Loss of Soil Organic Matter
and Its Restoration
By William A. Albrecht
Professor of Soils
University of Missouri
CENTURIES before there was any science that acquainted people
with the intricacies of plant nutrition, decaying organic matter,
as in manure or other forms, was recognized as an effective agent
in the nourishment of plants. The high productivity of most virgin
soils has always been associated with their high content of organic
matter, and the decrease in the supply with cultivation has
generally been paralleled by a corresponding decrease in
productivity. Even though we can now feed plants on diets that
produce excellent growth without the use of any soil whatever, yet
the decaying remains of preceding plant generations, resolved by
bacterial wrecking crews into simpler, varied nutrients for
rebuilding into new generations, must still be the most effective
basis for extensive crop production by farmers. Soil organic matter
is one of our most important national resources; its unwise
exploitation has been devastating; and it must be given its proper
rank in any conservation policy as one of the major factors
affecting the levels of crop production in the future.
The stock of organic matter in the virgin soils taken over by
the homesteading pioneers was a heritage from an extensive past.
Its accumulation in our northern soils began with the recession of
the last glacier, possibly some 25,000 years ago, and continued
long enough to ripen the residues into compounds that were ready to
be used quickly by growing plants.
With the departure of the ice sheet and the consequent general
rise in temperature, the glacial residue of pulverized rock offered
minerals in solution for plant growth. As the plants found nitrogen
to combine with these minerals, they grew, died, and began to
accumulate in the soil. Then, as the rate of rock weathering
increased, bringing a larger supply of soluble minerals, the
accumulation of plant remains became correspondingly larger.
Finally, when the rocks were more completely weathered so that they
provided less mineral stock, or very little, an equilibrium point
was reached at which the accumulated organic matter held in
combination most of the minerals that could be turned into soluble
forms. Thereafter the supply of soluble minerals became a limiting
factor in plant growth.
Wherever there was poor drainage and limited aeration of the sod
cover, or where there were heavily wooded soils of relatively
level, glaciated topography, more complete simplification of this
accumulated store of plant nutrients was very slow. In other words,
the organic matter that held the major stock of previously mobile
nitrogen and minerals now kept these essentials stored in compounds
not simple enough for prompt consumption by growing plants. This
represented a very large supply of nutrients not far from the
condition in which growing plants could use them. Unable to decay
completely or to accumulate much more, they were poised as it were
for rapid conversion, when a slight change in conditions occurred,
into forms of maximum utility for plant growth.
But with the removal of water through furrows, ditches, and
tiles, and the aeration of the soil by cultivation, what the
pioneers did in effect was to fan the former simmering fires of
acidification and preservation into a blaze of bacterial oxidation
and more complete combustion. The combustion of the accumulated
organic matter began to take place at a rate far greater than its
annual accumulation. Along with the increased rate of destruction
of the supply accumulated from the past, the removal of crops
lessened the chance for annual additions. The age-old process was
reversed and the supply of organic matter in the soil began to
decrease instead of accumulating.
Fuel for the Plant-Food Production Factory
Organic matter may well be considered as fuel for bacterial
fires in the soil, which operates as a factory producing plant
nutrients. The organic matter is burned to carbon dioxide, ash, and
other residues. This provides carbonic acid in the soil water, and
the solvent effect of this acidified water on calcium, potassium,
magnesium, phosphates, and other minerals in rock form is many
hundreds of times greater than that of rain water. At the same time
the complex constituents of the organic matter are simplified, and
nitrogen in the ammonia is released and converted into the nitrate
form. This, very briefly, is the complicated process of
decomposition, from which carbon dioxide results as the major
simplified end product, together with a host of others in smaller
amounts. This gas is released in such large quantities from the
soil that the supply in the atmosphere over the earth is maintained
at a constant amount.
