|
Why Is Organic Matter So Important?
Why are soils which in our father's hands were
productive
now relatively impoverished?
J. L. Hills, C. H. Jones, and C. Cutler, 1908
Organic matter
functions in a number of key roles to promote crop growth. It also is
a critical part of a number of global and regional cycles.
A fertile soil
is the basis for healthy plants, animals, and humans. Soil organic matter
is the very foundation for healthy and productive soils. Understanding
the role of organic matter in maintaining a healthy soil is essential
for developing ecologically sound agricultural practices. It's true
that you can grow plants on soils with little organic matter. In fact,
you don't need any soil at all! [Although gravel or sand hydroponic
systems without soil can grow excellent crops, large-scale systems of
this type are usually neither economically or ecologically sound.] It's
also true that there are other important issues aside from organic matter
when considering the quality of a soil. However, as soil organic matter
decreases, it becomes increasingly difficult to grow plants, because
problems with fertility, water availability, compaction, erosion, parasites,
diseases, and insects become more common. Ever higher levels of inputs
fertilizers, irrigation water, pesticides, and machinery are required
to maintain yields in the face of organic matter depletion. But if attention
is paid to proper organic matter management, the soil can support a
good crop without the need for expensive fixes.
The organic
matter content of agricultural topsoil is usually in the range of 1
to 6 percent. A study of soils in Michigan demonstrated potential crop-yield
increases of about 12 percent for every 1 percent organic matter. In
a Maryland experiment, researchers saw an increase of approximately
80 bushels of corn per acre when organic matter increased from 0.8 to
2 percent.
What Makes Topsoil?
Having a good amount of topsoil is important.
But what gives topsoil its beneficial characteristics? Is
it because it's on TOP? If we bring in a bulldozer and scrape
off one foot of soil, will the exposed subsoil now be topsoil
because it's on the surface? Of course, everyone knows that
there's more to topsoil than its location on the soil surface.
Most of the properties we associate with topsoil good nutrient
supply, tilth, drainage, aeration, water storage, etc. --
are there because topsoil is rich in organic matter and contains
a huge diversity of life.
|
You might wonder
how something that's only a small part of the soil can be so important
for growing healthy and high-yielding crops. The enormous influence
of organic matter on so many of the soil's properties biological, chemical,
and physical makes it of critical importance to healthy soils (figure
4.1). Part of the explanation for this influence is the small particle
size of the well-decomposed portion of organic matter the humus. Its
large surface area-to-volume ratio means that humus is in contact with
a considerable portion of the soil. The intimate contact of humus with
the rest of the soil allows many reactions, such as the release of available
nutrients into the soil water, to occur rapidly. However, the many roles
of living organisms make soil life an essential part of the organic
matter story.
|
Figure
4.1 Adding organic matter results in many changes.
Modified from Oshins, 1999.
|
Plant Nutrition
Plants need 18 chemical elements for their growth carbon (C), hydrogen
(H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur
(S), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), boron
(B), zinc (Zn), molybdenum (Mo), nickel (Ni), copper (Cu), cobalt (Co),
and chlorine (Cl). Plants obtain carbon as carbon dioxide (CO2) and
oxygen partially as oxygen gas (O2) from the air. The remaining essential
elements are obtained mainly from the soil. The availability of these
nutrients is influenced either directly or indirectly by thepresence
of organic matter. The elements needed in large amounts carbon, hydrogen,
oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur
are called macronutrients. The other elements, called micronutrients,
are essential elements needed in small amounts.
Nutrients from decomposing organic matter. Most
of the nutrients in soil organic matter can't be used by plants
as long as they exist as part of large organic molecules. As soil
organisms decompose organic matter, nutrients are converted into
simpler, inorganic, or mineral forms that plants can easily use.
This process, called mineralization, provides much of the nitrogen
that plants need by converting it from organic forms. For example,
proteins are converted to ammonium (NH4+)
and then to nitrate (NO3-). Most plants will
take up the majority of their nitrogen from soils in the form of
nitrate. The mineralization of organic matter is also an important
mechanism for supplying plants with such nutrients as phosphorus
and sulfur, and most of the micronutrients. This release of nutrients
from organic matter by mineralization is part of a larger agricultural
nutrient cycle (see figure 4.2). For a more detailed discussion
of nutrient cycles and how they function in various cropping systems,
see chapter 7.
|
Figure
4.2 The cycle of plant nutrients. |
Addition
of nitrogen. Bacteria living in nodules on legume roots convert
nitrogen from atmospheric gas (N2) to forms that the plant can use directly.
