Soil Requirements and Efficient Use

Some of the most significant field and laboratory investigations relating to the use of phosphate fertilizers have been concerned with the determination of the phosphorus requirement of different soil types under various farming systems, and with more efficient methods of use. The results have given farmers a sounder basis for determining the need for phosphorus and estimating the amount required.

That the efficiency in use of phosphate fertilizers depends also on other soil conditions and on crop requirements is illustrated by some recent investigations in Mississippi and Iowa. In the South, oats and corn often have not shown profitable responses from phosphate fertilizers despite the low amounts of soluble phosphates in the soils and low average yields. The recent work of Russell Coleman in Mississippi shows, however, that this lack of response to phosphate may be due largely to inadequate amounts of nitrogen. Where no nitrogen was applied in fertilizers, an application of 200 pounds of superphosphate (16-percent P201) an acre did not increase the yields; but where 48 pounds of nitrogen was used, the phosphate increased the yield of oats by 17.3 bushels and that of cotton by 215 pounds of seed cotton an acre.

Similar results were obtained by L. B. Nelson, Kirk Lawton, and C. A. Black in Iowa. In 22 experiments with oats conducted in different parts of the State in 1945 they found that the use of superphosphate (20-percent P,05) at the rate of 200 pounds an acre increased the yields by only 3.1 bushels an acre where no nitrogen had been applied, but by 7.8 bushels an acre where 40 pounds of nitrogen was used.

Studies have also been continued on better methods of applying phosphate fertilizers. In general, band placements have been found to be most satisfactory, but the best methods vary considerably with the kind of soil, the crop, and the kind and amount of fertilizer used. On soils that combine strongly with the phosphate to make it relatively unavailable to crops, the application of soluble phosphates in bands rather than broadcast has been found to be particularly advantageous. With relatively insoluble forms of phosphate, however, mixing of the fertilizer with the soil appears desirable.

Investigations on the deep placement of fertilizers have been stimulated by the desire to increase the rate of fertilization in order to obtain maximum production. This is particularly true of nitrogen fertilization. In experiments of W. H. Metzger and Floyd Davison in Kansas, the placement of phosphate fertilizer in the row at a depth of 6 inches resulted in considerably greater yields of sorghum than did more shallow placement. Experiments with corn in several Midwestern States, however, have shown no such advantage. It remains to be established, therefore, whether deep placement of phosphate is desirable, especially where only small amounts are applied and where the plants are likely to suffer from phosphorus deficiency before the root system contacts the deep placement zone. The greatest advantage of deep placement would seem to be in the case of nitrogen fertilizers and in areas of limited summer rainfall.

Residual Value of Phosphate Fertilizers

Phosphate fertilizers have long been used along with barnyard manure, because of the fact that manure is low in phosphorus as compared with nitrogen and potassium. Investigations by Alvin R. Midgley and David E. Dunklee in Vermont have re-emphasized the value of the practice. They found that greater increases in crop yields result from applying the manure and phosphate together than when applied separately.

When soluble phosphate fertilizers are added to soils they form new compounds that are largely insoluble in water and only partly available to plants. For that reason usually not more than 10 to 20 percent of the phosphorus in fertilizers is used by the crop to which it is applied. The remainder accumulates in the soil although much may be lost by erosion. A major problem in the efficient use of phosphate fertilizer in areas of intensive use, therefore, is to know the amount of phosphorus that accumulates, how it may be kept in forms most available to plants, and to what extent fertilizers may be needed by succeeding crops. There is some evidence to show that on soils used largely for such crops as citrus, potatoes, tobacco, and vegetables, a large accumulation of phosphorus has taken place.

Early in 1944 soil scientists at Beltsville initiated a cooperative study with a number of State agricultural experiment stations for a comprehensive study of the problem. Samples of soil collected from 425 fields in the important potato-producing areas of Alabama, Maine, Maryland, North Carolina, New Jersey, New York, and Virginia were analyzed for total and readily soluble phosphorus. Field experiments are also being conducted on selected farms for determining to what extent present changes in phosphate fertilization practices might be warranted. The results obtained thus far show that the plowed layer of soil in many of the older potato fields has accumulated as much as 800 pounds of phosphorus. There has also been a marked increase in readily soluble phosphorus, the amount depending on the chemical characteristics of the soil and on the farm management practices followed. The longer the soil has been farmed under intensive fertilization the greater has been the accumulation of phosphorus.

Other evidence that the needs for phosphate fertilizers may be materially affected by past fertilization is shown by the results of a 15-year experiment conducted at the Alabama Agricultural Experiment Station under the direction of Garth W. Volk and L. E. Ensminger. Where the superphosphate (20-percent P,O,) application of 300 pounds an acre annually was reduced one-half after 5 years, the yield of cotton remained fairly stable at about 95 percent of the yield obtained where the 300-pound annual application was continued. Where the applications of phosphate were omitted after the first 5 years, the yields dropped to about 85 percent within 3 years and to 65 percent within 8 to 10 years. Even after 8 years, however, the yields were still about twice as high as where no phosphate had been applied during the entire 15-year period.

The amounts of superphosphate used in this experiment were, of course, much higher than those usually applied in a legume-livestock or grain system of farming. Not only is the amount of residual phosphorus less where the rates of fertilization are lower, but the, availability of the residual phosphorus is also lower. Moreover, the very fine sandy loam soil on which the experiment was conducted was relatively low in clay. Soils high in clay, especially those high in reactive iron and aluminum compounds, usually show low residual effects from applied phosphate.