amendments are fibrous and will allow the soil retain some water while
draining excess water. Don’t add sand to clay soil because it can create a soil
structure similar to concrete.[23]
SOIL SCIENCE
SOIL AMENDMENTS
Organic amendments
Wood products can use the nitrogen in the soil and lead to nitrogen
deficiencies. Microorganisms, like bacteria, use nitrogen to break down the
wood in decomposition, lasting months to years. Throughout the decomposing
process, the nitrogen is released back for the plants to use.[23] In short, wood
products will temporarily rob the soil of nitrogen. Wood products that have a
greater surface area, like sawdust, will have a much more rapid decomposition
and it will be a greater consumer of nitrogen in the soil than solid wood.[23]
To avoid the complication of nitrogen deficiency in using wood products as
amendments, compost them in a pile. Add nitrogen rich materials to the pile
like grass clipping and manure to accelerate the decomposing of the wood.[23]
SOIL SCIENCE
SOIL AMENDMENTS
Organic amendments
Sphagnum peat, also called peat moss, is a soil amendment that raises the
soil’s ability to retain water. It also increases the cation exchange ratio and
increases soil acidity.
Sphagnum peat is harvested from bogs in Canada and the northern United
States. The harvest rate greatly exceeds the rate that the peat bogs can
replenish the eco-system, so it is considered a semi-renewal resource.[23][24]
Biosolids are byproducts of sewage treatment. The benefits of biosolids are
that biosolids are rich in nitrogen, phosphorus, potassium and they add organic
matter.[25] Biosolids are sold cheap or given away for free in some cities.
The main concerns about biosolids are pathogens, salts and heavy metal
content like cadmium and lead.[23][25] There are different classes of biosolids
and only class A is approved for use in production agriculture. It has been
treated to reduce the bacterial content.[23] Use caution when considering
biosolids because of the high salt and heavy metals.
SOIL SCIENCE
SOIL AMENDMENTS
Organic amendments
Manure is harmful to plants when it is not been aged or composted because of
the high amount of ammonia. Aged or composted manure has much less
ammonia.[23][26] Composted manure is high in organic content, but does not
have very much nitrogen, phosphorus or potassium.[26] Manure can contain a
lot of salt.[23] Manure should not be applied to a growing area with a crop that
is going to be harvested in four months or sooner, because of the threat of
pathogens like E. coli.[23] Manure from omnivores, like pigs, should always be
composted to kill parasites that humans can contract.[26]
• Aged manure is manure that has been in a pile for at least six months. It is
not mixed and there can be excess salt that accumulates in the manure pile.
The weed seeds in aged manure are often viable.[23]
• Composted manure is manure that has been through multiple heating
cycles and it has been mixed in between the heating cycles.[23][26] The heat
cycles are generally between 145 to 160 F.[26] Composted manure has very few,
if any, living weed seeds in it.[23][26]
SOIL SCIENCE
SOIL AMENDMENTS
Organic amendments
Plant compost comes from organic material, like grass clipping and leaves, that
is collected by cities. Instead of sending the organic material to the landfills, the
cities compost the material and sell it.[27] Plant compost can be relatively
inexpensive to buy. There are no regulations or standards as to what is
considered compost, so it is a good idea to choose your plant compost
carefully. Composted plant materials are usually very low in salt.[23]
Compost from organic plant material contains a lot of plant nutrients. When
composted, bacteria break down the plant material into material known as
humus. Humus increases the nutrients in the soil while also creating a soil that
has tilth.[14][23][27]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Correcting soil for a Nitrogen deficiency can be done by first giving the plants
fish emulsion. This is done by either spraying a diluted solution of fish emulsion
on the plants themselves,[15] or fish emulsion can by applied to the ground.[28]
Other fertilizers like dried blood, rabbit manure, cottonseed meal, or
commercially prepared organic fertilizers offer quick relief to the nitrogen
deficient plants.
Compost that is enriched with manure, organic material, tea and coffee
grounds, feathers, garden waste, and kitchen waste will make an excellent
long-term source of nitrogen.[14][15]
It is possible to over fertilize with nitrogen. When over fertilized with nitrogen,
plants become weak, because of water-filled tissues and stems. The stem can
easily break during windy conditions. Aphid infestation is another symptom as
aphids like plant that receive excessive nitrogen.[15]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Phosphorus is an nutrient that is captured by cations like calcium and iron.
