100 AGRICULTURE 



tables, like lettuce and radishes, in which quick growth is 

 desired. 



Three kinds of phosphates. Phosphates are those fer- 

 tilizers that contain the element phosphorus, in the form 

 of phosphoric acid. There are three kinds of phosphate, 

 that are of very different value. The first is natural or 

 raw phosphate, sometimes called Tennessee phosphate, 

 Florida phosphate, or floats. It is simply the phosphate 

 rock just as it is dug or brought up by dredges from its 

 place in phosphate beds, except that it has been ground 

 into a very fine powder. Since roots generally cannot 

 absorb much of this form of phosphate because it will not 

 dissolve in pure water, it is called insoluble phosphate. 



Acid phosphate is so called because it is made by ad- 

 ding sulfuric acid to the raw or natural phosphate. This 

 acid so changes the phosphate that roots can immediately 

 absorb it. The phosphate in acid phosphate is called 

 soluble. There is a third or intermediate form that plants 

 can use. This and the soluble phosphate are added to- 

 gether and called the available phosphoric, that is, the kind 

 that plants can use promptly. 



Acid phosphate usually contains from 12 to 16 per 

 cent of available phosphoric acid, that is from 24 to 32 

 pounds of available phosphoric acid in every 2OO-pound 

 bag. The farmer can afford to pay fully one third more 

 for the acid phosphate with 16 per cent than for that with 

 only 12 per cent of available plant-food. He will need 

 less of the high-grade than of the low-grade fertilizer, and 

 thus will save freight and expense of hauling and handling. 



Although raw or crude phosphate cannot be dissolved 



