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Amount of Organic Matter In Soils
The depletion of the soil humus supply is apt
to be a
fundamental cause of lowered crop yields.
J.H. Hills, C.H. Jones, and C. Cutler, 1908
The amount of organic matter in any particular soil
is a result of a wide variety of environmental, soil, and agronomic
influences. Some of these, such as climate and soil texture, are
naturally occurring. Human activity also influences soil organic
matter levels. Tillage, crop rotation, and manuring practices all
have profound effects on the amount of soil organic matter. Pioneering
work on the effect of natural influences on soil organic matter
levels was carried out in the U.S. more than 50 years ago by Hans
Jenny.
The amount of organic matter in soil is a result of
all the additions and losses of organic matter that have occurred
over the years (figure 5.1). In this chapter, we will look at why
different soils have different organic matter levels. Anything that
adds large amounts of organic residues to a soil may increase organic
matter. On the other hand, anything that causes soil organic matter
to decompose more rapidly or be lost through erosion may deplete
organic matter.
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Figure 5.1 Additions and losses
of organic matter from soils. |
If additions are greater than losses, organic matter increases.
When additions are less than losses, there is a depletion of soil
organic matter. When the system is in balance, and additions equal
losses, the quantity of soil organic matter doesn't change over
the years.
NATURAL FACTORS
Temperature
In the United States, it is easy to see how temperature affects
soil organic matter levels. Traveling from north to south, average
hotter temperatures lead to less soil organic matter. As the climate
gets warmer, two things tend to happen (as long as rainfall is sufficient):
more vegetation is produced because the growing season is longer,
and the rate of decomposition of organic materials in soils also
increases, because soil organisms work more efficiently in warm
weather and for longer periods of the year. This increasing decomposition
with warmer temperatures becomes the dominant influence determining
soil organic matter levels.
Rainfall
Soils in arid climates usually have low amounts of organic matter.
In a very dry climate, such as a desert, there is little growth
of vegetation. Decomposition may be very low when the soil is dry
and microorganisms cannot function well. When it finally rains,
a very rapid burst of decomposition of soil organic matter occurs.
Soil organic matter levels generally increase as average annual
precipitation increases. With more rainfall, more water is available
to plants and more plant growth results. As rainfall increases,
more residues return to the soil from grasses or trees. At the same
time, soils in high rainfall areas may have less soil organic matter
decomposition than well-aerated soils decomposition is slowed by
restricted aeration.
Soil Texture
Fine textured soils, containing high percentages of clay, tend to
have naturally higher amounts of soil organic matter than coarse
textured sands or sandy loams. The organic matter content of sands
may be less than 1 percent; loams may have 2 to 3 percent; and clays
from 4 to more than 5 percent. The strong bonds that develop between
clay and organic matter seem to protect organic molecules from attack
and decomposition by microorganisms. In addition, fine textured
soils tend to have smaller pores and have less oxygen than coarser
soils. This also causes reduced decomposition of organic matter.
The lower rate of decomposition in soils with high clay contents
is probably the main reason that their organic matter levels are
higher than in sands and loams.
Soil Drainage and Position in the Topography
Some soils have a compact subsoil layer that doesn't allow water
to drain well. Decomposition of organic matter occurs more slowly
in poorly aerated soils, when oxygen is limited or absent, than
in well-aerated soils. For this reason, organic matter accumulates
in wet soil environments. In a totally flooded soil, one of the
major structural parts of plants, lignin, doesn't decompose at all.
The ultimate consequence of extremely wet or swampy conditions is
the development of organic (peat or muck) soils, with organic matter
contents of over 20 percent. If organic soils are artificially drained
for agricultural or other uses, the soil organic matter will decompose
very rapidly. When this happens, the elevation of the soil surface
actually decreases. Some homeowners in Florida were fortunate to
sink corner posts below the organic level. Originally level with
the ground, those homes now perch on posts atop a soil surface that
has decreased so dramatically the owners park under their homes.
Soils in depressions at the bottom of hills are often
wet because they receive runoff, sediments (including organic matter),
and seepage from up slope. Organic matter is not decomposed as rapidly
in these landscape positions as in drier soils farther up slope.
