IMPROVEMENT OF THE SOIL 83 



gen. Occasional deposits are found, however, that 

 are rich in. this element. Stable manures, night 

 soils, and other animal excrements, owe their value 

 as fertilizers largely to the nitrogen which they 

 contain. Vegetable substances, useful as fertilizers 

 on account of the nitrogen they contain, are the 

 residues left from the extraction of various vegetable 

 oils. Castor-bean pumice and cotton-seed meal are 

 the two most widely used for this purpose. For 

 many years cotton seed and cotton-seed meal were 

 the cheapest sources of nitrogen in the South, but 

 now they are so extensively used for feeding cattle 

 that the price rules higher. The immense impor- 

 tance as a fertilizer of the vegetable nitrogen con- 

 tained in leguminous plants has already been suffi- 

 ciently emphasized. 



Phosphoric acid as found in fertilizers is always 

 combined with lime. It is mostly derived from 

 bones and from certain phosphatic rocks that are 

 mined very extensively in South Carolina and 

 Florida. It occurs in three conditions, known com- 

 mercially as water soluble, citrate soluble, and in- 

 soluble phosphate. The first two are both supposed 

 to be immediately available as plant food, and both 

 are rated at about the same value. The extent to 

 which the so-called insoluble phosphate becomes dis- 

 solved in the soil and thus becomes available de- 

 pends on the fineness to which it is ground and the 

 amount of vegetable matter in the soil. The 

 chemical changes taking place in connection with 

 the fermentation of vegetable matter aid consider- 

 ably in its solution. The insoluble phosphate from 