There are a number of free-living bacteria that also fix nitrogen.
Storage of
nutrients on soil organic matter. Decomposing organic matter can
feed plants directly, but it also can indirectly benefit the nutrition
of the plant. A number of essential nutrients occur in soils as positively
charged molecules called cations (pronounced cat-eye-ons). The ability
of organic matter to hold onto cations in a way that keeps them available
to plants is known as cation exchange capacity (CEC). Humus has many
negative charges. Because opposite charges attract, humus is able to
hold onto positively charged nutrients, such as calcium (Ca++), potassium
(K+), and magnesium (Mg++) (see figure 4.3a). This keeps them from leaching
deep into the subsoil when water moves through the topsoil. Nutrients
held in this way can be gradually released into the soil solution and
made available to plants throughout the growing season. However, keep
in mind that not all plant nutrients occur as cations. For example,
the nitrate form of nitrogen is negatively charged (NO3-) and is actually
repelled by the negatively charged CEC. Therefore, nitrate leaches easily
as water moves down through the soil and beyond the root zone.
|
Figure 4.3 Cations held on organic matter and
clay. |
Clay particles also have negative
charges on their surfaces (figure 4.3b), but organic matter may be the
major source of negative charges for coarse and medium textured soils.
Some types of clays, such as those found in the southeastern United
States and in the tropics, tend to have low amounts of negative charge.
When these clays are present, organic matter may be the major source
of negative charges that bind nutrients, even for fine textured (high
clay content) soils.
Protection
of nutrients by chelation. Organic molecules in the soil may also
hold onto and protect certain nutrients. These particles, called "chelates"
(pronounced key-lates) are byproducts of active decomposition of organic
materials and are smaller than those that make up humus. In general,
elements are held more strongly by chelates than by binding of positive
and negative charges. Chelates work well because they bind the nutrient
at more than one location on the organic molecule (figure 4.3c). In
some soils, trace elements, such as iron, zinc, and manganese, would
be converted to unavailable forms if they were not bound by chelates.
It is not uncommon to find low organic matter soils or exposed subsoils
deficient in these micronutrients.
Other ways
of maintaining available nutrients. There is some evidence that
organic matter in the soil can inhibit the conversion of available phosphorus
to forms that are unavailable to plants. One explanation is that organic
matter coats the surfaces of minerals that can bond tightly to phosphorus.
Once these surfaces are covered, available forms of phosphorus are less
likely to react with them. In addition, humic substances may chelate
aluminum and iron, both of which can react with phosphorus in the soil
solution. When they are held as chelates, these metals are unable to
form an insoluble mineral with phosphorus.
Beneficial
Effects of Soil Organisms
Soil organisms are essential for keeping plants well supplied with nutrients
because they break down organic matter. These organisms make nutrients
available by freeing them from organic molecules. Some bacteria fix
nitrogen gas from the atmosphere, making it available to plants. Other
organisms dissolve minerals and make phosphorus more available. If soil
organisms aren't present and active, more fertilizers will be needed
to supply plant nutrients.
A varied community
of organisms is your best protection against major pest outbreaks and
soil fertility problems. A soil rich in organic matter and continually
supplied with different types of fresh residues is home to a much more
diverse group of organisms than soil depleted of organic matter. This
greater diversity of organisms helps insure that fewer potentially harmful
organisms will be able to develop sufficient populations to reduce crops
yields.
Soil Tilth
When soil has a favorable physical condition for growing plants, it
is said to have good tilth. Such a soil is porous and allows water to
enter easily, instead of running off the surface. More water is stored
in the soil for plants to use between rains and less soil erosion occurs.
Good tilth also means that the soil is well aerated. Roots can easily
obtain oxygen and get rid of carbon dioxide. A porous soil does not
restrict root development and exploration. When a soil has poor tilth,
the soil's structure deteriorates and soil aggregates break down, causing
increased compaction and decreased aeration and water storage. A soil
layer can become so compacted that roots can't grow. A soil with excellent
physical properties will have numerous channels and pores of many different
sizes.