However, soils on a steep slope will tend to have low amounts of
organic matter because the topsoil is continually eroded.
Type of Vegetation
The type of plants that grow on a soil over the years affects the
soil organic matter level. The most dramatic differences are evident
when soils developed under grassland are compared with those developed
under forests. On natural grasslands, organic matter tends to accumulate
in high amounts and to be well distributed within the soil. This
is probably a result of the deep and extensive root systems of native
grasses. Their roots have high "turnover" rates, for root
death and decomposition constantly occurs as new roots are formed.
The high levels of organic matter in soils that were once in grassland
explains why these are some of the most productive soils in the
world. By contrast, in forests, litter accumulates on top of the
soil, and surface organic layers commonly contain over 50 percent
organic matter. However, subsurface mineral layers in forest soils
typically contain from less than 1 to about 2 percent organic matter.
Acidic Soil Conditions
In general, soil organic matter decomposition is slower under acidic
soil conditions than at more neutral pH. In addition, acidic conditions,
by inhibiting earthworm activity, encourage organic matter to accumulate
at the soil surface, rather than distributed throughout the soil
layers.
HUMAN INFLUENCES
Soil erosion removes topsoil rich in organic matter
so that, eventually, only subsoils remain. Crop production obviously
suffers when part or all of the most fertile layer of the soil is
removed. Erosion is a natural process and occurs on almost all soils.
Some soils naturally erode more easily than others and the problem
is also greater in some regions than others. However, agricultural
practices accelerate erosion. Nationwide, soil erosion causes huge
economic losses. It is estimated that erosion in the United States
is responsible for annual losses of $500 million in available nutrients
and $18 billion in total soil nutrients.
Unless erosion is very severe, a farmer may not even
realize that a problem exists, but that doesn't mean that crop yields
are unaffected. In fact, yields may decrease by 5 to 10 percent
when only moderate erosion occurs. Yields may suffer a decrease
of 10 to 20 percent or more with severe erosion. The results of
a study of three midwestern soils, shown in table 5.1, indicate
that erosion greatly influences both organic matter levels and water-holding
ability. Greater amounts of erosion decreased the organic matter
contents of these loamy and clayey soils. In addition, eroded soils
stored less available water than soils experiencing little erosion.
Organic matter also is lost from soils when organisms
decompose more organic materials during the year than are added.
This occurs as a result of practices such as intensive tillage and
growing crops that produce low amounts of residues (see below).
Tillage Practices
Tillage practices influence both the amount of topsoil erosion and
the rate of decomposition of soil organic matter. Conventional plowing
and disking of a soil to prepare a smooth seedbed breaks down natural
soil aggregates and destroys large, water-conducting channels. The
soil is left in a physical condition that allows both wind and water
erosion.
The more a
soil is disturbed by tillage practices, the greater the potential
breakdown of organic matter by soil organisms. During the early years
of agriculture in the United States, when colonists cleared the forests
and planted crops in the East and farmers later moved to the Midwest
to plow the grasslands, soil organic matter decreased rapidly. In
fact, the soils were literally mined of a valuable resource organic
matter. In the Northeast and Southeast, it was quickly recognized
that fertilizers and soil amendments were needed to maintain soil
productivity. In the Midwest, the deep, rich soils of the tall-grass
prairies were able to maintain their productivity for a long time
despite accelerated soil organic matter loss and significant amounts
of erosion. The reason for this was their unusually high original
levels of soil organic matter.
Rapid soil
organic matter decomposition by soil organisms usually occurs when
a soil is worked with a moldboard plow. Incorporating residues, breaking
aggregates open, and fluffing up the soil allows microorganisms to
work more rapidly. It's something like opening up the air intake on
a wood stove, which lets in more oxygen and causes the fire to burn
hotter. In Vermont, we found a 20-percent decrease in organic matter
after five years of growing corn on a clay soil that had previously
been in sod for a long time. In the Midwest, 40 years of cultivation
caused a 50-percent decline in soil organic matter. Rapid loss of
soil organic matter occurs in the early years, because of the high
initial amount of active ("dead") organic matter available
to micro-organisms. After much of the active portion is lost, the
rate of organic matter loss slows considerably.
