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
occurred over the years (figure 3.1). In this chapter, we will
is the result of a wide variety of environmental, soil, and
look at why different soils have different organic matter
agronomic influences. Some of these, such as climate
levels. While we will be looking mainly at the total amount
and soil texture, are naturally occurring. Agricultural
of organic matter, keep in mind that all three “types” of
practices also influence soil organic matter levels.
organic matter—the living, dead, and very dead—serve crit-
Tillage, crop rotation, and manuring practices all can
ical roles and the amount of each of these may be affected
have profound effects on the amount of soil organic
differently by natural factors and agricultural practices.
matter. Hans Jenny carried out pioneering work on the
Anything that adds large amounts of organic residues
effect of natural influences on soil organic matter levels
to a soil may increase organic matter. On the other hand,
in the U.S. more than sixty years ago.
anything that causes soil organic matter to decompose
The amount of organic matter in a soil is the result of
more rapidly or be lost through erosion may deplete
all the additions and losses of organic matter that have
organic matter.
losses
additions
soil organic
crop residues
CO (respiration
2
of soil organisms)
manures
matter
erosion
composts
Figure 3.1. Additions and losses of organic matter from soils.
Photo by Jerry DeWitt
23
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
organic matter. As the climate gets warmer, two things
tend to happen (as long as rainfall is sufficient): More
storAGe of orGAnIc MAtter In soIL
vegetation is produced because the growing season is
organic matter is protected in soils by:
• Formation of strong chemical organic matter—clay
longer, and the rate of decomposition of organic materi-
(and fine silt) bonds
als in soils increases because soil organisms work more
• Being inside small aggregates (physical y protected)
rapidly and are active for longer periods of the year at
• Conversion into stable substances such as humic ma-
higher temperatures. Faster decomposition with warmer
terials that are resistant to biological decomposition
temperatures becomes the dominant influence deter-
• Restricted drainage, sometimes related to texture,
mining soil organic matter levels.
that reduces the activity of the organisms that need
oxygen to function
Rainfal
• Char produced by incomplete burning
Soils in arid climates usually have low amounts of
Large aggregates are made up of many smal er ones
organic matter. In a very dry climate, such as a desert,
that are held together by sticky substances and fungal
there is little growth of vegetation. Decomposition is
hyphae. Organic matter in large aggregates—but
also low because of low amounts of organic inputs and
outside of the small aggregates that make up the
low microrganism activity when the soil is dry. When it
larger ones—and freely occurring particulate organic
finally rains, a very rapid burst of decomposition of soil
matter (the “dead”) are available for soil organisms to
organic matter occurs. Soil organic matter levels gener-
use. However, poor aeration resulting from restricted
ally increase as average annual precipitation increases.
drainage because of a dense subsurface layer, compac-
With more rainfall, more water is available to plants,
tion, or being in the bottom of a slope may cause a
and more plant growth results. As rainfall increases,
low rate of use of the organic matter. So the organic
matter needs to be in a favorable chemical form
more residues return to the soil from grasses or trees. At
and physical location for organisms to use it; plus,
the same time, soils in high rainfall areas may have less
the environmental conditions in the soil—adequate
organic matter decomposition than well-aerated soils—
moisture and aeration—need to be sufficient for most
decomposition is slowed by restricted aeration.
soil organisms to use the residues and thrive.
Soil Texture
Fine-textured soils, containing high percentages of clay
If additions are greater than losses, organic matter
and silt, tend to have naturally higher amounts of soil
increases. When additions are less than losses, there is
organic matter than coarse-textured sands or sandy
a depletion of soil organic matter. When the system is in
loams. The organic matter content of sands may be less
balance and additions equal losses, the quantity of soil
than 1%; loams may have 2% to 3%, and clays from 4% to
organic matter doesn’t change over the years.
more than 5%. The strong chemical bonds that develop
between organic matter and clay and fine silt protect
NATURAl FAcTORS
organic molecules from attack and decomposition by
Temperature
microorganisms and their enzymes. Also, clay and fine silt
In the United States, it is easy to see how temperature
combine with organic matter to form very small aggre-
affects soil organic matter levels. Traveling from north
gates that in turn protect the organic matter inside from
to south, higher average temperatures lead to less soil
organisms and their enzymes. In addition, fine-textured
24
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
soils tend to have smaller pores and less oxygen than
coarser soils. This also limits decomposition rates, one
of the reasons that organic matter levels in fine-textured
soils are higher than in sands and loams.