Decomposition by micro-organisms within the soil is the reverse
of the process represented by plant growth above the soil. Growing
plants, using the energy of the sun, synthesize carbon, nitrogen,
and all other elements into complex compounds. The energy stored up
in these compounds is then used more or less completely by the
microorganisms whose activity within the soil makes nutrients
available for a new generation of plants. Organic matter thus
supplies the "life of the Soil" in the strictest sense.
When measured in terms of carbon dioxide output, the soil is a
live, active body. An acre of the better Corn Belt soil in Iowa
(365) or northern Illinois, for example, exhales more than 25
times as much of this gas per day as does an adult man at work.
Such a soil area burns carbon at a rate equivalent to 1.6 pounds of
a good grade of soft coal per hour. The heat equivalent evolved in
the same time would convert more than 17 pounds of water to steam
under 100 pounds pressure. A 40-acre cornfield during the warmer
portion of a July day is burning organic matter in the soil with an
energy output equivalent to that of a 40-horsepower steam engine;
every acre, in other words, may be roughly pictured as a factory
using the equivalent of 1 horsepower. Organic matter is the source
of the power without which the plant-food elements could not be
changed to usable forms.
Supply of Virgin Soil Organic Matter Decreasing
The depletion of the supply of organic matter by cultivation is
well illustrated by the report of a study made by Jenny
(185) in central Missouri in which an undisturbed virgin
prairie soil was compared to an adjoining field cropped to corn,
wheat, and oats for 60 years without the addition of manure or
fertilizer. No erosion had taken place, yet 38 percent of the
organic matter represented by the virgin soil had been lost during
that period because of cultivation. As a consequence of this loss
in organic matter, the soil structure was modified to an extent
that might be represented by reducing the number of granules that
were the size of particles of sand by 11 percent and increasing the
number that were the size of clay particles by 5.5 percent. The
loss of organic matter represents soil compaction, which hampers
the circulation of air and water and hinders tillage operations at
the same time that the function of the soil in plant nutrition is
disturbed. Thus in but 60 years, more than one-third of the organic
matter, representing centuries of accumulation, was destroyed and
the efficiency of the soil for crop production was reduced.
Resulting Loss in Nitrate Nitrogen
Soil organic matter is the source of nitrogen. In the later
stages of decay of most kinds of organic matter, nitrogen is
liberated as ammonia and subsequently converted into the soluble or
nitrate form. The level of crop production is often dependent on
the capacity of the soil to produce and accumulate this form of
readily usable nitrogen. We can thus measure the activity that goes
on in changing organic matter by measuring the nitrates. It is
extremely desirable that this change be active and that high levels
of nitrate be provided in the soil during the growing season.
A study of the nitrate levels under corn in a Missouri silt loam
during 13 years reveals a gradual decline in the production of
nitrates (5). During the first 5 years of the test this soil
increased its nitrates in the spring to the maximum of more than 20
pounds per acre as early as May. During a similar period only 2
years later, this maximum had been reduced to 18 pounds, and it was
not attained until June; 3 years later the maximum was less than 16
pounds, attained in July; and 3 years after that, the maximum of 13
pounds was not reached until August (fig. 1). During continuous
cropping to corn without the addition of organic matter, the
maximum nitrate accumulation dropped to 65 percent of that in the
initial period, when the land had been in sod for some time. In
other words, though this soil had been in corn continuously for
only 13 years--which might seem equal to 52 years of a 4-year
rotation, with one crop of corn every 4 years-- its
nitrate-producing power, or its capacity to deliver this soluble
plant nutrient, had been reduced by 35 percent.
FIGURE 1.--Declining seasonal levels of nitrates and later
seasonal maxima with continued cropping to corn. (Averaged for
different succeeding 5-year periods.)
Such pronounced exhaustion is not limited to the corn crop,
which is readily associated with intensive tillage. The same thing
is true in the case of wheat. Its exhausting effects measured in
the same study (5) and shown in figure 2, were even greater
than those of corn. The nitrate level under wheat was constantly
lower than under corn for the corresponding period.
FIGURE 2.--Declining nitrate nitrogen levels in soils in wheat
and in corn as advancing 5-year season averages.