With the current
interest in reduced (conservation) tillage, growing row crops in the
future may not have such a detrimental effect on soil organic matter.
Conservation tillage practices leave more residues on the surface
and cause less soil disturbance than conventional moldboard plow and
disk tillage. In fact, soil organic matter levels usually increase
when no-till planters place seeds in a narrow band of disturbed soil,
while leaving the soil between planting rows undisturbed. The rate
of decomposition of soil organic matter is lower because the soil
is not drastically disturbed by plowing and disking. Residues accumulate
on the surface because the soil is not inverted by plowing. Earthworm
populations increase, taking some of the organic matter deeper into
the soil and creating channels that help water infiltrate into the
soil. Decreased erosion also results from using conservation tillage
practices.
Crop Rotations
and Cover Crops
At different stages in a rotation, different things may be happening.
Soil organic matter may decrease, then increase, then decrease, and
so forth. While annual row crops under conventional moldboard plow
cultivation usually result in decreased soil organic matter, perennial
legumes, grasses, or legume-grass forage crops tend to increase soil
organic matter. The turnover of the roots of these hay and pasture
crops, plus the lack of soil disturbance, allow organic matter to
accumulate in the soil. This effect is seen in the comparison of organic
matter increases when growing alfalfa compared to corn silage (figure
5.2) In addition, different types of crops result in different quantities
of residues returned to the soil. When corn grain is harvested, more
residues are left in the field than after soybeans, wheat, potatoes,
or lettuce harvests. Harvesting the same crop in different ways leaves
different amounts of residues. When corn grain is harvested, more
residues remain in the field than when the entire plant is harvested
for silage (figure 5.3).
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Figure 5.2 Organic carbon changes when growing
corn silage or alfalfa. Redrawn from Angers, 1992. |
Soil erosion is greatly
reduced and topsoil rich in organic matter is conserved when rotation
crops, such as grass or legume hay, are grown year-round. The extensive
root systems of sod crops account for much of the reduction in erosion.
Having sod crops as part of a rotation reduces loss of topsoil, decreases
decomposition of residues, and builds up organic matter by the extensive
residue addition of plant roots.
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Figure 5.3a Soil surface after harvest of corn
silage. Photos by Win Way. |
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Figure 5.3b Soil surface after harvest of corn
grain. Photos by Win Way. |
Use of
Organic Amendments
An old practice that helps maintain or increase soil organic matter
is to apply manures or other organic residues generated off the field.
A study in Vermont during the 1960s and 1970s found that between 20
and 30 tons (wet weight, including straw or sawdust bedding) of dairy
manure per acre were needed to maintain soil organic matter levels
when silage corn was grown each year. This is equivalent to 1 to 1 ½
times the amount produced by a large Holstein cow over the whole
year. Different manures can have very different effects on soil organic
matter and nutrient availability. They differ in their initial composition
and also are affected by how they are stored and handled in the field.
ORGANIC
MATTER DISTRIBUTION IN SOIL
In general,
more organic matter is present near the surface than deeper in the
soil (see figure 5.4). This is one of the main reasons that topsoils
are so productive, compared with subsoils exposed by erosion or mechanical
removal of surface soil layers. Much of the plant residues that eventually
become part of the soil organic matter are from the above-ground portion
of plants. When the plant dies or sheds leaves or branches, it deposits
residues on the surface. Although earthworms and insects help incorporate
the residues on the surface deeper into the soil and the roots of
some plants penetrate deeply, the highest concentrations still remain
within 1 foot of the surface.
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Figure 5.4 Examples of soil organic matter content
with depth. Modified from Brady and Weil, 1999. |
Litter layers that commonly
develop on the surface of forest soils may have very high organic
matter contents (figure 5.4a). Plowing forest soils after removal
of the trees incorporates the litter layers into the mineral soil.
The incorporated litter decomposes rapidly, and an agricultural soil
derived from a light, sandy texture forest soil in the north or a
silt loam in the southeast coastal plain would likely have a distribution
of organic matter similar to that indicated in figure 5.4b. Soils
of the tall-grass prairies have high levels of organic matter deep
into the soil profile (see figure 5.4c). After cultivation of these
soils for 50 years, far less organic matter exists (figure 5.4d).
