EXTENSION COURSE 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 sufficient 
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 of 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 
