Systems

Cropping Systems and Soil

W. H. Allaway.

Some of the first agricultural experiments in these United States consisted of comparisons of different cropping systems. Their results convinced many agricultural leaders that crop rotations that regularly included a legume sod crop were needed for lasting production.

But a Mississippi 4 H Club boy in 1955 raised 304 bushels of corn an acre, a new record, on a field that had been in corn for at least 6 consecutive years and had never produced more than about 15 bushels an acre before 1950.

Corn has been grown continuously on the Morrow plots at the University of Illinois alongside a 3-year rotation of corn, oats, and meadow since 1876. The results from these comparisons formed one of the cornerstones of the belief that rotations were essential to the maintenance of soil productivity.

But liberal amounts of nitrogen, phosphate, and potash fertilizers and lime were applied in 1955 to part of each plot in this experiment. Where this fertilizer was applied, the 79th consecutive crop of corn yielded 86 bushels an acre. It made 36 bushels an acre without the fertilizer. Corn on plots that had been in a corn-oats-meadow rotation with no soil treatment for 79 years made 63 bushels an acre without the fertilizer and 102 bushels where the nitrogen, phosphate, potash, and lime had been added in 1955. Plots that had been receiving regular treatments of manure, lime, and rock phosphate since 1904 yielded 79 bushels an acre without the extra fertilizer in 1955 and 98 bushels when they were liberally fertilized.

Corn plots of the corn-oats-meadow rotation, treated since 1904 with manure, lime, and rock phosphate, made 100 bushels an acre without the added fertilizer in 1955, and 101 bushels with the added fertilizer. Thus, on the Morrow plots, differences in soil productivity built up by 79 years of different cropping systems were reduced by one application of chemical fertilizer.

Other crops also produce well in continuous culture. Most of the sugarcane and much of the wheat, cotton, and rice raised the world over are grown on the same soil year after year.

On the other hand, crop rotations that include a legume or legume-grass sod at frequent intervals have contributed to the productivity of our soils and to the permanence of American agriculture. To be convinced of this, you need only to look at some of the fine general farms in Pennsylvania and Ohio, where rotation with clover or a grass-legume mixture has been used for many years. Today these farms are more productive than ever.

All this points up the fact that to maintain productivity many farmers can choose their rotation or cropping system from among many possibilities. The decision as to which method to choose is often of critical importance in determining farm income. In considering the choice, one should distinguish between a cropping system and a soil-management system.

A cropping system is the kind and sequence of crops grown on a given area of soil over a period of time. It may be a regular rotation of different crops, in which the crops follow a definite order of appearance on the land, or it may consist of only one crop, grown year after year on the same area. Other cropping systems may include different crops but lack a definite and Planned order in which the crops follow one another.

A soil-management system includes the cropping system plus the other practices, such as the use of fertilizers, terracing, irrigation, and drainage, that accompany the cropping system. When it comes to maintaining productivity, the effect of the cropping system may depend on what other practices are used with it in making up the system of soil management.

What effects might each crop have on the properties of the soil? Will any particular one of these crops make the soil better or worse for the next crop?

Some plants add to the supply of nitrogen, an important plant nutrient. Inoculated legumes can get a good share of the nitrogen they need by fixing uncombined nitrogen from the air. Nonleguminous crops must depend on chemically fixed sources of nitrogen from decomposition of the soil organic matter, manure, or chemical fertilizers. When a legume like alfalfa or a clover is grown and at least part of the crop is plowed under for the next crop, the supply of available nitrogen in the soil will be increased.

The amount of nitrogen a legume adds to the soil depends on its kind and the way it is managed. Soybeans are a legume, but most of the nitrogen they have taken from the air is carried off in the form of the proteins in the seed when the beans are harvested.

Alfalfa, sweetclover, red clover, and Ladino clover are among the more effective legumes for building up nitrogen. As a general rule, the more top growth turned under when a legume field is plowed, the more nitrogen is added to the soil. The amounts of nitrogen added to a field soil by legumes is not accurately known because they are hard to measure. Very likely a perennial such as alfalfa may add 100 pounds an acre of nitrogen to the soil if some of the top growth is turned.