ACTIVE
ORGANIC MATTER
The discussion
for almost all of this chapter has been about amounts of total organic
matter in soils. However, we should constantly keep in mind that we
are interested in each of the different types of organic matter in
soils the living, the dead (active), and the very dead (humus). We
don't just want a lot of humus in soil, we also want a lot of active
organic matter to provide nutrients and aggregating glues when it
is decomposed. We want the active organic matter because it supplies
food to keep a di verse population of organisms present. As mentioned
earlier, when forest or prairie soils were first cultivated, there
was a drastic decrease in the organic matter content. Almost all of
the decline was due to a loss of the active ("dead") part
of the organic matter. It is the active fraction that increases relatively
quickly when practices, such as reduced tillage, rotations, cover
crops, and manures, are used to increase soil organic matter.
LIVING
ORGANIC MATTER
In chapter 3, we talked about
the various types of organisms that live in soils. The weight of
fungi present in forest soils is much greater than the weight of
bacteria. In grasslands, however, there are about equal weights
of both. In agricultural soils that are routinely tilled, the weight
of fungi is less than the weight of bacteria. As soils become more
compact, larger pores are eliminated first. These are the pores
in which soil animals, such as earthworms and beetles, live and
function, so the number of such organisms in compacted soils decreases.
Different
total amounts (weights) of living organisms exist in various cropping
systems. In general, high populations of diverse and active soil organisms
are found in systems with more complex rotations that regularly leave
high amounts of crop residues and when other organic materials are
added to the soil. Organic materials may include crop residues, cover
crops, animal manures, and composts. Leaves and grass clippings may
be an important source of organic residues for gardeners. When crops
are rotated regularly, fewer parasite, disease, weed, and insect problems
occur than when the same crop is grown year after year.
On the other
hand, frequent cultivation reduces the number of many soil organisms
as their food supplies are depleted by decomposition of organic matter.
Compaction from heavy equipment causes harmful biological effects
in soils. It decreases the number of medium to large pores, which
reduces the volume of soil available for air, water, and populations
of organisms such as mites and springtails that need the large spaces
in which to live.
Sources
Angers, D.A. 1992. Changes in soil aggregation and organic carbon
under corn and alfalfa. Soil Science Society of America Journal
56: 12441249.
Brady, N.C., and R.R. Weil. 1999. The Nature and
Properties of Soils. 12th ed. Macmillan Publishing Co. New York,
NY.
Carter, V.G., and T. Dale. 1974. Topsoil and Civilization.
University of Oklahoma Press. Norman, OK.
Hass, H.J., G.E.A. Evans, and E.F. Miles. 1957. Nitrogen
and Carbon Changes in Great Plains Soils as Influenced by Cropping
and Soil Treatments. U.S. Department of Agriculture Technical
Bulletin 1164. U.S. Government Printing Office. Washington, D.C.
This is a reference for the large decrease in organic matter content
of Midwest soils.
Jenny, H. 1980. The Soil Resource. Springer-Verlag.
New York, NY.
Jenny, H. 1941. Factors of Soil Formation.
McGraw-Hill. New York, NY. Jenny's early work on the natural factors
influencing soil organic matter levels.
Magdoff, F.R., and J.F. Amadon. 1980. Yield trends
and soil chemical changes resulting from N and manure application
to continuous corn. Agronomy Journal 72:161164. See
this reference for further information on the studies in Vermont
cited in this chapter.
National Research Council. 1989. Alternative Agriculture.
National Academy Press. Washington, D.C.
Schertz, D.L., W.C. Moldenhauer, D.F. Franzmeier,
and H.R. Sinclair, Jr. 1985. Field evaluation of the effect of soil
erosion on crop productivity. pp. 917. In Erosion and Soil
Productivity. Proceedings of the national symposium on erosion
and soil productivity. Dec. 1011, 1984. New Orleans, LA. American
Society of Agricultural Engineers Publication 8-85. St. Joseph,
MI.
Tate, R.L., III. 1987. Soil Organic Matter: Biological
and Ecological Effects. John Wiley & Sons. New York, NY.
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