Compound Fertilizers

fertilizers containing two or all three of the three basic plant nutrients—N, P₂O_s, and K₂O—as well as microelements, such as B, Mn, Cu, Zn, and Mo.

Compound fertilizers are generally highly concentrated (containing boosted amounts of nutrients and little filler), so that they require less expenditure of labor and capital than do simple fertilizers for application, storage, and hauling. Compound fertilizers have good physical properties; they do not cake and they scatter well when applied by machine. The nutrient ratios of compound fertilizers vary, depending on the method of production, the initial components, and the requirements of the plants fertilized.

Compound fertilizers have been used extensively since 1950, especially in the United States, Canada, England, the Netherlands, Japan, France, and Italy, where they constitute more than 50 percent of all fertilizer produced. In the USSR, plans for 1971–75 called for an increase in the share of highly concentrated and complex fertilizers to 80 percent of all fertilizer production.

Compound fertilizers are classified as either two-element (phosphorus-potassium, nitrogen-phosphorus, and nitrogen-potassium) or three-element (nitrogen-phosphorus-potassium). Depending on the method of production, they are considered complex, complex-mixed, or mixed. Compound fertilizers are used for all crops, whereas complex fertilizers are used primarily for industrial crops, such as cotton and sugar beets.

Complex fertilizers are obtained through the chemical interaction of the initial components—ammonia, phosphoric acid, nitric acid, phosphorites, apatites, and natural potassium salts. They are produced in granulated form. The most common types of complex fertilizers in the USSR are Ammofos (nutrient content, 56–64 percent), Diammofos (71–74 percent), Nitrofos (38 percent), potassium nitrate (57 percent), Nitroammofoska (50–54 percent), and Nitrofoska (36 percent). The possibilities are promising for complex liquid fertilizers, potassium metaphosphate, and ammonium polyphosphate.

Complex-mixed fertilizers are produced by mixing finished fertilizers and treating them with sulfuric acid, nitric acid, and ammonia or ammoniate. The nutrient content in these fertilizers reaches 58 percent, depending on the initial components. Complex-mixed fertilizers for sugar beets are produced in the USSR that contain 4, 16, and 8 percent and 3, 12, and 6 percent nitrogen, phosphorus, and potassium. There are also more concentrated fertilizers, with as much as 45 percent nutrient content.

Mixed fertilizers are produced by machine mixing finished fertilizers (primarily superphosphate with nitrogen fertilizers and potassium chloride). Mixing rules are observed in order to avoid nutrient loss. For example, ammonium nitrate and other ammonia fertilizers cannot be mixed with thermophosphates, ash, or other alkaline fertilizers, since this causes a loss of nitrogen; and ammonium nitrate cannot be mixed with urea because of the very high hygroscopicity of the mixture obtained. Neutralizing supplements, such as lime, dolomite, and cement dust are added to improve the physical properties of the mixtures. The best mixed fertilizers are obtained when granulated components are mixed. The nutrient ratio to be used in mixed fertilizers depends on crop requirements and soil characteristics. For example, a 1 : 1 : 1 ratio of nitrogen, phosphorus, and potassium (that is, N, P₂O₅, and K₂O) is the basic fertilizer for grain crops, sugar beets, potatoes, and vegetables on soddy podzols, gray forests, and chernozems, and a 1 : 1.5 : 1 ratio is applied at planting time for grains, vegetables, and industrial crops.