The use of ammonium phosphates as separate materials increased from some 19,000 tons in 1939 to 336,300 tons in 1955. Smaller amounts were used in mixed fertilizers. Almost 80 percent of the ammonium phosphates directly applied to the soil were guaranteed to contain 16 percent of nitrogen and 20 percent of available phosphoric oxide. About equal amounts of the remaining material had N P₂O₅ ratios of 11-48 and 13-39.

A fourth ammonium phosphate came on the market in 1955. It is diammonium phosphate and contains approximately 21 percent of nitrogen and 53 percent of phosphoric oxide.

Another of the newer sources of phosphoric oxide is calcium metaphosphate, which contains about 63 percent of phosphoric oxide. It was developed by the Tennessee Valley Authority. A glasslike material, it is produced by treating phosphate rock with phosphoric oxide vapor. The phosphoric oxide in this fertilizer material is only slightly soluble in water but almost completely available to growing plants. Production increased from about 4,000 tons in 1939 to 56,700 tons in 1954.

Potassium chloride has been the major source of fertilizer potash and has been marketed in 50-percent and 60-percent grades. The 60-percent grade accounted for 82.2 percent of the agricultural potash of American origin in this form in 1939. By 1955, potassium chloride was supplying nearly 94 percent of the total potash for agriculture; most of it was distributed as the 60-percent grade. Nearly all the remaining 6 percent of total potash was in the form of potassium sulfate or sulfate of potash magnesia.

Increasing quantities of liquid mixed fertilizers, containing as high as 32 percent of plant nutrients, have been manufactured in recent years. Consumption of liquid mixed fertilizers amounted to 27,500 tons in 1954. Consumption in California accounted for about three-quarters of the total consumption in the United States.

ADVANCES in the manufacture and processing of materials and mixtures have improved the quality of fertilizers and helped hold down the costs.

A continuous ammoniator developed by the Tennessee Valley Authority in 1954 may be used in continuous processes for the manufacture of mixed fertilizers and in the granulation of mixed fertilizers.

Granulation consists of forming small grains or masses of fertilizer mixtures. The ingredients of the mixture are thoroughly mixed with sufficient moisture to make them semiplastic or sticky. The granules are formed by rolling or tumbling and drying to hard masses.

Granulation processes have been improved and are widely accepted as one means of improving the physical character of fertilizers. Consumer demand for granular fertilizers has increased steadily. Many materials and large tonnages of fertilizer mixtures are granulated because of the greater ease with which they can be handled. They have a lower caking tendency and less dusting loss, and can be distributed more uniformly in the field. The more hygroscopic granular materials may be coated with a conditioning agent an inert substance used to prevent the granules from sticking together. The granular products are packed in moisture-resistant bags to facilitate storage and handling.

Modern methods of granulation also make it possible to use more ammonia and ammoniating solutions, which are employed to ammoniate superphosphate and to supply a relatively inexpensive form of nitrogen in mixed fertilizers. Formerly their use was limited by the amount of ammonia that could be absorbed by the superphosphate without reversion of available phosphoric oxide.

Modern techniques, however, permit additions of ammonia in excess of this amount because of the simultaneous use of sulfuric or phosphoric acid. The acids neutralize any excess ammonia to form ammonium sulfate or phosphate. The reaction liberates heat, which aids the agglomeration and the drying of the finished product.

New uses for the more plentiful supplies of phosphoric and nitric acids have opened up in the fertilizer industry. Concentrated superphosphate can be manufactured in normal superphosphate equipment by treating phosphate rock with phosphoric acid rather than sulfuric acid. Some phosphoric acid is neutralized with ammonia to make ammonium phosphates. At least two companies have manufactured high-analysis mixed fertilizers by the nitric acid-sulfuric acid or nitric acid-phosphoric acid treatment of phosphate rock. The necessary potash for the finished product is added as a soluble salt to the process slurry.

Ways of handling fertilizers also have changed. The farmer once had to buy and handle bags of fertilizer weighing at least 80 pounds. The use of liquid fertilizers, which can be handled in bulk, has helped to relieve him of some of this chore. Solid fertilizers can be bought in bulk. Shipments of solid fertilizers in this form in 1954 were 8.25 percent of the total shipments of solid fertilizer. Sometimes the mixture is formulated to the farmer's specifications.

Modern techniques in production and handling and the acceptance by the farmer of higher analysis fertilizers have enabled the manufacturer to supply the farmer with fertilizer that is relatively inexpensive in comparison with the other materials he must buy. The index numbers (based on data for 1910-1914 as 100) of the prices paid by farmers for fertilizers were 101 and 155 in the calendar years 1939 and 1954, respectively. Corresponding numbers for feed were 93 and 226; for farm machinery, 155 and 313; and for seed, 92 and 226.

As the demand for fertilizer increases, new problems will arise. The solutions of the problems may lead to more highly concentrated fertilizers, the development of new materials, and better and cheaper methods of manufacturing fertilizers.