PLANT NUTRITION 273 



nicnialidii. Dcc()ni])(),sitioii of stored manure is a continuous jirocess, 

 the rale ol \vhich varies according to cmi\ ironmental conditions. 



Decomposition of manure. 1 lie liquid portion of manure is rela- 

 tively rich in nitrogen, cliiefly in the form of urea (CO(N 1^2)2)- 

 Through the activities of microorganisms, urea is broken down to am- 

 monia and carbonic acid, which may unite to form ammonium carl)o- 

 nate ((NH4)2C03) or the acid carbonate (NH4HCO3). Both com- 

 pounds are unstable, their instability being increased by increased 

 temperatures, alkalinity, and drying. When ammonia evolves in tire 

 presence of a limited amount of carbon dioxide, the reaction of tlie 

 decomposing material may reach pH 9.5. A slightly acid reaction, 

 equal to pH 6.6, practically inhibits decomposition of the urea. 



In addition to the readily decomposable urea, manure contains 

 proteins. Some of these come from the feed or are present in the litter; 

 others are of bacterial origin. Proteins undergo hydrolysis with the 

 liberation of amino acids, which, through biological oxidation, yield 

 nitrogen in the form of ammonia or elemental nitrogen. Numerous 

 other compounds arise as a result of protein degradation. These in- 

 clude indole, skatole, mercaptans, hydrogen svdfide, and amines, in 

 addition to various organic acids. 



The manurial solids contain considerable quantities of lignin, cellu- 

 lose, hcmicellulose, and related compounds. The rate and course of 

 decomposition of the carbohydrates and related compounds vary 

 with the emironmenlal conditions. Decomposition is extremely rapid 

 at relatively high temperatures with sufficient air supply and adequate 

 moistme. An excess of moisture, through its interference with aera- 

 tion, limits the reaction. 



When anaerobic rather than aeiobic conditions prevail, there is an 

 accumulation of intermediate compounds resulting from the decom- 

 position of carbohydrates, proteins, and related compoimds which in- 

 clude various organic acids, ammonia, hydrogen, methane, and carbon 

 dioxide. The acids tend to combine with ammonia, forming am- 

 monium salts, and thus reduce the loss of nitrogen through the vola- 

 tilization of ammonia. At the same time the pH of the manure is 

 lowered. 



During the decomposition of manure there is a simplification of 

 numerous compounds and the synthesis of others. The final result of 

 the synthesis of organic compounds is the formation of humus, a com- 

 plex so resistant to decay that it may be considered almost stable. 



Chemical freatment of manure. Concern over the raj^id loss of 

 ammonia from manure has led to the use of chemical preservatives. 



