By the 1800’s, phosphorus was known to be essential to plant growth. The only common sources of the mineral—bones and rock phosphate—were very insoluble. Then it was discovered that treating either bones or rock phosphate with sulfuric acid increased the solubility, hence the availability to plants, and the phosphorus industry was born. Today the most common phosphorus fertilizer—ordinary superphosphate—is manufactured by treating rock phosphate with sulfuric acid. Following are descriptions of phosphate fertilizers used by home gardeners.
A natural, mined mineral, the actual percentage of phosphorus varies according to the source. It is mined in many locations throughout the U.S. and world. The phosphate content ranges between 27 and 41 percent (0–27–0, 0–41–0).
Rock phosphate is most useful on acid soils. Even in those cases, it should be applied in quantities two to three times greater than recommended for superphosphate.
For short season plants that need phosphorus, rock phosphate is too slowly available to be of much use. In soils with a pH above 7, the rock phosphate becomes available so slowly it is of negligible value.
Bone meal is composed of bones from slaughterhouses that have been boiled and steamed to remove fats and protein (and nearly all the nitrogen), and ground into flour. Bone meal is more soluble than rock phosphate, but much less soluble than superphosphate. It contains about 24 percent phosphorus and less than 1 percent nitrogen.
The waste product of sea birds and bats, guano was at one time a very important fertilizer. It typically contains about 9 percent water-soluble phosphate as well as 13 percent nitrogen. The phosphorus contained in guano is a very soluble form plants can use.
Unfortunately guano supplies are finite and the richest deposits have already been depleted. It is still available in limited quantities.
Superphosphate (Single Superphosphate)
Superphosphate, 0–20–0, is rock phosphate treated with sulfuric acid to make it more soluble. Besides phosphate, it also contains calcium and sulfur, both essential nutrients.
Used alone, it is a good phosphorus source. Approximately 90 percent is water-soluble. A large quantity of superphosphate is used to manufacture dry, powdered, or granular complete fertilizers.
Concentrated Superphosphate (Triple Superphosphate)
As with ordinary superphosphate, rock phosphate is the initial ingredient, but the gypsum is removed from the reaction products. The resulting fertilizer is almost 98 percent water soluble and has a formula of 0–45–0.
This is the most concentrated form of phosphorus normally available to home gardeners. The convenience of greater concentration is an advantage; however, it includes little additional sulfur. A prime use is for the manufacture of mixed complete fertilizers.
These are made by combining ammonia with phosphoric acid. The most common kinds are monoammonium phosphate (MAP) 11–48–0 and diammonium phosphate (DAP) 21–53–0, or mixtures of the two.
Generally, the phosphorus content of these fertilizers is equally available with ordinary or concentrated superphosphate. Also, they have the advantage of supplying nitrogen at the same time. These dissolve in water and have an acid reaction in the soil because of the ammonium component. Ammonium phosphates are widely used in the manufacture of complete fertilizers.