Soil Drainage and Position in the Landscape
Decomposition of organic matter occurs more slowly
in poorly aerated soils. In addition, some major plant
compounds such as lignin will not decompose at all in
anaerobic environments. For this reason, organic matter
tends to accumulate in wet soil environments. When
conditions are extremely wet or swampy for a very long
Figure 3.2. Root systems of annual wheat (at left in each panel) and
wheatgrass, a perennial, at four times of the year. Approximately
period of time, organic (peat or muck) soils, with organic
25% to 40% of the wheatgrass root system dies back each year, adding
considerable amounts of organic matter, and then grows back again.
matter contents of over 20%, develop. When these soils
Compared to annual wheat, it has a longer growing season and has much
are artificially drained for agricultural or other uses,
more growth both above ground and below ground. Wheatgrass was 12
and 21 months old when the first and last photos were taken. Photo by
the soil organic matter will decompose rapidly. When
the Land Institute.
this happens, the elevation of the soil surface actually
decreases. Homeowners on organic soils in Florida
normally sink the corner posts of their houses below the
rates, for root death and decomposition constantly occur
organic level to provide stability. Originally level with
as new roots are formed. Dry natural grasslands also
the ground, some of those homes now perch on posts
frequently experience slow-burning fires from lightning
atop a soil surface that has decreased so dramatically
strikes, which contribute biochar that is very resistant
that the owners can park their cars under their homes.
to degradation. The high levels of organic matter in soils
Soils in depressions at the bottom of hills receive
that were once in grassland partly explain why these
runoff, sediments (including organic matter), and seep-
are now some of the most productive agricultural soils
age from upslope and tend to accumulate more organic
in the world. By contrast, in forests, litter accumulates
matter than drier soils farther upslope. In contrast, soils
on top of the soil, and surface organic layers commonly
on a steep slope or knoll will tend to have low amounts of
contain over 50% organic matter. However, subsurface
organic matter because the topsoil is continually eroded.
mineral layers in forest soils typically contain less than
2% organic matter.
Type of Vegetation
The type of plants that grow on the soil as it forms
Acidic Soil Conditions
can be an important source of natural variation in soil
In general, soil organic matter decomposition is
organic matter levels. Soils that form under grassland
slower under acidic soil conditions than at a more
vegetation generally contain more organic matter and
neutral pH. In addition, acidic conditions, by inhibit-
a deeper distribution of organic matter than soils that
ing earthworm activity, encourage organic matter to
form under forest vegetation. This is probably a result
accumulate at the soil surface, rather than distributing
of the deep and extensive root systems of grassland
throughout the soil layers.
species (figure 3.2). Their roots have high “turnover”
25
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
root vs. AboveGround resIdue contrIbutIon to soIL orGAnIc MAtter
Roots, already being well distributed and in intimate contact with the soil, tend to contribute a higher percentage of their weight to the more persistent organic matter (“dead” and “very dead”) than above-ground residues. In addition, compared to aboveground plant parts, many crop roots have higher amounts of materials such as lignin that decompose relatively slowly.
One experiment with oats found that only one-third of the surface residue remained after one year, while 42% of the root organic matter remained in the soil and was the main contributor to particulate organic matter. In another experiment, five months after spring incorporation of hairy vetch, 13% of the aboveground carbon remained in the soil, while close to 50% of the root-derived carbon was still present. Both experiments found that the root residue contributed much more to particulate organic matter (active, or “dead”) than did aboveground residue.