Legumes often are grown with grasses. When such a mixture is plowed for the next crop, the organisms that decompose the carbonaceous residues of the grasses utilize the simple nitrogen compounds that the decomposing legume residues liberate. The net effect is to prolong the release of simple nitrogen compounds for use by the next crop. It may help to prevent losses of the legume nitrogen by leaching and provide some of the nitrogen requirements of nonlegume crops.

In most experiments with crop rotations, yields of grains have been higher when they were rotated with legumes than when they were grown steadily, without rotation. In many of the experiments one could not determine exactly the extent to which the increased yields were due to the nitrogen the legume added to the soil and the extent to which the increases were due to improved soil structure or fewer insects and diseases.

Sometimes nitrogen fixed by legumes can meet most of the requirements for the other crops grown. In some instances it may be best to depend largely on nitrogen fertilizers or manure. In a few soils, high in organic matter, the release of nitrogen by decomposition of the native organic matter may meet most of the requirements of the crops.

Some crops cause marked declines in available nitrogen. If the part of the crop harvested and removed from the soil is high in nitrogen, the removal by itself may represent a critical depletion of the supply of nitrogen. If the crop is one that leaves a large carbonaceous residue and the residue is returned to the soil, the organisms decomposing it will temporarily tie up most of the simple compounds of nitrogen in the soil. This nitrogen will be kept in forms unavailable to succeeding crops until carbonaceous residues decompose.

Crops have different effects on the supply of mineral nutrients. Some crops can feed better than others on the less available forms of some of the mineral nutrients. When these strong feeders are grown, turned under, and decomposed, the minerals they took up are converted to readily available forms for the following crops.

E. E. DeTurk, of Illinois, discovered that sweetclover takes up phosphorus from rock phosphate more readily than corn and wheat do. When sweetclover was turned under, the supply of readily available phosphorus in the surface soil was increased. In addition to their ability to convert relatively insoluble phosphorus minerals into forms more readily available to other crops, sweet-clover, alfalfa, and some other deep-rooted legumes tend to bring phosphorus up from the deeper soil layers and deposit it on or near the surface of the soil if not all of the top growth is harvested.

Buckwheat also can feed on difficultly available forms of mineral nutrients and convert them to more readily available forms.

A system "a bush-fallow" of alternating food crops with forest is practiced on many areas in tropical countries. A great deal remains to be learned about the mechanisms involved in it, but at least part of the beneficial effect seems to come from nutrients the forest trees transfer from deeper horizons to the surface soil.

Obviously the effect of plants upon the availability of nutrients in soil is different in different soils. Thus, Dr. DeTurk reported that sweetclover had markedly improved the available phosphorus status of soil in some of the Illinois experiment fields but had relatively little effect on other fields.

Heavy and frequent applications of available nutrients in the form of fertilizers on some soils has made less urgent the need to include one of the "strong feeder" crops in the rotation in order to make sparingly soluble minerals available.

DIFFERENT CROPS have different effects on soil structure. The way soils take in water and permit it to move through the profile depends on the way the soil particles are arranged into granules or aggregates. The storage of water in the soil, use of this water by plants, and the exchange of gases between the soil pores and the atmosphere also depend on soil structure.

Conditions that provide large pores within the soil mass for rapid movement of water and air, combined with small pores for the storage of water are desirable, in general. The grouping of individual soil particles into aggregates, which resist the slaking action of water and resist crushing during tillage operations, is an essential feature of good soil structure.

When an intertilled crop is grown, the stirring of the soil in preparation of the seedbed and cultivation tends to break down the structure of the soil, destroy many of the aggregates or granules, and reduce the proportion of large pores.

Because decomposing organic matter provides substances that help to cement soil particles into stable aggregates, the crops that return large amounts of organic residue usually have a beneficial effect on structure.

The roots of certain crops also affect the structure of the lower horizons. The thick taproots of crops such as kudzu, sweetclover, and alfalfa penetrate some soils to considerable depth. When they die and decompose, they leave a channel or pore through which excess water can drain from the profile and down which the roots of the next crop can grow more easily.