After the First World War, agriculture was depressed. It was hard to adjust total production downward from the great increases stimulated by the war. Farmers could scarcely make ends meet, and soil depletion increased.

The land-grant colleges were well established and were becoming better, but they had to cover all phases of agriculture in their teaching programs, and the attention to the different phases in research and off-campus education followed roughly the same proportion as in resident teaching. Thus the effort devoted to soil science was small in relation to the growing soil problems on farms.

H. H. Bennett, of the Department of Agriculture, started writing dramatically in the middle 1920's about the growing soil problem, which became worse when a general depression grew: He got attention where others had failed. He did much to awaken the country to the problem.

Bennett and his staff developed a grouping of the many kinds of soil under the "use-capability" concept. They placed major emphasis on relating the kinds of soil to their needs in simple terms. This emphasis started the conservation program on a sound basis. It brought out the major conditions each land user must deal with and furnished a guide to the combinations of practices required for sustained production.

More than 1.6 million farmers plan the use and management of their soil and water resources through about 2,700 soil conservation districts. Other thousands of farmers each year cooperate with their districts.

America's greatly expanded industry has given farmers new tools and chemicals. Research programs in the manufacturing technology of fertilizers and in their use also have expanded.

Instead of basing the use of fertilizers on the analyses of harvested crops, according to the balance-sheet theory of putting in what plants take out, the aim now is to supplement the soil in order to develop and to maintain a balanced supply of plant nutrients at economic levels.

This level of fertility for many kinds of soil is far higher than that of the natural soils. Certain nutrients must be added to the soils in amounts well above those removed annually by plants. Few arable soils require fertilizers containing all the plant nutrients. Some soils need several. Others require only one or two, even after long use. The amounts removed by plants and animals are important, but we have learned to take account of the amounts in the soil that are becoming available to plant roots and of processes going on in the soils that affect these supplies.

Conceivably we may be able some day to draw up a true balance sheet fora soil. To do so we shall need to understand better the mechanics of several processes and know how to measure them more precisely.

These include besides analyses of plants, whole soils, and drainage waters the amounts of nutrients in dusts that fall onto or are washed into the soil by rain, and those removed; the amounts removed by normal soil erosion and those added by the incorporation of new materials into the soil from the underlying rock material; the sources of nitrogen compounds in the soil; and the rate at which nutrients become available or unavailable to plants under different conditions.

These processes are influenced by drainage, irrigation, total amounts of nutrients and the balance between the nutrients and other substances, and different species of growing plants. All are modified by cultural practices.

The use of fertilizer and soil erosion are related. Erosion may change the chemical nature of soils, but the great damage is to soil structure. Commonly a mellow surface soil is removed, and a subsoil difficult to cultivate is uncovered.

The erosion of sloping soil is stimulated by tillage or weakened plant growth by anything that exposes a bare soil to the direct action of wind and rain. On any highly erodible soil, even a highly fertile soil, exposure of the soil through cultivation results in erosion hazards. On many moderately erodible soils, however, the extent of erosion depends upon the vigor of the plant growth under good management practices, other than fertilization.

With low fertility and weak plants, erosion is serious; with high fertility and vigorous plant growth, it is not. Since the use of naturally infertile sloping soils, or of those that have been allowed to become infertile, means weak vegetation, erosion is increased. Thus the accelerated erosion of such infertile soils is more an effect of low fertility than it is a cause of the infertility.

A first step in the control of runoff and erosion on such soils, we have learned, is adequate fertilization. A fertile soil has a wider range of adapted crops, including grasses and legumes, than an infertile one. The problem may be solved with vigorously growing plants or with such plants and supplementary engineering works that would be ineffective by themselves or economically impractical at low levels of crop yields.

As a result of the developments in the use and supplies of fertilizers, a part of our emphasis in the study and teaching of soil management has been shifting from soil fertility to the physical properties of soils. In contrast to the early 1920's, few farm managers with adequate capital and skill need to allow soil fertility to be a limiting factor. We have methods of testing soils and recommendations for fertilizing for most kinds of soils. We have reasonably good fertilizer materials at prices that generally have been going down.

In fact, the modern farmer is not primarily concerned with the productivity of his soil when it is first plowed. He is concerned with the responsiveness of the soil to management the physical condition of the soil, the nature and stability of its structure, which controls its permeability by roots and water, and the maintenance of an adequate amount of available moisture.

THUS, FROM THE LORE, skills, and experience of many generations and the experiments, theories, and researches, we have learned a great deal about soils and how to use them.

We have learned that soils have depth and area. They can be defined, understood, classified, and mapped. Soil maps give us the means for firm connections between the experimental plots and the millions of fields. The capability of the soils for use can be predicted as a basis for farm decisions and the classification of land.

We have learned that any kind of soil is a complex combination of characteristics, no one of which has meaning by itself and apart from the others. A whole group of characteristics, each influencing the others, makes up the soil that will respond, for better or for worse, to the care we give it.

We have learned that the most efficient systems of soil management are combinations of practices, fitted to the unique kinds of soil in ways that realize the benefits of the many interactions among the separate processes and the several characteristics of the soil.

We have learned that soil-management practices can be adjusted with increasing accuracy to specific fields. Broadly defined combinations of treatments, or "shotgun" recommendations, can be replaced with specific ones that avoid the wasteful use of power, chemicals, and water.

We have learned that many of our so-called poor soils can be built up and maintained for efficient production and that there are few conflicts between those systems of management that give the greatest economic return and those that insure the continued improvement and conservation of the soils.

We have learned that many kinds of soil that gave low yields with the practices of only a generation ago can be used efficiently now. If those soils are protected under grass or trees, the United States can increase its acres of cropland very greatly if the need arises. Thus we have many choices in soil use and no real need to use unresponsive or high-risk soils for cultivated crops.

What we seek is not some kind of mythical natural balance between farmers and the soils they cultivate, but a cultural balance in which we use with understanding and precision all the tools of modern science, engineering, and economics.