HUMAN INFlUENcES
year than are added. This occurs as a result of practices
Loss of topsoil that is rich in organic matter by erosion
that accelerate decomposition, such as intensive tillage
has dramatically reduced the total amount of organic
and crop production systems that return low amounts
matter stored in many soils after they were developed for
of residues. Much of the rapid loss of organic matter
agriculture. Crop production obviously suffers when part
following the conversion of grasslands to agriculture has
of the most fertile layer of the soil is removed. Erosion
been attributed to large reductions in residue inputs,
is a natural process and occurs on almost all soils. Some
accelerated mineralization of organic matter because of
soils naturally erode more easily than others, and the
plowing, and erosion.
problem is greater in some regions than others. However,
agricultural practices accelerate erosion. It is estimated
Tillage Practices
that erosion in the United States is responsible for annual
Tillage practices influence both the amount of topsoil
losses of about a billion dollars in available nutrients and
erosion and the rate of decomposition of organic matter.
many times more in total soil nutrients.
Conventional plowing and disking of a soil to prepare a
Unless erosion is severe, a farmer may not even
smooth seedbed break down natural soil aggregates and
realize a problem exists. But that doesn’t mean that crop
yields are unaffected. In fact, yields may decrease by
Table 3.1
Effects of Erosion on Soil Organic Matter and Water
5% to 10% when only moderate erosion occurs. Yields
Organic
Available Water
may suffer a decrease of 10–20% or more with severe
Soil
Erosion
Matter (%)
Capacity (%)
erosion. The results of a study of three midwestern soils
slight
3.03
12.9
(referred to as Corwin, Miami, and Morley), shown in
Corwin
moderate
2.51
9.8
severe
1.86
6.6
table 3.1, indicate that erosion greatly influences both
slight
1.89
16.6
organic matter levels and water-holding ability. Greater
Miami
moderate
1.64
11.5
amounts of erosion decreased the organic matter content
severe
1.51
4.8
of these loamy and clayey soils. In addition, eroded soils
slight
1.91
7.4
stored less available water than minimally eroded soils.
Morley
moderate
1.76
6.2
Organic matter also is lost from soils when organ-
severe
1.60
3.6
isms decompose more organic materials during the
Source: Schertz et al. (1985).
26
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
destroy large, water-conducting channels. The soil is left
tillage practices leave more residues on the surface and
in a physical condition that is highly susceptible to wind
cause less soil disturbance than conventional moldboard
and water erosion.
plow–and–disk tillage. In fact, soil organic matter levels
The more a soil is disturbed by tillage practices, the
usually increase when no-till planters place seeds in a
greater the potential breakdown of organic matter by
narrow band of disturbed soil, leaving the soil between
soil organisms. During the early years of agriculture in
planting rows undisturbed. Residues accumulate on
the United States, when colonists cleared the forests and
the surface because the soil is not inverted by plowing.
planted crops in the East and farmers later moved to
Earthworm populations increase, taking some of the
the Midwest to plow the grasslands, soil organic matter
organic matter deeper into the soil and creating chan-
decreased rapidly. In fact, the soils were literally mined
nels that also help water infiltrate into the soil. The ben-
of this valuable resource. In the Northeast and Southeast,
eficial effects of minimizing tillage on soil organic matter
it was quickly recognized that fertilizers and soil amend-
levels are often observed quickly at the soil surface;
ments were needed to maintain soil productivity. In the
but deeper changes are much slower to develop, and
Midwest, the deep, rich soils of the tall-grass prairies were
depletion at depth is sometimes observed. In the upper
able to maintain their productivity for a long time despite
Midwest there is conflicting evidence as to whether a
accelerated loss of soil organic matter and significant
long-term no-till approach results in greater accumula-
amounts of erosion. The reason for this was their unusu-
tion of soil organic matter (SOM) than a conventional
ally high reserves of soil organic matter and nutrients at
tillage system when the full profile is considered. In
the time of conversion to cropland.
contrast, significant increases in profile SOM have been
Rapid decomposition of organic matter by organ-
routinely observed under no-till in warmer locations.
isms usually occurs when a soil is intensively tilled.
