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II. Factors Influencing Soil Structure
SOIL MANAGEMENT FOR CIONEZ~BVAJFION
absorption, run-off and resulting erosion losses are comparatively low.
Individual soil particles tend to be bound into stable aggregates. Water
falling on the soil surface does not bring about dispersion of the aggregates, with resulting crust formation. Individual soil particles held in
aggregates do not move downward to clog soil pores and thus limit water
and air movement. Favorable air-water relationships encourage deep
root penetration and thus contribute to drought resistance in the crops
As cultivation of these soils continues, there is a gradual deterioration
of structural qualities. The rate and, in a large measure, the extent of
such structural deterioration varies with the intensity of cultivation and
the cropping system followed. Numerous examples have been reported
showing a decided reduction in organic matter and nitrogen as a result
of cultivation operations. In addition to other effects, losses of organic
matter are ordinarily accompanied by decreases in the extent of aggregation. Jenny (1933) reports a 38 per cent decrease in organic matter
content of Putnam silt loam in Missouri after sixty years of cultivation.
A 33 per cent decrease in available bases and a corresponding decrease
in degree of aggregation occurred during the same period. Other soils
showed similar reductions in organic matter content and in aggregation
under conditions of cultivation.
B r a a e l d (1936) has reported a total porosity of 60.3 per cent and
a yield of 80 bushels per acre of corn from the first crop following sod.
An immediately adjacent area that had been under cultivation showed
a porosity value of 50.5 per cent and a corn yield of 20 bushels per acre.
Porosity of a clay soil was shown to have been decreased by as much as
18 per cent during forty years of cultivation.
Anderson and Browning (1949) show data on physical and chemical
changes resulting from cultivation of six Iowa soils. Physical properties
investigated included permeability, aeration porosity, volume weight,
and degree of aggregation. In this report, effects of cultivation on permeability of the soils were variable. In regard to other physical properties, it is shown that cultivation has resulted in decreased air-porosity,
considerably reduced aggregation, and an increase in volume weight.
Data on changes in nitrogen content indicate a substantial reduction in
organic matter content in all cases as a result of cultivation operations.
Retzer and Russell (1941) show a general deterioration of soil physical conditions as a result of cultivation operations. Marked changes in
physical condition of a clay soil under cultivation are reported by Page
and Willard (1946). The upper 3 feet of the profile, under virgin conditions, weighed 70.8 pounds per cubic foot. Following cultivation for
an extended period, this value increased to 86.5 pounds. In addition to
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this change in bulk density, the porosity of the soil decreased from 57.3
per cent to 47.6 per cent. It is interesting to note that these changes in
structure were not limited to the plowed layer but extended to a depth
of 3 feet in the profile. The report emphasizes the effect of these changes
in physical properties of the soil on air and water capacity and on drainage conditions within the profile.
The preceding discussion indicates that cultivation operations, in
general, tend to reduce the extent of aggregation, reduce the volume of
pore space, and increase the weight of soil per unit volume. The content
of organic matter is ordinarily reduced as these physical changes occur.
The data indicate that the cultivation process, by loosening the soil and
temporarily increasing the volume of air space, leads to more rapid decomposition of organic matter than occurs under sod cover. As the
organic materials are decomposed, soil aggregates tend to become dispersed. Breakdown of aggregates permits closer packing of soil particles, with the accompanying decrease in porosity and increase in density
2. Mechanical Compuction in Cultivation
I n addition to the above factors, the direct effect of machine operation on soil compaction should be noted. During the past few decades
the use of horse-drawn field equipment has largely given way to motordriven equipment. As the size and power of tractors increased, larger
and heavier cultivating and harvesting equipment came into .use. Such
equipment has not only contributed to soil structure deterioration
through more vigorous and more frequent cultivation but ha5 also resulted in serious compaction resulting directly from wheel traffic.