Concurrently with the foregoing measurements showing the decline
of nitrates, careful chemical analyses were made of the same kind
of soil nearby under fallow conditions with an annual spring
plowing. The surface soil alone lost 2,300 pounds of organic matter
per acre, as shown in figure 3 (4). A nearby plot in a
3-year rotation of corn, wheat, and clover, with all the crops
removed during a period beginning 2 years earlier and extending 2
years longer--a total of 17 years--lost 800 pounds of organic
matter. [Footnote:] From unpublished data supplied by M. F.
Miller Missouri Agricultural Experiment Station. Regardless
of the presence or absence of a crop, the failure to add organic
matter and regular tillage of the soil mean a depletion of the
original stock of organic matter at a very significant rate, even
where there is no erosion. Where erosion removes the body of the
surface soil itself, the rate of depletion is much greater.
Lower Crop Yields and Land Values
In addition to carrying nitrogen, the nutrient demanded in
largest amount by plants, soil organic matter either supplies a
major portion of the mineral elements from its own composition, or
it functions to move them out of their insoluble, useless forms in
the rock minerals into active forms within the colloidal clay.
Organic matter itself is predominantly of a colloidal form
resembling that of clay, which is the main chemically active
fraction of the soil. But it is about five times as effective as
the clay in nutrient exchanges. Nitrogen, as the largest single
item in plant growth, has been found to control crop-production
levels, so that in the Corn Belt crop yields roughly parallel the
content of organic matter in the soil (184). On a Missouri
soil with less nitrogen than that corresponding to 2 percent of
organic matter (40,000 pounds of organic matter per acre of plowed
surface soil) an average yield of 20 bushels of corn per acre can
hardly be expected. For yields approaching 40 bushels, roughly
double the amount of organic matter is required. With declining
organic matter go declining corn yields and therefore lower
earnings on the farmers investment. Thus the stock of organic
matter in the soil, particularly as measured by nitrogen, is a
rough index of land value when applied to soils under comparable
conditions. According to studies in Missouri, for example, the
lower the content of organic matter of upland soil, the lower the
average market value of the land.
FIGURE 3.--Decrease of organic-matter content in a fallow,
untreated soil in contrast to the gain in soil treated with 2 1/2
tons of red clover annually, representing over 500 pounds annual
increase in organic matter per acre.
Problem of Maintaining a Liberal Supply of Soil Organic
Matter
Though the rapid depletion in the Corn Belt, for example, of the
soil organic matter and soil fertility in the pioneer period of a
hundred years may be alarming, there is consolation in the fact
that this high rate of depletion will not continue. As is true for
all biochemical processes, the early rate of consumption is rapid,
which gives a sudden decrease. Then the rate of consumption falls
off, so that the loss in the second period will perhaps be less
than half that in the first. In the third stage the loss will
possibly be half that of the second. Long-continued experiments,
accompanied by soil analyses, prove that the organic-matter content
of a soil will reach a fixed level characteristic of the
surrounding climatic conditions. After a period of heavy loss,
then, we may expect a fairly constant level during a long period of
continued cultivation. This situation is well illustrated by the
decline in nitrogen content shown in figure 5 of the article, Soil
Nitrogen (p.369). In other words, we may anticipate a further
decline in productivity from the present relatively high levels,
followed by a more constant level, which will be proportionate to
the lower content of organic matter, determined by the environment
in each particular region.
Maintaining Versus Increasing the Organic Matter
The following questions naturally arise: What should be the
content of organic matter in a soil? Should the present level be
raised or merely maintained economically? These are questions of
decided significance in determining policies in soil
management.
Attempting to hoard as much organic matter as possible in the
soil, like a miser hoarding gold, is not the correct answer.
