EXTEISrSION COUKSE IN SOILS. 49 



quantities of the element must be applied in this form than it is 

 expected a single crop will remove. When 300 to 400 pounds of 

 ground steamed bone meal are used per acre, it will supply sufhcient 

 phosphorus for from three to five crops, depending largely on kind 

 and yield. 



Natural phosphates. — The chief supply of phosphorus for soil 

 improvement is from natural phosphate beds. These are widely 

 distributed over the earth, the most important deposits being in 

 the United States, Canada, France, Spain, Norway, and north 

 Africa. More than half of the world's output of these phosphates is 

 produced in the United States. The principal phosphate beds in 

 this country which have been worked are in Florida, Tennessee, and 

 South Carolina. Enormous deposits, however, have recently been dis- 

 covered in adjacent parts of Utah, Idaho, and Montana. Natural 

 phosphate deposits are prepared in two ways for application to the 

 soil, (1) by grinding the material to an extremely fine condition 

 which is known and sold as raw phosphates or floats; and (2) by 

 treating the ground material with sulphuric acid so as to form acid 

 phosphate or superphosphate. 



Raw phosphate or floats. — Rock phosphate varies greatly in con- 

 tent of phosphorus, ranging from 9 to 18 per cent, though the usual 

 limits are 11 to 15 per cent. Even when ground to extreme fineness 

 this material is dissolved in the soil with very great difficulty and 

 becomes available to crops slowly. Certain crops, however, have 

 greater power to secure their phosphorus from this source than others. 

 The chief process by which this material is made available is through 

 the action of carbon dioxid set free by the decomposition of organic 

 matter in the soil. It is very necessary, therefore, that this material 

 be used only v/hen it is intimately mixed with some form of actively 

 decomposing vegetable matter. This occurs when it is thoroughly 

 incorporated with barnyard manure or applied as a top-dressing on 

 some green-manuring crop which is being plowed under, or is applied 

 to a soil naturally containing large quantities of vegetable matter, 

 such as peat or muck soils. When used under these conditions rock 

 phosphate is often as profitable to crops having a long period of growth 

 as either of the other forms mentioned. From 500 to 1,000 pounds 

 per acre of finely ground phosphate is commonly applied once in 

 three or four years. 



Acid phosphate. — In order to make the phosphorus or rock phos- 

 phate more readily available than in its natural condition it is very 

 generally treated with sulphuric acid. Crude sulphuric acid and raw 

 rock phosphate are mixed in about equal proportions, so that the 

 percentage of phosphorus in the mixture is about one-half that in the 

 raw rock phosphate, though essentially all of it is made available to 

 21862°— Bull. 355—16 4 