When captured, phosphorus becomes unavailable to plants to use. For this
reason, growers must add phosphorus fertilizer to compost and the soil that
the plants grow in. the most common phosphorus fertilizer is phosphate rock.
Spread phosphate rock on the soil that the plants grow in and mix some into a
compost pile when it is worked with. Phosphate rock is barely soluble and
requires living organisms to convert the phosphate into a form that plants can
use. Other phosphorous rich material include wood ash, bone meal, citrus
waste, cottonseed meal, manure, fish waste, and dried blood.
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Potassium is an element that serves as a nutrient for plants. It is bound in
compounds that are commonly referred to as potash. These compounds can
come from organic or inorganic sources. Inorganic sources of potash are mined
as salts or from burning material with potassium in it. Organic sources of
potash include wood ash, aged or composted cow manure, cottonseed meal,
aged poultry manure, and plant compost.[15][29]
Some gardeners use wood ash as an effective amendment to maintain a
correct amount of potash in the soil. Potash in the form of wood ash acts as a
liming material and it can be easy to over fertilize the soil with ash. The best
practice, when working with wood ash, is to mix it into a compost pile over
time. It is also a good idea to monitor the pH of the compost pile, as wood ash
increases the pH. Always check the pH of a soil that is going to be fertilized with
wood ash. Some crops, like blueberries, need an acidic soil and wood ash
would change that soil into a more alkaline soil.[29]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Calcium deficiencies in the soil can be treated by adding to the growing area
bone meal, oyster shells, wood ash, and compost. Mixing compost or wood ash
into the soil are some of the fastest ways to add calcium to the soil. Dolomitic
limestone is a compound that can be used to treat a calcium or Magnesium
deficiency, because the compound has both elements.[15] Soil that has a pH
below 6.2 will have a decrease calcium and magnesium availability to the
plant.[17]
Sulfur deficiencies are uncommon, though they can occur in sandy soil. If a soil
test does indicate a sulfur deficiency, it can be easily fixed by adding a diluted
solution called sulfate of potash-magnesia.[15] This solution can be bought at
most gardening shops.
Zinc deficiencies can occur in any soil regardless of pH. The best way to fix a
zinc deficiency is to add biodegraded organic materials like composted manure
or plant compost. Seaweed fertilizers and phosphate rock both contain traces
of zinc and they can be used to increase the amount of zinc in the soil.[15]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Boron deficiencies can be temporarily corrected by giving the plants a solution
containing seaweed fertilizer. A long term solution is to incorporate phosphate
rock, small amounts of sawdust, oak leaves, peat moss, or other acidic organic
material into the compost that will go into growing area in the future.[15]
Manganese deficiencies are different than most nutrient deficiencies because
adding organic matter does very little to help. The most common reason for a
manganese deficiency is that the soil pH is too alkaline, making the nutrient
unavailable for the plants roots to use.[15][17] Nutrient availability to the plant,
for manganese, diminishes as the soil becomes more alkaline and increases
above 6.5 pH.[17]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Iron deficiencies, like manganese, are often due a soil being too alkaline for
proper nutrient availability. Iron availability decreases in a soil pH above 6.2.[17]
Iron needs to be captured in organic material for the plants to use, in a process
called chelation. When the plants absorb the organic material, they absorb the
iron as well. If the soil pH is above 6.2, iron nutrients have difficulty in bonding
to organic material for absorption by plant roots.[15]
To correct an iron deficiency, add chicken manure or other composted manures
to the soil. Adding garden and kitchen wastes, greensand, dried blood, and
seaweed to the compost pile for future use will provide all of the iron that most
plants need.[15]
SOIL SCIENCE
SOIL AMENDMENTS
Correcting specific nutrient deficiencies
Copper deficiencies in plants are often found growing in alkaline soil, over 7.5,
though it can occur in all soil.[15][17] In correcting soil that is copper deficient,
use composted manure that is mixed into the soil. For immediate relief for the
plant, a grower can spray a solution of seaweed onto the shoots of the plant.[15]
Molybdenum deficiencies are common in soil that is acidic. The nutrient
becomes increasing unavailable for the plants to use as the pH deceases from
7.0.[17] Correcting a molybdenum deficiency can be done by increasing the soil
pH through liming, for very acidic soil. If the soil pH has to be low due to crop
needs, consider growing a cover crop like vetch to capture nitrogen that the
plants can use.[15] Cover crops are crops that are grown with the intent of tilling
them under. By tilling the cover crop underground, a grower can put nitrogen
into the ground for a future crop. A molybdenum deficiency looks like a
nitrogen deficiency because molybdenum is used by roots to convert nitrogen
into a form that plants can use. By using a cover crop, a grower can reduce the
amount of molybdenum that their plants will need to use in a growing
season.[14][15] Here is a video that talks about cover-cropping.
