How to develop and maintain soil productivity? Information on broad land use, cropping system, tillage, water control and fertilization.
DEVELOPING AND MAINTAINING SOIL PRODUCTIVITY
The methods used to make a good arable soil and to maintain it depend upon the kind of soil the farmer or gardener starts with. Practices that promote the ideal arable soil on one kind of soil are sometimes ruinous on another kind, even on the same farm. Unfortunately, there are no known practices by which many kinds of soil can be made productive, although man is learning how to use more and more of them. Eight kinds of decisions that are involved in a good system for soil conservation are discussed below.
Broad Land Use. In the process of developing a rational plan for soil use, decisions are made on uses for forestry, longtime pastures, and crops requiring tillage, according to the basic capabilities of the soils. The costs of the practices necessary for their sustained use are considered, together with the expected income from the harvests over a long period—generally 10 years or more.
Cropping System. Each cultivated tract needs a cropping system that is adapted to the soil and makes its maximum contribution to the production unit—farm, plantation, or garden. Commonly this means a rotation of crops, including grasses to develop good structures in the surface soil, and deeply rooted plants to maintain structure in the subsoil. Continuous culture of some kinds of plants encourages soil-borne diseases, yet if structure can be maintained otherwise and there are no diseases, continuous culture of one crop may be best. Thus, repeated corn-growing on a sloping, slowly permeable soil leads to ruinous soil erosion, yet some smooth fields in the tropics have grown sugarcane year after year for as much as 300 years with gradually increasing yields.
Crop rotations have merit mainly as they help spread the work load and help maintain the ideal soil. Roughly one half of the arable soils in the world, including meadows, should perhaps be used under rotations of adapted crops. Many soils in the tropics need a period of 5 to 15 years in restoration shrub or forest for each 3 to 6 years in crops. In semiarid regions on unirrigated brown and chestnut soils, crops of barley or wheat are alternated with clean tillage or fallow. For one year weeds are kept down and soil moisture accumulates, while the next year the crop grows with the stored water and the current rainfall. In the driest places of such arid regions, one crop may need to be planted after two years of clean fallow. Unless well managed, such fallow fields present a hazard of soil blowing, which leads to great dust storms during widespread droughts.
Tillage. Tillage methods are needed to make the surface soil receptive of water; keep the surface cloddy where dry winds may strike the bare surface; incorporate needed lime, organic matter, and fertilizer into the soil; maintain good structure in the subsoil; and control weeds. Generally farmers still till the soil too much. The surface soil is often finely pulverized and made ready to blow away in dry seasons and to puddle in moist periods. Excessive tillage, especially when the soil is wet, injures structure. With modern chemical weed killers, some tillage can be avoided. Heavy machines compact the subsoils of some soils and reduce the movement of water and air. Natural claypans and hardpans and man-made “traffic” pans in the subsoil can be broken by deep chisels. Commonly their breaking must be accompanied by deep fertilization and the growth of deeply rooted plants to be permanently successful. Without organic matter, the soil returns to a massive condftion.
Soils that have sandy surface materials that are subject to blowing can be improved by deep plowing during periods of moderate moisture if the subsoil has a good prismatic or blocky structure. However, deep plowing of sandy soils with loose sandy subsoils only worsens the situation by burying the organic matter that should be left in the very surface for protection.
Water Control, Use, and Disposal. Each field and garden needs an orderly system of water control and disposal. Some farmers have naturally well-drained soils and reasonably dependable rainfall, but many do not. The erosion hazard is much greater in some areas than others, even where the natural soils are similar. On the whole, for example, soil erosion is not a serious problem in the northwestern part of Europe, except on steep slopes because of the gentleness of the rains. In contrast, erosion is a serious hazard in much of the United States because of the great severity of the individual showers.
When rains come with great intensity, even permeable soils cannot absorb all of the water. Some runs off, and it must be made to flow away slowly and harmlessly by some combination of the following: close-growing plants, such as grass or trees or the two together; or alternate strips to cultivated crops, laid across the slope in strip-cropping. Thus instead of planting a whole field to one crop, the farmer may have alternate strips of cotton or corn; of barley, wheat, or oats; and of meadow. In following years he can alternate his crops so that each strip has a rotation. In addition, diversion terraces or ridges may be built to intercept excess water at the upper margin of the field and guide it to prepared, erosion-proof channels. Terraces can be built as ridges of earth or walls of masonry across the slopes to make long winding areas of soil with reduced slopes. The terraces slow down the water that falls on the field, thus giving more time for it to enter the soil and for the excess to flow away to prepared outlets.
Terraces must be kept in continual repair, because at a break in a terrace the water is concentrated, and a deep gully may form rapidly. Masonry terraces are used a great deal on hilly soils in southern Europe and in other places, but they are costly to build and are handicaps to the use of machinery. Then too, terraces are not useful if the soils are very thin, and the farmer must be sure to have a deep rooting zone after they are installed. Finally, by plowing and planting on the contour, at right angles to the slope instead of up and down the slope, runoff can be sufficiently slowed down on permeable soils.