Studies on both undisturbed and agricultural soils show
that as organic matter increases, soils tend to be less compact
and have more space for air passage and water storage. Sticky substances
are produced during the decomposition of plant residues. Along with
plant roots and fungal hyphae, they bind mineral particles together
into clumps, or aggregates. In addition, the sticky secretions of
mycorrhizal fungi those that infect roots and help plants get more
water and nutrients are important binding material in soils. The
arrangement and collection of minerals as aggregates and the degree
of soil compaction have huge effects on plant growth (see chapter
6). The development of aggregates is desirable in all types
of soils because it promotes better drainage, aeration, and water
storage. The one exception is for wetland crops, such as rice, when
you want a dense, puddled soil to keep it flooded.
Organic matter,
as residue on the soil surface or as a binding agent for aggregates
near the surface, plays an important role in decreasing soil erosion.
Surface residues intercept raindrops and decrease their potential to
detach soil particles. These surface residues also slow water as it
flows across the field, giving it a better chance to infiltrate into
the soil. Aggregates and large channels greatly enhance the ability
of soil to conduct water from the surface into the subsoil.
Most farmers
can tell that one soil is better than another by looking at them, touching
them, how they work up when tilled, or even by sensing how they feel
when walked on. What they are seeing or sensing is really good tilth.
For an example, see the photo on the back cover of this book. It shows
that soil differences can be created by different management strategies.
Farmers and gardeners would certainly rather grow their crops on the
more porous soil depicted in the photo on the right.
Since erosion
tends to remove the most fertile part of the soil, it can cause a significant
reduction in crop yields. In some soils, the loss of just a few inches
of topsoil may result in a yield reduction of 50 percent. The surface
of some soils low in organic matter may seal over, or crust, as rainfall
breaks down aggregates, and pores near the surface fill with solids.
When this happens, water that can't infiltrate into the soil runs off
the field, carrying valuable topsoil (figure 4.4).
|
Figure 4.4 Changes in soil surface and water-flow
pattern due to soil crusting. |
Large soil pores, or channels,
are very important because of their ability to allow a lot of water
to flow rapidly into the soil. Larger pores are formed a number of ways.
Old root channels may remain open for some time after the root decomposes.
Larger soil organisms, such as insects and earthworms, create channels
as they move through the soil. The mucus that earthworms secrete to
keep their skin from drying out also helps to keep their channels open
for a long time.
Protection
of the Soil Against Rapid Changes in Acidity
Acids and bases are released as minerals dissolve and organisms
go about their normal functions of decomposing organic materials
or fixing nitrogen. Acids or bases are excreted by the roots of
plants, and acids form in the soil from the use of nitrogen fertilizers.
It is best for plants if the soil acidity status, referred to as
pH, does not swing too wildly during the season. The pH scale is
a way of expressing the amount of free hydrogen (H+)
in the soil water. More acidic conditions, with greater amounts
of hydrogen, are indicated by lower numbers. A soil at pH 4 is very
acid. Its solution is 10 times more acid than a soil at pH 5. A
soil at pH 7 is neutral there is just as much base in the water
as there is acid. Most crops do best when the soil is slightly acid
and the pH is around 6 to 7. Essential nutrients are more available
to plants in this pH range than when soils are either more acidic
or more basic. Soil organic matter is able to slow down, or buffer,
changes in pH by taking free hydrogen out of solution as acids are
produced or by giving off hydrogen as bases are produced. (For discussion
about management of acidic soils, see chapter
18.)
Stimulation
of Root Development
Microorganisms in soils produce numerous substances that stimulate plant
growth. Humus itself has a directly beneficial effect on plants (figure
4.5). Although the reasons for this stimulation are not yet understood,
certain types of humus cause roots to grow longer and have more branches,
resulting in larger and healthier plants. In addition, soil microorganisms
produce a variety of root-stimulating substances that behave as plant
hormones.
|
Figure 4.5 Corn grown in nutrient solution
with (right) and without (left) humic acids. In this experiment
by Rich Bartlett and Yong Lee, adding humic acids to a nutrient
solution increased the growth of tomatoes and corn and increased
the number and branching of roots. Photo by R. Bartlett. |
Darkening
of the Soil
Organic matter tends to darken soils. You can easily see this in coarse-textured
sandy soils containing light-colored minerals. Under well-drained conditions,
a darker soil surface allows a soil to warm up a little faster in the
spring. This provides a slight advantage for seed germination and the
early stages of seedling development, which is often beneficial in cold
regions.
