Phosphoric acid is most essential in growing cereal crops, but it is scarcely less important in vegetable gardening. Soils deficient in phosphorus fail to give large yields and the crops are also slow in maturing. In vegetable gardening, more importance is attached now to the use of phosphoric acid than ever before, and the gardener should not lose sight of the fact that most soils are lacking in this plant food.

Rock phosphates are the chief sources of supply. They vary from 12 to 18 per cent in available phosphoric acid. Raw and steamed animal bone are also in common use, and bone tankage is employed by some vegetable growers. Thomas slag, also known as iron phosphate and Thomas phosphate, which contains 15 to 20 per cent of phosphoric acid, is another desirable form.