THE VEGETABLE GARDEN 



Phosphorus is used by plants in the form of 

 phosphoric acid, and there are several forms in 

 which it can be had. The organic forms of nitrogen, 

 which I have mentioned, contain a little phosphoric 

 acid. The most common and most highly con- 

 centrated form of phosphoric acid on the market 

 now is superphosphates, or soluble phosphates. 

 These are derived from the phosphate rock se- 

 cured in South Carolina, Florida, and Tennessee. 

 They are seldom used in their natural state, be- 

 cause but little of the phosphoric acid in them is 

 available as plant food. It is made available by 

 grinding and treating with sulphuric acid. These 

 treated phosphate rocks contain twenty-five to 

 thirty per cent of available phosphoric acid. 



Thomas slag, a by-product in the manufacture 

 of steel, has been used frequently as a source of 

 phosphoric acid, but as the supply is limited it 

 cannot always be obtained. It contains about 

 nineteen or twenty per cent of available phos- 

 phoric acid and six or seven per cent which is in- 

 soluble in soil water. 



Bone contains a great deal of phosphoric acid 

 about twenty-two per cent and it also contains 



about four per cent of nitrogen. Bone can be se- 



[38] 