Color and Organic Matter
In Illinois, a hand-held chart has been developed
to allow people to estimate percent soil organic matter. Their
darkest soils almost black indicates from 3.5 to 7 percent
organic matter. A dark brown soil indicates 2 to 3 percent
and a yellowish brown soil indicates 1.5 to 2.5 percent organic
matter. (Color may not be as clearly related to organic matter
in all regions, because the differences in the amount of clay
and the types of minerals present also influence soil color.)
|
Protection Against Harmful
Chemicals
Some naturally occurring chemicals in soils can harm plants. For example,
aluminum is an important part of many soil minerals and, as such, poses
no threat to plants. As soils become more acidic, especially at pH levels
below 5.5, aluminum becomes soluble. Some soluble forms of aluminum,
if present in the soil solution, are toxic to plant roots. However,
in the presence of significant quantities of soil organic matter, the
aluminum is bound tightly and will not do as much damage. Organic matter
is the single most important soil property that reduces pesticide leaching.
It holds tightly onto a number of pesticides. This prevents or reduces
leaching of these chemicals into groundwater and allows time for detoxification
by microbes. Microorganisms can change the chemical structure of some
pesticides, industrial oils, many petroleum products (gas and oils),
and other potentially toxic chemicals, rendering them harmless.
ORGANIC MATTER AND NATURAL CYCLES
The Carbon
Cycle
Soil organic matter plays a significant part in a number of global cycles.
People have become more interested in the carbon cycle because the buildup
of carbon dioxide (CO2) in the atmosphere is thought to cause global
warming. Carbon dioxide is also released to the atmosphere
when fuels, such as gas, oil, and wood, are burned. A simple version
of the natural carbon cycle, showing the role of soil organic matter,
is given in figure 4.6. Carbon dioxide is removed from the
atmosphere by plants and used to make all the organic molecules necessary
for life.
|
Figure 4.6 The role of soil organic matter in
the carbon cycle. Losses of carbon from the field are indicated
by the dark border around the words describing the process.
|
Sunlight provides
plants with the energy they need to carry out this process. Plants,
as well as the animals feeding on plants, release carbon dioxide
back into the atmosphere as they use organic molecules for energy.
The largest
amount of carbon present on the land is not in the living plants, but
in soil organic matter. That is rarely mentioned in discussions of the
carbon cycle. More carbon is stored in soils than in all plants, all
animals and the atmosphere. Soil organic matter contains an estimated
four times as much carbon as living plants. As soil organic matter is
depleted, it becomes a source of carbon dioxide for the atmosphere.
When forests are cleared and burned, a large amount of carbon dioxide
is released to the atmosphere. There is a potentially larger release
of carbon dioxide following conversion of forests to agricultural
practices that rapidly deplete the soil of its organic matter. There
is as much carbon in 6 inches of soil with 1 percent organic matter
as there is in the atmosphere above a field. If organic matter decreases
from 3 percent to 2 percent, the amount of carbon dioxide in the atmosphere
could double. (Of course, wind and diffusion move the carbon dioxide
to other parts of the globe.)
The Nitrogen
Cycle
Another important global cycle in which organic matter plays a major
role is the nitrogen cycle. This cycle is of direct importance in agriculture,
because available nitrogen for plants is commonly deficient in soils.
Figure 4.7 shows the nitrogen cycle and how soil organic matter enters
into the cycle. Some bacteria living in soils are able to "fix"
nitrogen, converting nitrogen gas to forms that other organisms, including
crop plants, can use. Inorganic forms of nitrogen, like ammonium and
nitrate, exist in the atmosphere naturally, although air pollution causes
higher amounts than normal. Rainfall and snow deposit inorganic nitrogen
forms on the soil. Inorganic nitrogen also may be added in the form
of commercial nitrogen fertilizers. These fertilizers are derived from
nitrogen gas in the atmosphere through an industrial fixation process.
