This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is one of the most popular phosphatic manures.
It is obtained by treating substances containing tricalcium phosphate with sulphuric acid. At first superphosphate was made from bone ash and bone black, but the great bulk is now obtained from natural minerals (phosphorites, coprolites, apatites). Many millions of tons are produced annually, about 800,000 tons being manufactured in the United Kingdom. Commercial samples contain from 25 to 40 per cent of soluble phosphates, but there is great variation. When buying, the soluble phosphates only should be taken into account, the insoluble phosphates not being highly valued.
When superphosphate is applied to the soil, the phosphates, being soluble in water, are well distributed amongst the soil particles by a shower of rain. Then a change takes place. The soluble phosphate reverts into an insoluble state owing to the carbonate of lime (or chalk) and the compounds of iron and alumina present in the soil. This change prevents the phosphates from being washed out of the soil too readily.
Superphosphate is an acid manure, and therefore tends to use up the available lime in the soil (see p. 161).