Healthy soil and sustainable growing

["Tradingpost" <tradingpost@lobo.net>]

This is the entire piece from Albrecht I mentioned. It was originally printed 
in the book,  Soils and Men, Yearbook of Agriculture 1938, U.S. Dept. of 
Agric. A great classic I was fortunate to get my hands on.

paul tradingpost@lobo.net
------------------------------------------

Loss of Soil Organic Matter and Its Restoration By William A. Albrecht
Professor of Soils
University of Missouri
http://www.soilandhealth.org/01aglibrary/010120albrecht.usdayrbk/lsom.html

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.

Declining seasonal nitrates

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.

Declining nitrate nitrogen

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.

Decrease of organic matter

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.

-------------------------------------------------------------------------
     Table 1. Moisture content at successive depths under sod and
                          under cultivated soil
-------------------------------------------------------------------------
                  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
-------------------------------------------------------------------------

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.]