1 88 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



tion of phosphoric acid has been made. One thousand pounds of 

 16 per cent, acid phosphate would represent an application of 80 

 parts per million of phosphoric acid, and this heavy application 

 would not increase very much the phosphoric acid removed from 

 soils of very high fixing power. 



A soil containing 100 parts per million of phosphoric acid, with 

 a low acid consumed, and with a fixing power of less than 50, 

 probably contains a corresponding amount of calcium phosphate 

 accessible to the roots of plants. 



too 



30 



fiO 



40 



eo 



/ ? 3 4 5-6 7-3 3-10 1 1- IB 



GROUP. BASED ON ACTIVE. PHOSPHOR/C ACID 



Fig- 39 Relation of the active phosphoric acid of the soil to the phosphoric 



acid withdrawn by crops in pot experiments, expressed 



as bushels corn per acre. 



A soil containing 100 parts per million of phosphoric acid, with 

 an acid consumed of 20 per cent., may or may not expose much 

 phosphoric acid to the roots of plants. It is impossible to say 

 how much of it is protected by the calcareous material. 



It is impossible to distinguish phosphoric acid in its several 

 different forms. For example, suppose plots were fertilized 

 with equal quantities of phosphoric acid, Thomas phos- 

 phate, phosphate rock, acid phosphate, and apatite. We 

 could not expect to find a relation between the phosphoric 



