COMMERCIAL FERTILIZERS 509 



Ground fish is an excellent form of nitrogen, and is as 

 readily available as blood but has a lower nitrogen content. 



Tankage is highly variable in composition, and the con- 

 centrated tankage, being more finely ground, undergoes 

 more readily the decomposition necessary for the utiliza- 

 tion of the nitrogen. Crushed tankage contains from 3 to 

 12 per cent of phosphoric acid, in addition to its nitrogen. 



Leather meal and wool-and-hair waste when untreated 

 are in such a tough and undecomposable condition that 

 they may remain in the soil for years without losing their 

 structure. They are not to be recommended as manures. 



429. Availability of organic nitrogenous fertUizers. — 

 The forms in which combined nitrogen is available to 

 most agricultural plants has already been stated to be 

 nitrates, ammonium salts, and certain organic compounds. 

 Of the latter the simple compounds, as urea, appear to be 

 most readily taken up by plants. Decomposition is there- 

 fore a necessary process for most of these fertilizers, and 

 their usefulness is, in general, proportional to the readi- 

 ness with which aerobic decomposition proceeds, or to the 

 proportion of available compounds that they contain in 

 their original condition. Guano, for instance apparently, 

 contains much nitrogen that is available without further 

 decomposition. Dried blood quickly decomposes and 

 soon forms available substances, consisting of the simpler 

 organic nitrogenous compounds, ammonia and nitrates. 

 The decomposition process is a biological one, arising from 

 the action of micro5rganisms that first break down the 

 complicated organic compounds, forming simpler ones, 

 and finally carry the nitrogen into the form of ammonia, 

 then to nitrous acid, and at last to nitric acid. 



Numerous attempts have been made to determine the 

 relative availability of the nitrogen in various organic 