The writer an d his associates found wide differences in the bulk
density and porosity of Sassafras loam depending on the intensity of
wheel traffic. An area in potato production showed a n average bulk
density of 1.33 for samples taken in the plant row. Samples from row
middles, which had normal tractor and sprayer traffic, averaged 1.46 in
bulk density. Air-filled pore space was 24.2 per cent in the row and 15.9
per cent in the middle.
Jamison et al. (1950) in soil bin tests found that soil compaction from
pneumatic tractor tires varied with the soil moisture content and with
the initial looseness of the soil. On dry, compact soil little increase in
bulk density occurred under the tire action. On loose, moist soil, more
or less comparable to seedbed conditions, bulk density was increased
from 0.94 to as much as 1.39 by the movement and pressure of the tractor
Free et al. (1947) found that soil compaction, under a given set of
SOIL MANAQEMENT FOR CONSEE~VATIONAND PRODUCTIVITY
conditions, was reduced as the organic matter content of the soil increased. Parker and Jenny (1945) emphasize the importance of organic
matter additions and the absence of cultivation for maintenance of soil
structural conditions. Either cover crops or manure additions greatly
increased water infiltration in orchard soils. Tractor traffic and heavy
disking brought about significant increases in volume weight and reductions in soil porosity. Elimination of cultivation and regular use of
cover crops reduced resistance to penetration, reduced core weights, and
brought about marked improvement in the rate and amount of water
Weaver (1950) and Weaver and Jamison (1951) found that maximum compaction of Cecil clay and Davidson loam under implement
traffic occurred at a moisture content near the optimum for plowing.
It is emphasized that tractor and other heavy machinery operations
should be performed a t moisture contents as low as practicable for the
work to be done.
It has long been recognized that organic matter content is related to
physical properties of the soil. A high level of aggregation or granulation in the soil is usually associated with plentiful supplies of good quality organic matter. Further evidence of the granulating effect of organic
matter occurs in the fact that organic materials must be made soluble
in order to obtain dispersion of soils for mechanical analyses.
It has been shown by Baver (1935) that a significant correlation
exists between the percentage of aggregates larger than 0.05 mm. and
the carbon content of a wide variety of soils. A higher correlation coefficient was found for aggregates larger than 0.1 mm., indicating that
organic matter tends to favor the formation of relatively larger aggregates. When various soils were grouped on the basis of clay content,
it was noted that the aggregating effect of organic matter was greater
in the soils containing the smaller amounts of clay. A high correlation
was found between organic matter content and aggregation in soils containing less than 25 per cent clay. A significant but much lower correlation was found for soils containing higher amounts of clay. The relatively greater effectiveness of organic matter in promoting aggregation
in soils of low clay content is of interest later in this report, where data
from a loamy sand soil are discussed in some detail.
Myers (1937) reports that colloidal organic matter is more effective
than clay in the formation of stable aggregates with fine quartz sand and
orthoclase particles. His results emphasize the importance of dehydration in the process of stable aggregate formation. The effectiveness of
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organic matter additions in bringing about the aggregation of silt and
clay particles has been reported by Kolodny and Neal (1941). Manure
applications and winter cover-crop practicea were & o w to increase aggregation of flne particles in the soil. Richards et al. (1948) in a subsequent report showed similar results. In addition, it was pointed out
that the aggregating effect of the treatments persisted to a greater or
less extent for a six-year period following cessation of the treatments.
McCalla (1945) suggests that the quantity of organic matter, within
reasonable limits, is less important than the quality in producing aggregate stability. This report indicates that structural stability is temporary, lasting only as long as the stabilizing decomposition products
continue to exist in the soil. These data appear to support the view,
expressed elsewhere in this discussion, that continuing aggregation and
structural stability in cultivated soils require regular additions of organic matter.
Uhland (1949)has shown increases in percolation rates and in volume
of pores drained at a given tension following crops of kudzu and alfalfa.
On a desurfaced Shelby silt loam, the bulk density of exposed surface
material was reduced from 1.31 to 1.00 at the end of twelve years in a
grass-legume sod mixture. Sassafras loam under continuous cultivation
in potatoes showed significant increases in degree of aggregation with
annual manure applications, M reported by Klute and Jacob (1949).