SOIL SCIENCE
ASSESSMENT QUESTIONS
 What is soil pedogenesis and what five factors are used in soil
pedogenesis?
 What are soil horizons? Which horizons do most plants use? Which horizon
is clay found in?
 Is there a difference in dark colored soil when compared with light colored
soil?
 What are the soil components? What ratio of soil air and soil water should
there be in uncompacted soil? How much organic matter should be in the
soil? What happens to soil that is compacted?
 What is tilth? What is the relationship between tilth and root
development?
 What is the difference between sand, silt and clay? What is loam?
SOIL SCIENCE
ASSESSMENT QUESTIONS
 What are essential plant nutrients? How many can you name?
 What are macronutrients? How are they different than micronutrients?
 Why is nitrogen important to plants? Why should growers pay close
attention to the nitrogen in the soil that their crops use?
 What is chlorosis?
 What is soil pH? What is considered a neutral pH?
 How do growers raise soil pH? How do they lower soil pH?
 What is the relationship between essential plant nutrients and soil pH?
 What is an ion? What is the difference between an anion and a cation?
 What is a cation exchange capacity (CEC)? How does a grower increase the
CEC of their soil?
SOIL SCIENCE
ASSESSMENT QUESTIONS
 How do growers test their soil for pH, CEC and essential plant nutrients?
 What are biosolids? What class is permitted for agricultural use? Why is it
allowed?
 What is the difference between aged manure and composted manure?
 How is plant compost different than composted manure? Which has more
salt?
 How long should a grower wait for harvesting vegetables that were treated
with composted manure?
 How does a grower increase the soil’s ability to retain water, while draining
excess water?
SOIL SCIENCE
REFERENCES
[25] Arnold,
K., Magai, R., Hoormann, R., & Miles, R. (1996). Safety and Benefits
of Biosolids. Retrieved from University of Missouri Extension:
http://extension.missouri.edu/publications/DisplayPub.aspx?P=WQ427
[18] Bickelhaupt, D. (2013). Soil pH: What it means. Retrieved from ESF:
http://www.esf.edu/pubprog/brochure/soilph/soilph.htm
[14] Bierman, P. M., & Rosen, C. J. (1999). Nutrient cycling and maintaining soil
fertility in fruit and vegetable crop systems. Retrieved from University of
Minnesota Extension:
http://www.extension.umn.edu/distribution/horticulture/M1193.html
[12] Clay soils. (2011). Retrieved from The Royal Horticultural Society:
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=620
[20] Dana, M. N., & Lerner, R. B. (2001). Collecting soil samples for testing.
