Phosphate Fertilizers

by W. H. PIERRE

Only 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 applied in the United States to phosphate rock, deposits of which had been found in South Carolina. Little was known then about the use of phosphate fertilizers, but information on the phosphate needs of American 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 application to the soil. American farmers spend about 200 million dollars annually for phosphorus, the important plant food element that is added to soils through the use of phosphate fertilizers.

Of the total amount of phosphorus found in soils, only a small percentage 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 fertilizers. 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 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 consumption 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 phosphorus contained in crops may be returned to the soil in manures, cropping 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 fertilizers 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 P205, and concentrated superphosphate, containing 40 to 45 percent P105. 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.

Calcium metaphosphate is produced when the elementary phosphorus issuing from a phosphate-reduction furnace is burned to P,O, and is allowed to react with phosphate rock heated to 1,200 C. Because it contains about 65 percent P205, it is a concentrated phosphate, as compared with ordinary superphosphate. A summary prepared by the Tennessee Valley Authority of 758 experiments conducted in the seven Tennessee Valley States with cotton, corn, small grains, and hay shows that the 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 percent P,O.5 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 maximum limits of 0.4 percent fluorine and 40-mesh size as standards that insure relatively high availability to crops along with economy of production. 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 superphosphate on calcareous soils and when applied as a top dressing.

The main advantage of these newer forms of phosphate, as well as of concentrated superphosphate that contains about 45 percent P205, 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.