FERTILIZERS, ROTATIONS FOR COTTON 87 



insoluble, whereas that in monocalcium phosphate is 

 easily soluble and therefore available to plants. The object 

 of the sulfuric acid treatment, therefore, is to render the 

 phosphoric acid soluble. Acid phosphate is also made by 

 treating ground bone with sulfuric acid. Ordinarily acid 

 phosphate contains from 12 to 16 per cent of soluble 

 phosphoric acid. As a rule, about one-fourth of acid phos- 

 phate consists of phosphates (chiefly monocalcium phos- 

 phate) while three-fourths consists of calcium sulfate and 

 impurities. The calcium sulfate is a soil stimulant and no 

 doubt, in many cases, the effects produced from applying 

 acid phosphate are partially due to the action of calcium 

 sulfate in making soluble certain mineral elements of plant- 

 food in the soil. The principal reason why acid phosphate 

 is so universally used by cotton growers is that it gives 

 quick results due to the readily soluble form in which the 

 phosphoric acid is contained. 



Raw rock phosphate. This material is used very little 

 by cotton growers. It consists of the finely ground phos- 

 phate rock without any acid treatment. Consequently 

 the phosphoric acid contained is very difficultly soluble. 

 On soils that are devoid of organic matter it produces 

 practically no results. However, experiments that have 

 been conducted at the Illinois, Ohio, Pennsylvania and 

 Maryland Experiment Stations, indicate that on soils well 

 provided with decaying organic matter, raw rock phos- 

 phate is a very profitable source of phosphoric acid. The 

 organic acids produced as by-products in the decomposi- 

 tion of the organic matter act upon the raw rock phosphate 

 changing a part of the phosphoric acid into an available 

 form. In the event that future investigations establish 

 the fact that raw rock phosphate may be made to mate- 

 rially increase the yield of cotton when applied to soils 



