mineral and organic substances that contain phosphorus and that are used for improving the nutrition of plants. Phosphorus fertilizers are the only means for replenishing phosphorus in soil. They are produced commercially from phosphate rock and apatites. Phosphorus fertilizers include such organic substances as bone meal and manure and such phosphorus-rich industrial wastes as basic slag and Thomas slag.

Phosphorus fertilizers were the first mineral fertilizers to be produced commercially. Superphosphate was first produced in 1842 in Great Britain (bone meal being the principal phosphorus fertilizer in the first half of the 19th century). In Russia, superphosphate was first produced in 1868. The production of fertilizer from phosphate rock began in 1855 in France. In Russia, A. N. Engel’gardt was the first to use phosphate rock in agriculture, beginning experiments in 1866 with ground phosphate rock on his estate, Batishchevo, in Smolensk Province. During the years 1867–69, D.I. Mendeleev studied the effect of phosphorus fertilizers on agriculture yields in Smolensk, St. Petersburg, Moscow, and Simbirsk provinces, and he stressed the importance of using ground phosphate rock and superphosphates in agriculture. Apatites were first processed into fertilizer on a commercial scale in the USSR in 1935 (the Khibiny deposit, the world’s largest).

By 1900, world production of phosphorus fertilizers was approximately 1 million tons (calculated on the basis of P₂O₅), of which about 20,000 tons were produced in Russia. In the 20th century, especially since 1950, the use of phosphorus fertilizers has increased markedly (Table 1). However, the increase in the consumption of phosphorus fertilizers has been less than the increases for nitrogen and potassium fertilizers, a fact explained by the insufficient supplies of phosphate rock.

The supply of phosphorus fertilizers per hectare (ha) of tillage in 1974 (in kg P₂O₅) was 16.5 worldwide, 198.4 in Belgium, 74 in Czechoslovakia, 66.8 in Great Britain, 56 in Poland, 53.6 in the German Democratic Republic, 24.1 in the United States, and 18.7 in the USSR. The number of varieties of phosphorus fertilizers is increasing.

 

Phosphorus fertilizers are divided into three groups on the basis of solubility. In the water-soluble group (ordinary, double, and ammoniated superphosphates), phosphorus is present as monobasic calcium phosphate, Ca(H₂PO₄)₂ · H₂O. The water-soluble fertilizers are produced for the most part in granulated form; they are used for application as base (ground) and starter (drill) fertilizers. The second group comprises citrate-soluble (soluble in an alkaline solution of ammonium citrate—Peterman reagent) and citric-soluble (soluble in citric acid) fertilizers. It includes dicalcium phosphate, Thomas slag, basic slag, defluorinated phosphate, and fused magnesium phosphate. Here, the phosphorus is present as dibasic calcium phosphate, CaHPO₄ · H₂O or as tetracalcium phosphate, Ca₄P₂O₅. Citrate-soluble and citric-soluble fertilizers are ground (base) fertilizers applied during autumn plowing and spring cultivation. The third group comprises the sparingly soluble fertilizers, that is, ground phosphate rock and bone meal, with phosphorus present as tricalcium phosphate, Ca₃ (PO₄)₂. These fertilizers are applied in large amounts to acid soils, where the sparingly soluble phosphates are converted into a form usable by plants. All phosphorus fertilizers are nonhygroscopic; they do not cake, and they are readily broadcast by fertilizer spreaders.

New highly concentrated phosphorus fertilizers (ammonium polyphosphates, potassium metaphosphates) containing from 50 to 80 percent P₂O₅ are promising. They are equal and, in some cases, superior to the standard forms of phosphorus fertilizers in effectiveness. In the United States and in several countries in Western Europe, liquid fertilizers based on polyphosphoric acids

have found use. These fertilizers permit the complete mechanization of fertilizer application, reduction of waste to a minimum, uniform incorporation into the soil, and addition of trace elements and pesticides. The characteristics of the major mineral phosphorus fertilizers are given in Table 2.

Phosphorus fertilizers increase crop yields, improve crop quality, stimulate the maturation of plants, and improve the resistance of plants to lodging and drought. Drought resistance has special importance for the USSR because major crop-producing regions are located in zones of insufficient moisture. The high effectiveness of phosphorus fertilizers has been established for all the soil and climatic zones of the Soviet Union and for all crops. The positive effect of phosphorus fertilizers is especially apparent with sufficient availability of nitrogen and potassium to plants and with deep incorporation of the fertilizer into the soil. The application of 60 kg of P₂O₅ (bulk broadcast) for winter wheat yields an additional 2–5 quintals (q) per ha. With spring wheat, 60–80 kg of P₂O₅ increases the yield by 1.5–2.5 q per ha. Owing to their low rate of diffusion, phosphorus fertilizers have a residual effect, measuring 6–8 years in arid regions and 2–3 years in regions of sufficient moisture.

The rate of phosphorus fertilizer application depends on the soil, crop, and availability of nutrients. In the USSR, 60–120 kg of P₂O₅ are applied (bulk broadcast application) to each hectare at the time of plowing or cultivation. At the time of sowing, 10–40 kg of P₂O₅ are applied as drill fertilizer. Topdressing with phosphorus, as a rule, is not effective on nonirrigated land.

In the irrigated regions of Azerbaijan and the republics of Middle Asia, 100–120 kg of P₂O₅ per ha increases the yield of raw cotton by 3–5 q per ha. In the sugar beet belt, the rate of 60–120 kg of P₂O₅ per ha results in the increase of the yield of sugar beets by 25–50 q per ha, and the sugar content is increased by 0.1–0.3 percent. Phosphorus fertilizers are the principal fertilizer for application on sunflower crops. On the chernozems of the Ukraine and Moldavia, in the forest-steppe regions of the RSFSR, and in the steppe region of the Northern Caucasus, 60 kg of P₂O₅ per ha increases the yield of sunflower seeds by 1–4.5 q per ha; 20 kg of P₂O₅ per ha applied as a starter at the time of planting, in some cases combined with 10 kg of N, brings an additional increase of 1.0–3.4 q per ha. With a sufficient amount of phosphorus, sunflowers also yield more oil. In rates of 90 kg per ha, phosphorus fertilizers increase the potato yield on sol-podzolic soils and chernozems by 25–30 q per ha; in addition, the starch content in tubers is increased by 0.6–1.2 percent. Phosphorus fertilizers are effective on other crops, including vegetables, fruits, and feed crops.
 

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