CROPS AND SOIL IMPROVEMENT 



stable manure than when used upon land that 

 remains deficient in organic matter. Applications 

 should be in large amount per acre 500 to 

 1000 pounds in order that the amount of readily 

 available phosphoric acid may meet the immediate 

 need of plants. Dependence should be placed 

 upon the readily available acid phosphate in all 

 instances until experiment on the farm shows 

 that the rock-phosphate is a cheaper source of 

 plant-food than the acid phosphate. 



Acid Phosphate. When animal bone is treated 

 with sulphuric acid, the result is an acid phos- 

 phate, but treated animal bone is so rare on the 

 market that it may be ignored. The acid phos- 

 phate on the market is rock-phosphate treated 

 with sulphuric acid to render its plant-food avail- 

 able. The content of phosphoric acid varies 

 because the original rock-phosphate varies, but 

 the most common grade on the market is guar- 

 anteed to contain 14 per cent available phosphoric 

 acid, and 1 to 2 per cent insoluble. Some acid 

 phosphate is guaranteed to contain 16 per cent 

 available phosphoric acid, and some runs down to 

 10 per cent available. 



An acid phosphate contains quickly available 

 plant-food. A prejudice exists against it on ac- 



[180] 



