152 SOILS AND FERTILIZERS 



meal has been so low in price that its use as a fertilizer 

 has been admissible. 



A ton of cottonseed meal, costing $20 and containing 

 2 per cent of phosphoric acid and 7 per cent of nitro- 

 gen, would be equivalent to 13.1 cents per pound for 

 the nitrogen, which is frequently cheaper than purchas- 

 ing some other nitrogenous fertilizer. 



169. Leather, Wool Waste, and Hair are rich in nitro- 

 gen, but on account of their slowness in decomposing 

 are unsuitable for fertilizer purposes. When present in 

 fertilizers they give poor field results. 



170. Available Nitrogen. One of the methods em- 

 ployed to detect, in fertilizers, the presence of inert 

 forms of nitrogen, as leather, is to digest the material in 

 prepared pepsin solution. 50 S.ubstances like dried blood 

 are nearly all soluble in the pepsin, while leather and other 

 inert forms are only partially so. The solubility of the 

 organic nitrogen in pepsin solution determines, to a great 

 extent, the value of the material as a fertilizer. 51 