Organic matter functions mainly as it is decayed and destroyed. Its
value lies in its dynamic nature. A soil is more productive as more
organic matter is regularly destroyed and its simpler constituents
made usable during the growing season. Its mere presence in the
soil is of value during certain stages of decay, when it influences
soil structure and water relations and when it functions in holding
plant food in readily available form much more effectively than
does any mineral fraction of the soil. The objective should be to
have a steady supply of organic matter undergoing these processes
for the benefit of the growing crop. Up to the present, the
policy--if it can be called a policy--has been to exhaust the
supply, rather than to maintain it by regular additions according
to the demands of the crops produced or the soil fertility removed.
To continue very long with this practice will mean a further sharp
decline in crop yields.
The level of organic-matter content to be maintained is not the
same for all regions. It varies according to climate. Professor
Jenny in his studies of virgin organic matter of soils (184)
has pointed out that--within regions of similar moisture
conditions, the organic matter content of upland, terrace, and
bottomland soil, including both prairie and timber vegetation,
decreases from north to south. For each fall of 10° C. (18°
F.) in annual temperature, the average organic matter content of
the soil increases two or three times, provided the
precipitation-evaporation ratio is kept constant.
Thus from south to north the level of organic matter in the soil
becomes naturally higher. In the northern section of the Temperate
Zone with its moderate rate of vegetative growth and moderate
production of organic matter, the longer periods of lower
temperature lessen decay and increase accumulation by carry-over
from season to season. In the southern section, even though the
growing season is longer and produces more vegetation, yet there is
also a longer season for decay, and it proceeds at a much more
rapid rate. Because the rate of decay doubles and trebles for every
rise of 10° C. (18° F.) in temperature, the destruction of
organic matter is more complete and there is little accumulation.
Its nature, particularly its composition, is also different. It
shows a narrower carbon-nitrogen ratio (184) and a greater
resistance to further simplification.
The level of organic matter in the soil of the temperate regions
rises with lower annual temperatures, and also with increased
moisture. The level is also higher in grasslands than in timbered
soils under equal moisture conditions. The same amount of moisture
in the North with its lower temperature is more effective in
bringing about an increase of soil organic matter than in the South
with its higher temperature. Hence sod crops are more effective
restorers of organic matter in the northern than in the southern
part of the North Temperate Zone. The climate of the region must be
considered in determining the level of organic matter to be
maintained in the soil. Changes in altitude must also be considered
insofar as these correspond to climatic variations.
In northern Missouri, for example, virgin soils are in a
condition of natural equilibrium at an organic-matter content of
3.54 percent; in southern Missouri at 2.20 percent; in southern
Minnesota at 4.44 percent; and in Arkansas at 1.96 percent. In
terms of pounds per plowed acre, the figures are: For southern
Minnesota, 88,800 pounds; northern Missouri, 70,800 pounds;
southern Missouri, 44,000 pounds; and Arkansas, 39,200 pounds.
These figures represent the natural equilibrium between the
production of organic matter by native vegetation and its
destruction by micro-organisms. The balance figure is determined in
the main by the temperature-rainfall combination, or climate. It
would be folly, according to these data, for the farmer in Arkansas
to attempt to increase organic matter in his soil to the level
common in the soil tilled by the Minnesota farmer. Likewise the
problem of increasing the organic matter will be simpler for the
farmer in the North, where even with the same amount of moisture,
the lower temperature is influential in preserving more of the
organic matter added to the soil.
Cultivation of the soil and extended periods without a
vegetative cover decrease the content of organic matter below that
considered natural, or virgin, for the locality. The degree of
exhaustion of organic matter to levels below the virgin stock
represents the possibilities of improvement. But these
possibilities also are affected by climate. In the northern
sections both temperature and moisture conditions are favorable to
restoration, and the growing of legumes and the addition of green
manure are very effective in this direction, as experimental
results demonstrate. Farther south, restoration is more difficult,
and it may even be impossible to restore the organic matter
profitably and permanently to levels even approaching virgin
conditions. However, the longer growing season permits two crops a
year, one of which may be a legume for green manure, and this makes
it possible to provide organic matter and a turn-over of nitrogen
regularly even when the level cannot be raised.
