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III. Soil Management for Structure Maintenance

III. Soil Management for Structure Maintenance

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SOIL MANAGEMENT FOR CONSERVATION AND PRODUCTIVITY



391



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



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protection afforded by leaves and stems against raindrop impact and to

the influence of root activity on soil physical conditions.

The effectiveness of different crops at various growth stages in intercepting rainfall has been shown by Haynes (1938). Ellison (1944,1948)

has emphasized the influence of raindrop impact on bare soil in dispersing soil aggregates, in compacting the surface layer, and in initiating

runoff and erosion. Ekern (1950) reports on a study concerning the

energy relationships of drop impact comparable to that found in natural

rainfall. He reports that this is a mechanism of sufficient quantitative

importance to be responsible for much of the accelerated sheet erosion

from cultivated soils.

Bmer (1948, p. 182) quotes data reported by Wollny in 1874 showing that rye, peas, and vetch protected the soil to such an extent that the

noncapillary porosity of a calcareous sandy soil was 34-53 per cent

higher than that of an adjacent unprotected soil. This report indicates

that canopy protection is particularly effective in maintaining the content of large pores in the soil. The deterioration of structure resulting

from raindrop impact was evident more as a decrease in the volume and

number of large pores than as a decrease in total porosity, An example

given shows the total porosity of an unprotected soil to be about 6 per

cent less than that of a soil protected by vegetation. Nonoapillary porosity, however, was reduced by 31 per cent on the unprotected soil area.

Structure deterioration and cornpaction of the soil surface is a matter

of considerable importance from the standpoints of both crop production

and water conservation. Compacted surface layers, resulting in crust

formation, may interfere seriously with seed germination and successful

establishment of seedling stands. In addition, the reduced rate of water

absorption through the compacted layer often results in excessive rates

and amounts of surface runoff, regardless of the structural and watertransmitting properties of the soil immediately below the surface layer.

Protection of the soil surface against aggregate destruction is one of several factors responsible for the effectiveness of grass-legume mixtures

and other close-growing vegetation in both structure maintenance and

conservation of soil and water.

It is difficult, if possible at all, to distinguish between direct and indirect influences of root growth on physical conditions of the soil. It

seems obvious that organic matter introduced at various soil depths

through root growth would, upon eventual decomposition, exert a favorable influence on soil structure. This, in fact, is the only practicable

means of introducing fresh organic materials at levels below plow depth,

In addition, the pressures exerted by growing roots, the binding action of

fine fibrous roots, and the wetting and drying cycles resulting from water



SOIL MANAGEMENT FOR CONSERVATION AND PRODUCTIVITY



393



usage and replenishment would all be expected to contribute to improved

aggregation and general striictural properties of the soil.

2. Crop Rotations



It has been pointed out in the foregoing sections that soil organic

matter is one of the important factors influencing soil structural conditions. It is also indicated that cultivation tends to bring about deterioration of structure. This latter effect appears to be the result of increased

rates of organic matter decomposition, reduced amounts of vegetative

residues returned to the soil, exposure o€ the soil surface to rainfall action, and compaction of the soil by cultivation machinery. Doubtless

other factors are involved.

Despite the unfavorable effects of cultivation on soil structure, there

seems no doubt that cultivation will continue to be necessary a t some

level of intensity in many agricultural enterprises. The problem, then,

is to devise crop and soil management systems which permit the production of cultivated crops and, at the same time, provide sufficient organic

matter additions to maintain favorable soil structural conditions. There

is little possibility and, for that matter, no particular need to maintain

the organic matter level in cultivated soils at levels found commonly in

virgin soils. The real need is for a cropping system which provides additions of good-quality organic material to the soil a t regular intervals.

It is then desirable, from the standpoint of soil structure maintenance,

that decomposition of the organic material proceed a t a reasonably rapid

rate. Organic matter decomposition, in addition to the beneficial effects

resulting from nutrient release, is desirable and necessary under average conditions in order to maintain the soil in a well-aggregated condition which is resistant to erosion and in order to maintain a productive

condition. The accumulation of a large amount of organic material that

was highly resistant to decomposition would not necessarily contribute

to good physical condition of the soil o r to soil productivity. The desirable condition, as pointed out, is one where a system of crop and soil

management is so arranged that additions of good-quality organic material are made a t regular intervals. The normal decomposition processes

that contribute to good physical condition of the soil can then proceed

without danger of the eventual exhaustion of soil organic supply.

A practical and time-proved method of maintaining favorable physical conditions in cultivated soils is through the rotation of cultivated

crops with grass-legume sods or other close-growing, noncultivated crops.

