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.
Phosphatic manures are derived from various sources, and are valuable because they induce the earlier production of flowers and fruits. They are mainly useful for the supply of phosphoric acid, which is an ingredient of every part of a plant, and exists in considerable quantities in some, such as the Cauliflower, the Radish, Peas, and Beans. There are fair supplies of phosphoric acid in the soil, as much as 2750 lb. to the acre being recorded at Rothamsted in a field that had not been manured for fifty years. As already stated, from 20 to 125 lb. of phosphoric acid per acre is a sufficient supply for most crops. These quantities may be liberated by deep cultivation and the addition of stable manure, and by the judicious application of some of the following "artificials", chiefly remarkable for their phosphates.