330 SOUTHERN FIELD CROPS 



in the cotton-belt is- acid phosphate. This usually con- 

 tains 14 to 16 per cent of available phosphoric acid, but 

 both lower and higher grades than this may be obtained. 

 Add phosphate is manufactured by adding sulphuric 

 acid to the finely ground phosphate rock, or raw phos- 

 phate. The sulphuric acid is employed in order to make 

 the phosphoric acid promptly available to plants. As 

 the sulphuric acid has no fertilizing value, it serves to 

 dilute the original phosphate rock. Hence, acid phos- 

 phate usually contains only about half as large a percentage 

 of phosphoric acid as the raw phosphate from which it 

 was made. But nearly all of the phosphoric acid in acid 

 phosphate is in a soluble or available condition. 



Raw phosphate consists of the finely ground phosphate 

 rock without treatment with any acid. Among the 

 names given to it are crude phosphate, ground phosphate 

 rock, and floats. It usually contains from 26 to 30 per 

 cent of total phosphoric acid. All of this is insoluble, 

 and hence not in a form to be immediately used by the 

 roots of plants. 



As ground phosphate rock contains about twice as much 

 total phosphoric acid as does acid phosphate, and in some 

 localities costs less than half as much per ton, it would 

 be desirable to use the raw phosphate if it could be made 

 available. 



303. Effects of different phosphates on cotton. Re- 

 peated experiments in many states have shown that 

 cotton can make some use of raw phosphate, but that 

 acid phosphate usually is much more effective. However, 

 experiments have also shown that the raw phosphate 

 becomes more quickly available if it is mixed with large 