We are confronted, then, by three facts: (1) The stock of
organic matter in the soil is being exhausted at an alarming rate;
(2) the exhaustion is still in its early stages in some of the more
recently developed agricultural areas; and (3) there are no
climatic handicaps that prohibit restoration. These facts mean
corresponding--and inescapable--responsibilities. The Nation should
be made aware of the rapid rate at which the organic matter in the
soil is being exhausted. Farm-management practices should be
adopted that will at least maintain, and in as many cases as
possible even increase, the supply of this natural resource in the
soil. The maintenance of soil organic matter might well be
considered a national responsibility.
Interrelation of Soil Organic Matter with Nitrogen and
Minerals
At first thought, the problem of restoring soil organic matter
may not seem difficult according to simple mathematical
calculations. If a soil in virgin condition contained 44,000 pounds
of organic matter per acre and 35 percent of this has been
exhausted during 60 years of cultivation, the apparently simple
solution would be to add 15,400 pounds of dry material to the soil,
or an amount of organic matter equivalent to the weight lost. The
addition of the equivalent of some 7 3/4 tons of dry matter in the
form of manure, legumes, straw, and other farm-waste products might
seem to be a satisfactory solution. But the virgin organic matter
that has been lost was very different in nature and effects from
the material considered to replace it. In kind and composition, the
organic matter used for restoration should be as close as possible
to that which was lost, at least in terms of effective results.
Building Soil Organic Matter Largely a Nitrogen Problem
Soil bacteria, the agents of decomposition, use carbon mainly as
fuel and nitrogen as building material for their bodies and for the
production of the intricate organic compounds that result from
their activity. Fresh organic matter is characterized as a rule by
a large amount of carbon in relation to nitrogen. It has a wide
carbon-nitrogen ratio, in other words; or so far as the bacteria
are concerned, a wide ratio of fuel to building material. Such
fresh material--straw, for example--may have a ratio that is too
wide, so that it decomposes very slowly. If the ratio is less wide,
decomposition may be more actively carried on. The carbon will then
be rapidly used up as fuel while the nitrogen is held or treasured
without appreciable loss.
Thus when decay has proceeded to the point where the
carbon-nitrogen ratio is significantly decreased, a residue of a
more stable nature is produced. Thereafter the carbon-nitrogen
ratio is narrower and remains more constant. This corresponds more
nearly to the condition that holds in the case of the organic
matter in virgin soils. Its further decay, which is slow because of
the relatively low level of carbon, liberates nitrogen in place of
storing or preserving it. Because of its high carbon content, the
decomposition of fresh organic matter requires additional soluble
nitrogen to be used as building material by the micro-organisms,
which obtain it from the soil, often exhausting the supply to a
degree that is damaging to a growing crop. The amount of increase
in organic material corresponds, in the main, to the amount of
nitrogen available. The extra carbon in the fresh material is lost
from the soil. Thus when soils are given straws, fodders, and
similar crop residues of low nitrogen content, only small increases
in soil organic matter can result--in the main, only as large as
the added nitrogen will permit. Many tons of common farm residues
and wastes per acre are needed to produce a single additional ton
of organic matter in the soil.
The restoration of soil organic matter, then, is a problem of
increasing the nitrogen level or of using nitrogen as a means of
holding the carbon and other materials. This is the basic principle
behind the use of legumes as green manures. In building up the
organic content of the soil itself, it will often be desirable to
use legumes and grasses rather than to add organic matter, such as
straw and compost, directly. If legumes and grasses are to be
successfully grown on many of the soils of the humid regions of
this country it will be necessary, first, to properly fertilize and
lime the soil. Legumes use nitrogen from the air instead of the
soil, and thus serve to increase the amount in the soil when their
own remains are added to it. Commercial nitrogen used as treatment
on straw for the production of artificial manure in compost piles,
or when plowing under straw in the field after the combine, may be
considered in the same category. Small amounts of added nitrogen
may in this way make possible the use of large amounts of
carbonaceous matter in restoring the soil. Thus the European farmer
first "makes" his manure by composting the fresh straw-dung mixture
from the barn and then treats it intermittently with the
nitrogen-bearing liquid manure or urine from the same source and
the nitrogen-rich leachings from the manure pit. He does not
consider the fresh, strawy barn waste manure in the strictest sense
until the surplus carbon has been removed through the heating
process, and the less active manure compounds become similar to
those of the soil organic matter. In a similar way, it should be
understood that the soil organic matter can be "made" or built up
only as the nitrogen supply is raised and combined with
carbonaceous material in a more narrow ratio.