Retrieved from Purdue University Cooperative Extension Service:
http://www.hort.purdue.edu/ext/HO-71.pdf
SOIL SCIENCE
REFERENCES
[23] Davis,
G. J., & Whiting, D. (2013). Choosing a soil amendment. Retrieved
from Colorado State University Extension:
http://www.ext.colostate.edu/pubs/Garden/07235.html
[3] Decomposers. (2013). Retrieved from Nature Works:
http://www.nhptv.org/natureworks/nwep11b.htm
[28] Earle, C. (2010). Fish emulsion as a fertilizer. Retrieved from
Gardenguides.com: http://www.gardenguides.com/96891-fish-emulsionfertilizer.html
[27] Environmental Benefits. (2013). Retrieved from Environmental Protection
Agency: http://www.epa.gov/compost/benefits.htm
[15] Hildreth, J. (1994). Restoring soil nutrients. Retrieved from Mother Earth
News: http://www.motherearthnews.com/organic-gardening/soilnutrients-zmaz94jjzraw.aspx?PageId=1#ArticleContent
[9] Ideal Soil. (2013). Retrieved from Doctor Dirt :
http://www.doctordirt.org/teachingresources/idealsoil
SOIL SCIENCE
REFERENCES
[5] Jones,
A. J., Sorensen, B., & Dierberge, B. (2010). Soil and water resources.
Retrieved from University of Nebraska:
http://aged.unl.edu/c/document_library/get_file?uuid=dd22a998-89f044f0-abad-caf838c36bb2&groupId=4403449&.pdf
[26] Logan, M. (2010). How to compost manure. Retrieved from
gardenguides.com: http://www.gardenguides.com/78016-compostmanure.html
[16] Mengel, D. B. (1993). Fundamentals of soil cation exchange capacity.
Retrieved from Purdue University Cooperative Exchange Service:
http://www.extension.purdue.edu/extmedia/AY/AY-238.html
[1] Pidwirny, M. (2006). Soil Pedogenesis. In Fundamentals of Physical
Geography (2nd ed.). Retrieved 2013, from PhysicalGeography.net:
http://www.physicalgeography.net/fundamentals/10u.html
[6] Pidwirny, M. (2013). Soil. Retrieved from The Encyclopedia of Earth:
http://www.eoearth.org/view/article/156081/
SOIL SCIENCE
REFERENCES
[24] Priesnitz,
W. (2011). Does peat moss have a place In the ecological garden?
Retrieved from Natural Life Magazine:
http://www.naturallifemagazine.com/0712/asknlpeat.html
[19] Rehm, G. (2009). Soil cation ratios for crop production. Retrieved from
University of Minnesota Extension:
http://www.extension.umn.edu/distribution/cropsystems/DC6437.html
[29] Risse, M., & Harris, G. (1999). Best management practices for wood ash used
as an agricultural soil amendment. Retrieved from Soil Acidity and Liming:
http://hubcap.clemson.edu/~blpprt/bestwoodash.html
[7] Ritter, M. (2009). Soil systems. Retrieved from The Physical Environment:
http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/soil_systems/
soil_development_profiles.html
[21] Self, R. (2013). Soil Testing. Retrieved from Colorado State University
Extension: http://www.ext.colostate.edu/PUBS/crops/00501.html
SOIL SCIENCE
REFERENCES
[22] Self,
R. J. (2013). Soil Test Explanation. Retrieved from Colorado State
University Extension: http://www.ext.colostate.edu/pubs/crops/00502.html
[8] Soil components. (2007). Retrieved from Montana Natural Resources
Conservation Service:
http://www.mt.nrcs.usda.gov/technical/ecs/water/lid/soilcomponents.html
[4] Soil Formation. (2013). Retrieved from The University of Hawai‘i at Manoa:
http://www.ctahr.hawaii.edu/MauiSoil/a_factor_form.aspx
[11] Soil Texture. (2011). Retrieved from Department of Environment and
Primary Industries:
http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_texture
[13] Soil types. (2010). Retrieved from Lawson Fairbanks:
http://www.lawsonfairbank.co.uk/soil-types.asp
[2] Weathering. (2013). Retrieved from National Geographic:
http://education.nationalgeographic.com/education/encyclopedia/weather
ing/?ar_a=1
SOIL SCIENCE
REFERENCES
[17] Whiting,
D., Card, A., Wilson, C., & Reeder, J. (2011). Soil pH. Retrieved from
Colorado State University Extension:
http://www.cmg.colostate.edu/gardennotes/222.html
[10] Whiting, D., Card, A., Wilson, C., Moravec, C., & Reeder, J. (2011). Managing
soil tilth: Texture, structure, and pore space . Retrieved from Colorado State
University Extension: http://www.cmg.colostate.edu/gardennotes/213.html