Bulk density values decreased in all cases at the higher manure applications as compared with unmanured areas.
Lute at d. (1946) report an interesting case of differences between
cultivated crops in regard to effects on structural properties of the soil.
It seems reasonable to expect that the growth of cultivated crops would
tend t o bring about improvement in soil structure similar to that effected
by grass-legume mixtures and other noncultivated plant growth. With
cultivated crops, however, such a tendency for structural improvement
is largely nullifled by the cultivation operations which accompany growth
of the crop. In the above report it is shown that soil porosity was the
same under a cotton-corn system without cover crop and a cotton-peanuts
system with winter cover. This appears to support the data of Martin
(1942) indicating that corn stover is a particularly favorable type of
organic material for improvement of soil structural conditions.
Uhland (1947) emphasizes the effect of grass and legume crops on
the stability of aggregation in the soil. Data are reported in terms of
the number of drops of water falling 30 cm. required to disperse soil
aggregates of a selected size formed under different cropping systems.
It is shown that an average of 6.2 drops dispersed the aggregates of a
soil that had been in continuous corn and 7.5 drops dispersed the aggre-
SOIL MANAGEMENT FOR COWSEWATION
gates found in a21 area that had been under clean fallow for thirteen
years. I n contrast to this condition of relatively unstable aggregation,
it took 41.2 drops to disperse aggregates from second-year meadow and
40.2 for samples from a thirteen-year old alfalfa treatment. No significant difference in the water-holding capacity of a fine sandy loam resulted from the use of winter cover crops, as reported by McVickar,
et al. (1946).
Alderfer and Merkle (1941a) found that structural breakdown in the
soil varied directly with the intensity of cultivation. Soil areas under
a corn, oats, wheat, clover rotation showed deterioration in physical conditions as compared with areas under sod. I n turn, areas under continuous cultivation were in less favorable structural condition than were
the areas in rotation. The stability of aggregates was found to be positively correlated with the organic matter content of the soil. Bulk density values showed an inverse relationship to aggregate stability and
organic matter content.
Woodruff (1939) reports that cultivated soils receiving regular manure applications or soils cropped to a rotation including the regular
use of grass-legume mixtures, were found to possess a higher state of
aggregation than that in untreated soils cropped continuously. H e further points out that the physical properties of a cultivated soil imparted
by organic matter may be more closely associated with past crop history
and soil treatments than with the properties of the virgin profile.
Metzger and Hide (1938) point out the improvement in soil aggregation resulting from two years growth of alfalfa and of sweet clover under
field conditions. Hide and Metzger (1939) found little or no improvement in soil aggregation resulting from the growth of bluegrass for a
four-month period under greenhouse conditions. When finely ground
wheat straw or alfalfa was mixed with the soil, the improvement in 'aggregation was similar to that occurring under growth of the crop.
Waksman and Martin (1939) found that ground alfalfa o r straw
increased the degree of aggregation of a Coastal Plain soil. The alfalfa
produced a n earlier and greater effect than did the straw. This result
appears to be related to the relatively more rapid rate of decomposition
of the alfalfa. 'Peele (1940) has shown the positive relationship between
microbial activity and the resulting rate of organic matter decomposition
and the rate and extent of soil aggregate formation.
It is not intended, in the foregoing section, to imply that organic
matter content and activity in the soil is the only factor of importance
bearing on the formation of stable aggregates. The development and
maintenance of stable granulation of soil particles obviously is influenced
by wetting and drying, freezing and thawing, clay content, numerous
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chemical reactions, and a variety of other factors. The operation of
these factors, however, tends to be fixed by the nature and location of
the soil under consideration. Organic matter content is one of the few
factors, and possibly the only major factor, influencing aggregate formation and stabilization which lends itself to systematic manipulation in
the soil management program, Cropping systems can be designed to
provide regular additions of organic matter in cultivated soils and thus
permit continuing maintenance of favorable levels of particle aggregation.