It is only under conditions of this kind that beneficial effects
on crops may be expected through further decomposition. The manure
making of the Old World farmer turns the miscellaneous
straw-dung-urine mixture, of highly variable value, into a
standardized fertilizer for specific use. Our great variety of crop
wastes--straw, cornstalks, etc.--should be used in a similar way,
by adding nitrogen to bring about a proper adjustment with their
excess carbon. These neglected wastes will then provide extra and
valuable soil organic matter that will have beneficial rather than
possibly detrimental effects on crops.
Level of Minerals in Soil Influences Organic-Matter Supply
Bacterial activity does not occur in the absence of the mineral
elements, such as calcium, magnesium, potassium, phosphorus, and
others. These, as well as the nitrogen, are important: Recent
studies show that the rate of decomposition is reduced when the
soil is deficient in these elements. In virgin soils high in
organic matter, these elements also are at a high level, and are
reduced in available forms as the organic matter is exhausted. A
decline in one is accompanied by a decline in the other.
It has been held that calcium, for example, is instrumental in
retaining the organic matter in a stable form in the soil. Though
this seems doubtful in view of the fact that the addition of lime
to soils hastens the rate of loss of organic matter, calcium has a
decided influence on the growing crop and therefore on the amount
of material it adds to the soil when turned under. It has recently
been discovered that the fixation of nitrogen from the atmosphere
by legumes is more effective where high levels of calcium are
present in available form (3). Thus, if in calcium-laden
soils, excellent legume growth results and correspondingly large
nitrogen additions are made, such soils may be expected to contain
much organic matter. Liberal calcium supplies and liberal stocks of
organic matter are inseparable. The restoration of the exhausted
lime supply exerts an influence on building up the supply of
organic matter in ways other than those commonly attributed to
liming.
In the presence of lime (calcium) the legumes use other elements
more effectively, such as phosphorus (175) and probably
other nutrients. Thus heavier production results on soils rich in
minerals, including more intensive and extensive root
development--the most effective means of introducing organic matter
into the soil. The presence of large supplies of both organic
matter and minerals points clearly to the fact that the soils were
high in the latter when the former was produced. It seems logical
to ascribe causal significance to the minerals in the production of
organic matter, whether or not they are effective in preserving it.
If the soils that have lost their organic matter are to be
restored, the loss of minerals, which has probably been fully as
great, must be taken into account, and provision must be made to
restore these mineral deficiencies before attempting to grow crops
for the sake of adding organic matter.
How Can Soil Organic Matter Be Restored?
Conservation and restoration of soil organic matter as a
national problem calls for a program of soil and farm management in
which (1) needless losses are eliminated or reduced to a minimum,
and (2) the stock in process of consumption is regularly maintained
with attention to its possible economical increase. Experimental
results indicate the steps in such a program.
First attention should be given to eliminating accelerated
erosion. When, according to the long-continued soil erosion studies
at the Missouri Agricultural Experiment Station (263), the
entire plowed surface soil under continuous corn may be washed away
in 50 years, it would be foolhardiness to attempt soil building by
processes so slow as to make only an inch in hundreds of years.
Erosion can be eliminated, as the investigations have shown and
recent extensive erosion-control experience demonstrates, by sod
cover crops, reduction in the amount of tillage, and other
measures. The establishment of sod crops on badly eroded land often
requires proper fertilization and liming.