Bacteria and fungi convert the organic forms of nitrogen into ammonium
and different bacteria convert ammonium into nitrate. Both nitrate and
ammonium can be used by plants.
|
Figure 4.7 The role of soil organic matter in
the nitrogen cycle. Losses of nitrogen from the field are indicated
by the dark border around the words describing the process.
|
Nitrogen can be lost from a soil in a number of ways. When crops are
removed from fields, nitrogen and other nutrients also are removed.
The nitrate (NO3-) form of nitrogen leaches
readily from soils and may end up in groundwater at higher concentrations
than may be safe for drinking. Organic forms of nitrate as well
as nitrate and ammonium (NH4+) may be lost
by runoff water and erosion. Once freed from soil organic matter,
nitrogen may be converted to forms that end up back in the atmosphere.
Bacteria convert nitrate to nitrogen (N2) and nitrous
oxide (N2O) gases in a process called denitrification,
which occurs in saturated soils. Nitrous oxide (it's called a "greenhouse
gas") contributes to global warming. In addition, when it reaches
the upper atmosphere, it helps to decrease the ozone levels that
protect the earth's surface from the harmful effects of ultraviolet
(UV) radiation. So if you needed another reason not to apply excessive
rates of fertilizers or manures in addition to the economic costs
and the pollution of ground and surface waters the possible formation
of nitrous oxide should make you cautious.
The Water
Cycle
Organic matter plays an important part in the local, regional, and global
water, or hydrologic, cycle due to its role in promoting water infiltration
into soils and storage within the soil. Water evaporates from the soil
surface and from living plant leaves as well as from the ocean and lakes.
Water then returns to the earth, usually far from where it evaporated,
as rain and snow. Soils high in organic matter, with excellent tilth,
enhance the rapid infiltration of rainwater into the soil. This water
may be available for plants to use or it may percolate deep into the
subsoil and help to recharge the groundwater supply. Since groundwater
is commonly used as a drinking water source for homes and for irrigation,
recharging groundwater is important. When the soil's organic matter
level is depleted, it is less able to accept water, and high levels
of runoff and erosion result. This means less water for plants and decreased
groundwater recharge.
Sources
Allison, F.E. 1973. Soil Organic Matter and its Role in Crop
Production. Scientific Publishing Co. Amsterdam, The Netherlands.
Brady, N.C., and R.R. Weil. 1999. The Nature and
Properties of Soils. 12th ed. Macmillan Publishing Co. New York,
NY.
Follett, R.F., J.W.B. Stewart, and C.V. Cole (eds).
1987. Soil Fertility and Organic Matter as Critical Components
of Production Systems. Special Publication No.19. Soil Science
Society of America. Madison, WI.
Lucas, R.E., J.B. Holtman, and J.L. Connor. 1977.
Soil carbon dynamics and cropping practices. pp. 333451. In
Agriculture and Energy (W. Lockeretz, ed.). Academic Press.
New York, NY. See this source for the Michigan study on the relationship
between soil organic matter levels and crop-yield potential.
Oshins, C. 1999. An Introduction to Soil Health.
A slide set available at the Northeast Region SARE website: www.uvm.edu/~nesare/slide.html.
Powers, R.F., and K. Van Cleve. 1991. Long-term ecological
research in temperate and boreal forest ecosystems. Agronomy
Journal 83:1124. This reference compares the relative
amounts of carbon in soils with that in plants.
Stevenson, F.J. 1986. Cycles of Soil: Carbon,
Nitrogen, Phosphorus, Sulfur, Micronutrients. John Wiley
& Sons. New York, NY. This reference compares the amount of
carbon in soils with that in plants.
Strickling, E. 1975. Crop sequences and tillage in
efficient crop production. Abstracts of the 1975 Northeast Branch
American Society Agronomy Meetings. pp. 2029. See this
source for the Maryland experiment relating soil organic matter
to corn yield.
Tate, R.L., III. 1987. Soil Organic Matter: Biological
and Ecological Effects. John Wiley & Sons. New York, NY.
Top
|