It is common knowledge that soils generally possess favorable physical
properties when first brought under cultivation from a virgin condition.
Land operators who have cleared timberland or plowed virgin sod are
well aware of this fact. As cultivation continues on such areas, the soil
becomes less open and friable. Organic matter content is reduced with
a resulting tendency for the breakdown of soil aggregates. As dispersion of the aggregates progresses, the soil particles become more closely
packed together, bulk density increases, and the volume of pore space is
reduced. Aggregate dispersion commonly proceeds more rapidly a t the
soil surface than a t levels below the surface. This area, under cultivation
conditions, is exposed to the beating action of raindrops in addition to
exposure to other forces of structural decline. Difficulties with crust
formation following rainstorms tend to develop as dispersion of aggregates in the surface layer continues. This condition often introduces
problems of securing satisfactory seed germination and usually reduces
the' rate of water absorption, thus increasing runoff losses.
There are many examples in the literature showing the beneficial
effects of permanent vegetation and of the absence of cultivation on soil
physical conditions. The influence of grass-legume mixtures in promoting aggregation of the Hagerstown soil is reported by Alderfer
(1950). The value of sod in rotation with cultivated crops for the maintenance of soil structure is emphasized. Wilson et al. (1947) report
that the percentage of aggregates larger than 2.0 mm. under crops studied was continuous bluegrass > rotation meadow > rotation corn > continuous corn. With the methods of measurement used, the aggregates
formed under rotation meadow and rotation corn were less stable than
those under continuous bluegrass. With a four-year rotation of corn,
wheat, clover, and grass, Elson (1943) found that the soil under wheat
had the same percentage of macroaggregates (larger than 1.0 mm.) as
SOIL MANAGEMENT FOR CONSERVATION AND PRODUCTIVITY
that under corn. Soil under clover showed a 10.3 per cent increase in
aggregation over wheat areas, and the grass treatment brought about a
9.1 per cent increase over clover.
Alderfer and Merkle (1941b) have reported studies relating to the
structural stability and permeability of native forest soils compared
with cultivated areas of the same soil types. In general, it is shown that
soils under forest or under continuous bluegrass are relatively high in
permeability and organic matter content, with relatively low values for
volume weight. Soils under rotation of cultivated crops and grasslegume mixtures show intermediate values, whereas soils under continuous cultivation show comparatively low values for permeability, reduced
organic matter content, and relatively high values for volume weight. It
is pointed out that land under bluegrass or other sod-forming grasses,
if not subjected to compaction, may develop a degree of granulation
equal to or better than that found under forest conditions. Heavily
trampled and pastured sod, on the other hand, may become nearly as
compact and impervious as land used for intertilled crops. This report
emphasizes the fact that when soils are brought under cultivation there
is ordinarily a slow but significant breakdown of structure. This process
is greatest and most rapid when intensively tilled row crops dominate
the cropping system.
The deleterious effect of continuous cultivation on soil structure is
emphasized by Rynasiewicz (1945). Soil organic matter content and
degree of aggregation under different cropping systems varied in the
order of: onions and two years of mangels < onions and buckwheat
< onions and corn < onions and redtop < permanent grass sod. Positive correlation between degree of aggregation and onion yield is shown.
In general, the effect of tillage is to destroy soil structure through
the breakdown of aggregates and granules. The extent of the destructive effect varies widely with different soil types. Considerable variation
may occur within a given soil type depending on moisture conditions
a t the time of tillage and on the type and severity of cultivation operation carried out.
Growing crops influence the structure of the soil both directly and
indirectly. Most of the foregoing examples deal principally with the
indirect effects. These, in general, may be considered as the changes in
aggregation and porosity of the soil resulting from organic matter produced by plant growth. Certain very real direct effects occur, although
it often is difficult and may in some cases be impossible to evaluate them
separately from the influence of added organic matter. The two principal direct effects of vegetation on soil structure are due to the canopy