Bailey and Nixon (1948) state that “the ideal rotation is one that

achieves complete harmony between the farmer’s demand for cultivated

crops and the needs of the land for protection.” It is pointed out that



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0. R. NEAL



a good rotation is one that includes enough grass and legumes to reduce

soil and water losses, reduce leaching of plant nutrients, maintain organic matter supply, improve soil structure, and increase acre yields of

cultivated crops in the rotation. This rather large order seems entirely

within the realm of possibility. Effects of grass-legume sod in rotation

on organic matter and soil structure have been shown. The influence of

such rotation systems on soil and water losses and on yields of cultivated

crops is discussed below.

IV. INFLUENCB

OF ROTATIONS

ON CONSERVATION

AND PRODUCTIVITY

The influence of crop rotations and resulting differences in physical

condition of the soil on productivity and on soil and water losses is closely

interrelated. The improvement in aggregation and porosity of the soil

resulting from the sod rotation would be expected to increase the rate

and amount of water absorption during rainfall periods. This, in turn,

would decrease runoff and accompanying erosion and provide more favorable moisture conditions in the soil. Decreases in the amount of

erosion would reduce the extent of nutrient loss from the soil. Whitson

and Dunnewald (1916) reported that erosion sediment contained 3 times

the concentration of nitrogen and 2 times as much phosphorus as was

contained in the soil from which the sediment came. Miller and Krusekopf (1932) report similar data emphasizing the selective nature of the

erosion process. The writer (1944) showed that eroded material from

a loamy sand soil contained about 5 times as much organic matter and

and 1.4times as much K20 as compared

nitrogen, 3.1 times as much P205,

with the concentration of these materials in the surface soil. The soil

contained about 16 per cent silt and clay, whereas the eroded material

contained 58 per cent of these size fractions. Productivity of the soil

would thus be expected to be gradually reduced as the erosion process

continued. That this is the case has been reported by Uhland (1940),

who shows that corn yields varied inversely with the extent of erosion.

It was found by the writer (1943) that potato yields under identical

conditions of fertilization and cultural practices varied from 274 bushels

per acre on eroded areas to 343 bushels from relatively uneroded areas.

The extent of the reduction in productive capacity of the soil resulting from erosion is shown in a report by Lamb et aE. (1950). Areas in

corn receiving uniform fertilization of 1000 pounds per acre of 10-10-10

fertilizer show yields varying fsom 17 to 88 bushels per acre, depending

on the amount of past erosion. Other tests showed yield variations from

40 to 106 bushels and from 54 to 82 bushels per acre, depending on past

erosion. After two years of uniform cropping in an alfalfa, clover,



SOIL MANAGEMENT FOR CONSERVATION AND PRODUCTIVITY



395



timothy sod, areas of Honeoye soil showed variations in corn yield from

49 to 69 bushels per acre, depending on the amount of erosion prior to

the sod treatment.

The above and other reports have shown that, in general, past erosion

reduces crop yields as compared with areas under similar cultural conditions where less erosion has occurred. From a comparatively long-time

viewpoint, effects of grass-legume rotations in maintaining or increasing

yields thus appear to be due in part to conservation effects of the rotation system.

Effects resulting from the rotation of cultivated truck crops with

grass-legume mixtures at regular intervals are reported by Neal and Brill

(1951). It is pointed out that the practice of growing cultivated crops

in rotation with grass-legume mixtures or other close-growing, noncultivated crops has long been followed in certain agricultural areas. It is

commonly recognized that such cropping practices aid in soil organic

matter maintenance and in weed and disease control, and improve soil

productivity. More recently it has become evident that such practices

improve physical conditions of the soil, thus providing better aeration

and drainage and reducing runoff and erosion losses. I n many areas,

however, the above factors have been only of incidental importance in

determining the cropping system to be followed. Economic need has

been the primary consideration. I n general farming areas and on dairy

and other specialized livestock farms there is commonly a need both for

the cultivated grain crops and for the forage crops produced in a good

rotation. I n such enterprises cultivated crops are commonly grown in

regular rotation with small grain and with grass-legume mixtures. The

rotation study reported here, however, was carried out on a New Jersey

Coastal Plain soil used for vegetable crop production. The soil type is

a Freehold loamy sand. I n this and similar vegetable-producing areas,

little or no livestock is kept on many of the farms. The replacement of

horses, as a source of farm power, by tractors and trucks has removed

all need for grass and legume crops as animal feed on these farms. I n

this situation the decision as to whether or not cultivated crops will be

grown, in rotation with sod crops rests on the effects of such a rotation

on soil and water conservation, on the physical condition of the soil, and

on soil productivity. I n the absence of immediate economic need for

the forage crops, many Coastal Plain areas have been cultivated continuously during recent years in the production of vegetable crops. Despite heavier fertilization, improved methods of disease and insect

control, and generally improved crop varieties and cultural practices,

the acre yields of a variety of vegetable crops have declined under this

system of soil management, as shown by Carncross (1948). It appears



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III. Soil Management for Structure Maintenance

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