Sod crops have not been fully appreciated. Grasses have been the
stepchildren in the American crop family. They have not been
"cultivated" in the same sense as farm crops; they have been left
to themselves, to grow on soils often turned over to them because
depleted fertility made cereal cropping unprofitable. They have
been incidental in the farm program. Consequently, they have not
delivered their maximum in animal production and have often been
very inefficient feed. Land in grass was considered idle and
checked off the accounts, even if not recorded on the debit
side.
The Old World, with its longer agricultural experience, shows
that the lands still in good production today are those occupied by
sod crops regularly for a large part of the time, where clean, or
summer, cultivation has been reduced to a minimum. In France and
England only slightly more than one-fourth of the cultivated soils
are in clean cultivation. In Germany the figure is even less, and
there are vast acreages of permanent pastures in all these
countries. In the United States the area in clean cultivation and
row crops approaches one-half the cultivated land; and this in
regions where the rains are of torrential nature. We may well be
guided by Old World experience, which tells us that sod crops are a
paramount factor in holding the soil and maintaining its
productivity by their regular additions of organic matter. The
tough sod slice should be more fully appreciated as an asset in
terms of its organic matter rather than considered as a liability
because of the high power required to plow it.
Some recent studies suggest that we have not appreciated sod
crops in relation to moisture absorption and the storage of
moisture in the subsoil. The beneficial effects of sods turned
under for corn crops have usually been ascribed to nitrogen, when
possibly the important factor has been accumulated moisture in the
subsoil. Grass crops absorbed 87.4 percent of the rainfall, a
3-year rotation with one sod crop absorbed 85.5 percent, while
continuous corn absorbed only 69.6 percent, according to trials
extending over 14 years (263). This amounted to an increased
rainfall of 7.2 inches for grass and 6.4 inches for rotation as
compared to continuous corn. The difference in crop yield was more
significant than these figures indicate, since two-thirds of the
annual rainfall came in the 6 months of the growing season, or the
period when differences in rainfall mean increased yields.
Much of the extra water absorbed moves beyond the zone of
consumption by the shallow grass roots and is stored there. Thus
the deeper soil layer under sod, such as the third foot, carries
more water than the same layer under tilled soil. Moisture studies
of two such adjacent soils, no far distant from those under the
erosion study cited above, are interesting from this standpoint,
especially for the years 1934 and 1936, which were seasons of
deficient rainfall. Table 1 gives the moisture content as the
percentage of moisture in the successive 1-foot layers to a depth
of 3 feet.
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Table 1. Moisture content at successive depths under sod and
under cultivated soil
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First foot Second foot Third foot
Year and month --------------------------------------------------------
Sod Tilled Sod Tilled Sod Tilled
-------------------------------------------------------------------------
1934: Percent Percent Percent Percent Percent Percent
April 27.18 25.23 29.90 24.61 26.11 16.51
November 33.80 31.70 31.90 30.80 32.60 24.80
1936:
March 26.30 27.80 28.20 28.90 28.30 23.00
November 27.00 26.80 28.50 27.30 27.80 19.80
1937:
April 32.90 28.30 30.00 28.60 30.70 23.40
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The third-foot layer was much drier under the tilled soil than
under the sod during all of these studies. Its recovery of moisture
after rain was always delayed and its total water content never
equaled that in the third foot under sod. Though the first-foot
layer under sod had a lower moisture content than that under
tillage during 1 month (March 1936), in all samplings the moisture
supply of the second- and third-foot layers was greater under the
sod than under the tilled surface, with the most pronounced
differences in the third foot, varying from 5.3 to 9.5 percent.
These differences mean on the average that the third-foot layer
under sod is storing the equivalent of a 1.2-inch rainfall, which
it may supply to the sod crop, or to the deeper roots of the
following crop, in the drier summer season. This stored moisture
under sod should be considered as a factor in combating
droughts.
