402 SOIL PHYSICS AND MANAGEMENT 



tion. Large deposits are found in South Carolina, Florida, Ten- 

 nessee, Utah, and other states. This is mined and, when finely 

 ground, constitutes the raw rock phosphate of commerce. When 

 this phosphate is treated with an equal weight of sulfuric acid, the 

 resulting product is acid phosphate. This treatment renders most 

 of the phosphorus available. It contains from six to eight per cent 

 of the element phosphorus. 



Basic slag, a by-product formed in the manufacture of steel 

 from iron ores containing considerable phosphorus, has been ex- 

 tensively used in Europe as a source of phosphorus, but to no large 

 extent in this country. 



Forms Compared. Of these different sources, steamed bone 

 meal, acid and raw rock phosphate are most commonly used. 



Without entering into a lengthy discussion of the merits of each 

 of these, it may be said in general that upon soils low in organic 

 matter acid phosphate or steamed bone meal may be used to good 

 advantage. If the soil is well supplied with organic matter, finely 

 ground rock phosphate will be preferable, since the acids produced 

 by the decay of the organic matter render the phosphorus available. 

 Any form of quickly decaying organic matter, such as legumes, green 

 or barnyard manure, will aid in liberating the phosphorus. For im- 

 mediate results the rock phosphate should be applied before the 

 material is turned under. It may be added to the soil for the pur- 

 pose of helping to obtain a catch of clover. For best results with any 

 form of phosphate, limestone should be present in the soil. 



In the use of phosphorus on soils deficient in this element the 

 one purpose should be to increase the amount by applying more 

 than is used by the crops. A naturally fertile soil rarely contains 

 less than fourteen hundred to sixteen hundred pounds of the ele- 

 ment per acre in the plowed soil. 



Most upland soils, as shown by the tables on pages 392, 393, and 

 394, actually contain from eight hundred to twelve hundred pounds. 

 In the building up of these soils an excellent plan is to add a ton of 

 finely ground rock phosphate per acre every four to six years until 

 the amount has reached that of a normal fertile soil, or about 

 eighteen hundred to two thousand pounds in the surface seven inches 

 of an acre. After this is reached a sufficient amount should be 

 applied to replace that removed by the crops. 



The cost of a pound of the element phosphorus is a thing that is 

 frequently overlooked. In bone meal and acid phosphate the cost 

 of a pound of phosphorus was about twelve and one-half cents per 