Some of the most productive arable soils of both Europe and the United States have been made from naturally wet soils. However, other wet soils are useless for crops even with drainage. These should be left as wetlands for wildlife and swamp forests.
Early irrigation was carried out mostly on desertic soils crossed by surface or underground streams from rain-collecting mountains, but with the coming of cheap and abundant fertilizers, water has become the limiting factor even in so-called humid regions. Whereas farmers on desertic soils furnish nearly all the water for their crops through little ditches supplied from irrigation canals, farmers in humid regions commonly use sprinkler irrigation during the occasional periods of drought. In all cases of irrigation, the soil must have good drainage to prevent waterlogging. Thus many clayey desertic soils have to have drains as well as irrigation canals. Since irrigation structures are expensive, successful farming under irrigation requires the maintenance of nearly ideal soil, with both careful water control and a good balance of plant nutrients.
Use and Conservation of Organic Matter. Soil organic matter is helpful in maintaining the structure of soils, both in the surface soil and in the subsoil. It supplies energy for microorganisms, including some that fix nitrogen from the air. As it decomposes, it furnishes to plants a slow, continuous, balanced supply of plant nutrients. It has no mysterious function; it is useful only to the extent that it contributes to the ideal arable soil. Yet for many clayey soils there are no practical alternative methods for maintaining structure except through growing deeply-rooted legumes to get organic matter into the subsoil, and the continual incorporation into the upper soil of manure, compost, and crop residues balanced with nitrogen. Organic crop residues and the roots of grasses offer the farmer a way to stabilize the surface of sandy soils and other soils that are subject to severe blowing during dry periods.
Reaction Control. On acid soils the use of lime —most commonly finely ground limestone—is the first requisite for developing a good system of soil management. Until that is done the other practices are not effective, and the high-yielding protein-rich hays, so necessary for livestock and for the stability of the ideal rooting zone of many soils, cannot be grown.
Protection. Both soils and crops need to be protected against winds, beating rains, and diseases. Water-control devices and vigorous vegetation protect the soil against erosion. Besides the maintenance of a cloddy surface soil, windbreaks are needed where winds are very strong and soil blowing is a hazard. In some places, belts or strips of living trees serve best. ‘ Where trees do poorly or land is very expensive, barriers of woven canes or closely spaced pickets are used. Systems of crop rotations that change host plants also help to control many diseases and insects. For intensive vegetable and fruit production, soils may be treated with pesticides that destroy many types of nematodes, worms, and other organisms.
Fertilization. Essentially all soils used intensively require treatment with chemical fertilizers for economic production. Although some soils may not need them when first cultivated, most of them require at least phosphorus and nitrogen for economic yields. Next in general need is potassium, but none of the plant nutrients can be taken for granted.
In the 20th century, chemical fertilizers have been improved, cheapened, and made more abundant. Man’s present food supply and the efficiency of producing it would be impossible without fertilizers. Not only have the fertilizers been improved, but also the methods of soil testing. In most of the advanced countries, farmers and gardeners can have soil tests made and receive excellent advice on the fertilizers to apply. The soil test results alone, however, are not reliable without considering the kind of soil and what crops are to be grown.
Because of these developments man is less concerned with the “inherent” fertility of a soil than he once was. What matters more to th^ modern farmer is the physical condition of the soil and its water supply. Nutrient deficiencies can be corrected more easily than hardpans, slopes, and other difficult physical conditions. Thus the measure of the productivity of soil in practical agriculture is its response to management—the harvest to be had in relation to inputs of labor and materials. Many of the most productive soils now used give little harvest with simple clearing and cultivation alone, but with proper chemicals to produce an ideal nutrient balance and with devices for controlling the water, they may be very productive indeed. For some time yet man may expect greater increases in production from the soils now being used than from new soil being brought into use.
Soil Conservation. If a farmer or gardener can look at his management system over a 10-year period and not simply from one year to the next, he finds that those sets of practices that improve his soil are also the ones that give him the greatest return. This is a key fact for education on soil conservation.
Generalization about individual soil practices in the world, in a country, or even on one farm is impossible. Soils are unique. They respond individually to unique combinations of practices.
Both gardeners and farmers must beware of the advocates of any single practice above others. What is good for one soil may not be for another. Organic matter is much more important in some places than others. Some soils should be plowed deeply and others very little or not at all. Some soils must be devoted periodically to legume-grass meadows to remain productive, others not. The heavy applications of fertilizers that are best for some soils injure others.
The world has abundant soils of good quality. In some areas soils have deteriorated from erosion and other processes resulting from poor management, and much good soil is being lost to agriculture through the encroachment of urban areas—subdivisions, airports, factories, and the like—most of which could be put instead on soils that are unsuited to fanning. On the other hand, soil conservation is improving in many countries, and production from the soils now being used in the various countries could probably be increased by 20% to 100% with the arts of soil management now known to be practical. With the parallel development of industry, transport, and medical services, large remote areas could be brought into intensive use. A great agricultural development in the tropics may lie in the future.