Phosphate Fertilizers by W. H. PIERRE O.FNLY A LITTLE more than a century ago Sir John Lawes of England first produced soluble phosphate fertilizers by treating ground bones with sulfuric acid. About 25 years later his process was apphed in the United States to phosphate rock, deposits of which had been found in South Carolina. Little was known then about the use of phos- phate fertilizers, but information on the phosphate needs of Ameri- can soils gradually accumulated, and the phosphate fertilizer industry soon became well established. Today, nearly 4 million tons of phosphate rock are mined annually for the production of superphosphate fertilizers and for direct appHcation to the soil. American farmers spend about 200* million dollars annually for phosphorus, the important plant food ele- ment that is added to soils through the use of phosphate fertihzers. Of the total amount of phosphorus found in soils, only a small per- centage is in a form readily available for use by plants. Most of it is found in compounds from which plants cannot obtain sufficient amounts for rapid growth and maximum yields. Cropping results in a continuous removal of the most available soil phosphorus. Furthermore, as H. T. Rogers in Virginia and O. R. Neal in New Jersey have shown, it is this small but most valuable portion of the total soil phosphorus that is most readily lost by erosion. But the problem of phosphate fertilization is not simply one of adding to soils an amount of phosphorus equal to that removed by crops or lost by erosion. Soils differ greatly in the kinds of phosphorus compounds they contain and in their ability to pass along to the plant the phosphate added in fertihzers. Moreover, crops vary in their ability to use the phosphorus compounds of the soil and in their response to phosphate fertilizers. The kinds of phosphate fertilizers used and the way in which 554 PHOSPHATE FERTILIZERS 555 they are applied are other factors that affect the returns obtained. Only through a better understanding of such factors can farmers bring about the most efficient use of phosphate fertilizers and insure adequate con- sumption for soil improvement and conservation. The amount of phosphate as well as other fertilizers used on American farms reached an all-time peak in 1946, largely because of the great war-born demand for agricultural products and the relatively high level of farm income. Despite variations through the years, associated with changes in farm income, the trend in the use of phosphate fertilizers has been definitely upward. As with other fertilizers, the use of phosphate fertilizers varies in the different States. Even in the East, where rainfall is not the limiting factor in crop production, large differences exist; in 1943, seven Southeastern States used 224,609 tons of phosphorus in fertilizers, or more than 40 percent of the total used in the United States. The use of phosphate fertilizers in the various States is in sharp contrast to the calculated amounts of phosphorus removed in harvested crops. Five Corn Belt States—Ohio, Indiana, Illinois, Iowa, and Missouri—for example, removed in harvested crops in 1943 more than 30 percent of the 740,000 tons of phosphorus removed in the entire country, according to calculations made by J. H. Stallings. And even though half of the phos- phorus contained in crops may be returned to the soil in manures, crop- ping results in a much heavier drain of phosphorus from the soils of the Corn Belt and other North Central States than from those of the Eastern States. Eastern farmers use relatively large amounts of phosphate ferti- lizers because their soils are inherently less productive and have been farmed longer than the soils to the west and because of the kinds of crops they grow and their type of farming. Potatoes and truck crops usually give a high net return from fertilizers because of their high acre value, and ordinarily receive at least a ton of fertilizer to the acre. Tobacco and cotton get much more than are usually applied to grain and hay crops. Most of the phosphate fertilizer used in the United States consists of ordinary superphosphate, containing 16 to 22 percent PgOr,, and con- centrated superphosphate, containing 40 to 45 percent PsOr». Both materials contain monocalcium phosphate, a form readily available to crops. Finely ground phosphate rock also is used for direct application to the soil. It is less readily available to crops than is superphosphate, but when used with legumes in the cropping system it has proved effective in building up the productive level of acid, phosphorus-deficient soils. Since about 1934, extensive investigations have been in progress to determine the value of several new phosphate materials prepared by the Tennessee Valley Authority for use as fertilizers. Most of the experimental work was done with concentrated or triple superphosphate and with two new products, calcium metaphosphate and fused tricalcium phosphate. 556 YEARBOOK OF AGRICULTURE Calcium metaphosphate is produced when the elementary phosphorus issuing from a phosphate-reduction furnace is burned to P20r, and is allowed to react with phosphate rock heated to 1,200° C. Because it contains about 65 percent P2O0, it is a concentrated phosphate^ as com- pared with ordinary superphosphate. A summary prepared by the Ten- nessee Valley Authority of 758 experiments conducted in the seven Tennessee Valley States with cotton, corn, small grains, and hay shows that tiie yields with calcium metaphosphate averaged 99 percent as high as with ordinary superphosphate. On calcareous soils and on certain acid soils where it is not well incorporated into the soil, calcium metaphosphate has been found inferior to superphosphate. Fused tricalcium phosphate is produced by heating phosphate rock to 1,500°-1,600° C. in the presence of water vapor. The process causes a disruption of the apatite structure of the phosphate rock and the loss of fluorine, so that a tricalcium phosphate that contains about 30 per- cent P2O5 is formed. The degree to which fluorine is removed and the fineness of grinding influence the cost and the availability to crops. Experiments conducted since 1941, and summarized by the Tennessee Valley Authority, have resulted in the establishment of tentative maxi- mum limits of 0.4 percent fluorine and 40-mesh size as standards that insure relatively high availability to crops along with economy of produc- tion. In general, the availability of fused tricalcium phosphate on acid soils has been found to be slightly less than that of superphosphate. Like calcium metaphosphate it has been found to be inferior to superphos- phate on calcareous soils and when appUed as a top dressing. The main advantage of these newer forms of phosphate, as weU as of concentrated superphosphate that contains about 45 percent P2O5, is the economy in cost of transportation and handling. If further work shows that they can be produced as economically as superphosphate they should find greatly increased use. Because concentrated phosphates do not contain sulfur, that element would need to be added separately to sulfur-deficient soils if concentrated phosphates come into general use. Soil Requirements and Efficient Use Some of the most significant field and laboratory investigations relat- ing 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 anci 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 PHOSPHATE FERTILIZERS 557 have not shown profitable responses from phosphate fertihzers 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 nitrogren. Where no nitrogen was applied* in fertilizers, an application of 200 pounds of superphosphate (16-percent P2O5) 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-pcr- cent P2O5) 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 phos- phate 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 unavail- able 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 fertihzer 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 maxi- mum production. This is particularly true of nitrogen fertilization. In experiments of W. H. Metzger and Floyd Davison in Kansas, the place- ment of phosphate fertilizer in the row at a depth of 6 inches resulted in considerably greater yields of sorghum'than did more shallow place- ment.