The advantages of grass-sod crops as effective agencies for soil
restoration may be summed up as follows: They do much toward
guaranteeing a moisture supply for their own needs by absorbing
more of the rainfall. They add a heavy root growth annually that,
for native bluestem, for example, amounts to as much as 1.34 tons
per surface acre-foot, according to Weaver and Harman (453).
Because of the annual death of part of these roots, this is a
regular addition of organic matter that helps to maintain the
supply. On the untilled and less violently aerated soil, where the
higher moisture means lower temperatures, these conditions favor a
return to the original, or virgin, stock of organic matter in the
soil. At the same time, erosion is prevented both by the living
grass and by the spongy surface residue accumulated above the soil
from the dead plant tops of the previous season.
Sod crops are sufficiently effective in restoring soil organic
matter to offset the destructive influences of clean cultivation
and summer tillage. Unpublished data from studies by the Missouri
Agricultural Experiment Station show clearly the destructive
influence of summer fallow and, in contrast, the increase in
organic matter obtained through sod crops. When sod was used in an
ordinary 3-year crop rotation, with manure made from the crops
returned to the soil, there was no serious decline in the content
of organic matter, as shown by table 2.
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Table 2.--Gains and losses in soil organic matter (in pounds per acre of
surface soil) during 17 years, on areas under different systems of
cropping and management *
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Crop and management Gain Loss
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Pounds Pounds
Rotation--corn, wheat, clover--all crops removed ----- 800
Rotation--corn, wheat, clover--manure equivalent returned 3,200
Rye and cowpeas--turned under as green manure 1,200
Rye turned under--summer fallow ----- 14,400
Red clover continuously--all crops removed 3,600
Red clover continuously--all crops turned under 9,600
Alfalfa continuously--all crops removed 10,400
Grass sod, clipped--nothing removed 10,000
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* Unpublished data of M.F. Miller, Missouri Agricultural Experiment Station.
The provision of liberal supplies of soluble plant nutrients for
profitable cereal production demands tillage and the breaking down
of organic matter. The organic matter to be broken down should be
provided by sod crops, particularly legumes, used regularly in the
rotation. Permanent legume sods are effective agents, as this study
testifies, in building up the organic matter on soil containing
ample minerals--particularly when the crops are not removed.
Continuous red clover sod with no crop removal increased the
organic-matter content by a total of 9,600 pounds in 17 years, or
an average of 564 pounds a year. In similar soil in another plot
nearby that was given 2 1/2 tons of clover annually as a green
manure under fallow; the annual gain in organic matter amounted to
571 pounds a year (see fig. 3).
New Awareness and New Responsibility
American citizens are becoming conscious of the fact that loss
of fertility and the depletion of organic matter in the soil are
partly responsible for the menace of erosion. The first step in
remedying this situation is to restore fertility by the use of lime
and fertilizer. The second step is to put some lands permanently
into sod crops--legumes wherever possible, and the better
grasses--and to use sod more regularly in rotations on tillable
cropped lands. The conservation and use of such farm wastes as crop
residues and manures should be included as the third step.
If these practices are recommended as proper soil management by
all agricultural agencies, their adoption by individual farmers
will become so common that the rate of soil depletion will be
lessened. The need for long-time investments in materials that
build up the soil in organic matter and fertility should be
recognized in granting credit to farmers. Both owners and tenants
must accept responsibility for soil conservation and work for it
cooperatively. Unearned increment, the great wealth producer of the
past, should be recognized as largely responsible for the mining of
soil fertility and the burning up of soil organic matter until it
has reached such a low level that this source of wealth has an
extremely uncertain outlook in the future. The heritage of soil
fertility and organic matter that we are handing on to the next
generation is not large enough to be used lavishly. Careful
conservation and thrifty management will be imperative if it is to
yield even a moderate income.
[Source: U.S. Dept. of Agric., Soils and Men, Yearbook of
Agriculture 1938, pp. 347-360.]
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