Incorporating residues with a moldboard plow, breaking
Crop Rotations and Cover Crops
aggregates open, and fluffing up the soil allow microor-
Levels of soil organic matter may fluctuate during the
ganisms to work more rapidly. It’s something like open-
different stages of a crop rotation. SOM may decrease,
ing up the air intake on a wood stove, which lets in more
then increase, then decrease, and so forth. While annual
oxygen and causes the fire to burn hotter. In Vermont,
row crops under conventional moldboard-plow culti-
we found a 20% decrease in organic matter after five
vation usually result in decreased soil organic matter,
years of growing corn on a clay soil that had previously
perennial legumes, grasses, and legume-grass forage
been in sod for decades. In the Midwest, many soils lost
crops tend to increase soil organic matter. The high
50% of their organic matter within forty years of begin-
amount of root production by hay and pasture crops,
ning cropping. Rapid loss of soil organic matter occurs
plus the lack of soil disturbance, causes organic mat-
in the early years because of the high initial amount of
ter to accumulate in the soil. This effect is seen in the
active (“dead”) organic matter available to microorgan-
comparison of organic matter increases when growing
isms. After much of the active portion is lost, the rate of
alfalfa compared to corn silage (figure 3.3). In addi-
loss slows and what remains is mainly the already well-
tion, different types of crops result in different quanti-
decomposed “passive” or “very dead” materials. With
ties of residues being returned to the soil. When corn
the current interest in reduced (conservation) tillage,
grain is harvested, more residues are left in the field
growing row crops in the future should not have such a
than after soybeans, wheat, potatoes, or lettuce har-
detrimental effect on soil organic matter. Conservation
vests. Harvesting the same crop in different ways leaves
27
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
Having sod crops as part of a rotation reduces loss of
topsoil, decreases decomposition of residues, and builds
3.2
up organic matter by the extensive residue addition of
alfalfa
plant roots.
3.0
Use of Synthetic Nitrogen Fertilizer
2.8
Fertilizing very nutrient-deficient soils usually results in
greater crop yields. A fringe benefit of this is a greater
percent carbon 2.6
corn
amount of crop residue—roots, stems, and leaves—
2.4
resulting from larger and healthier plants. However,
nitrogen fertilizer has commonly been applied at much
0
1
2
3
4
5
higher rates than needed by plants, frequently by as
years
much as 50%. Evidence is accumulating that having
Figure 3.3. Organic carbon changes when growing corn silage or alfalfa.
extra mineral nitrogen in soils actually helps organisms
Redrawn from Angers (1992).
better decompose crop residues—resulting in decreased
different amounts of residues. When corn grain is har-
levels of soil organic matter. (See chapter 19 for a
vested, more residues remain in the field than when the
detailed discussion of nitrogen management.)
entire plant is harvested for silage or stover is used for
purposes like bioenergy (figure 3.4).
Use of Organic Amendments
Soil erosion is greatly reduced and topsoil rich in
An old practice that helps maintain or increase soil
organic matter is conserved when rotation crops, such
organic matter is to apply manures or other organic
as grass or legume hay, are grown year-round. The
residues generated off the field. A study in Vermont dur-
permanent soil cover and extensive root systems of sod
ing the 1960s and 1970s found that between 20 and 30
crops account for much of the reduction in erosion.
tons (wet weight, including straw or sawdust bedding)
a) corn silage
b) corn grain
Figure 3.4. Soil surface after harvest of corn silage or corn grain. Photos by Bill Jokela and Doug Karlen.
28
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
chAPter 3 aMount oF organiC Matter in soils
organic matter (%)
1
2
3
4
1
2
3
4
3
6
9
12
3
6
9
12
1 ft
depth
2 ft
3 ft
a) forest soil (litter layer on
b) agricultural soil
c) prairie soil
d) agricultural soil
top of mineral soil may be
(originally forest)
(originally prairie)
30% or more organic)
Figure 3.5. Examples of soil organic matter content with depth (note different scales for forest and prairie soils). Modified from Brady and Weil (2008).
of dairy manure per acre were needed to maintain soil
plant roots are believed to contribute more to a soil’s
organic matter levels when silage corn was grown each
organic matter than the crop’s shoots and leaves. But
year. This is equivalent to one or one and a half times
when the plant dies or sheds leaves or branches, depos-
the amount produced by a large Holstein cow over the
iting residues on the surface, earthworms and insects
whole year. Varying types of manure—like bedded,
help incorporate the residues on the surface deeper into
liquid stored, digested, etc.—can produce very different
the soil. The highest concentrations of organic matter,
effects on soil organic matter and nutrient availability.
h