Decomposition of Sweet Clover as a Green Manure 125 



retarded. It is also known that carbon dioxide may be used by micro- 

 organisms as a source of carbon, under which conditions its evolution 

 would not be a proper measure of the rate of decay. Hutchinson and 

 Milligan (1914) improved Wollny's method by supplementing the carbon 

 dioxide measurements with nitrate determinations. 



In consideration of the fact that the decay of organic matter is a 

 bacteriological process, the problem of its measurement has been attacked 

 from that standpoint. This mode of attack was first suggested by Remy. 

 The underlying principle of the method is the determination of the kind, 

 and the intensity of the functions, of the bacteria concerned. This is 

 accomplished by inoculating nutrient solutions with portions of the soil 

 in question and determining the metabolic products after a given period 

 of incubation. Many recent investigators have modified the original 

 method. Altho the Remy scheme and its modifications have been the 

 most widely adopted of the methods devised for studying the decay of 

 organic matter, the procedure contains many weaknesses, as pointed 

 out by Allen and Bonazzi (19 15). 



The stage of decay which gives the most information as to the avail- 

 ability of a green manure is nitrate formation, since it is as nitrate that 

 nitrogen is taken up by the plant. If nitrification has occurred, it is 

 evident that other processes have preceded it, to furnish forms of nitrogen 

 capable of being nitrified. The accumulation of nitrates in soils under 

 a green-manure treatment has been used as a measure of the availability 

 of the material turned under. 



Inasmuch as the measurement of nitrates formed was the method 

 finally adopted in the present investigation, it seems desirable to cite 

 instances when this procedure has been followed and considerations that 

 have led to its adoption. Early investigators believed that the presence 

 of organic matter retards nitrification. If this were so, the measurement 

 of nitrate accimiulation would be a poor method of studying the decom- 

 position of a green manure. Recent work, however, seems to have 

 disproved the earlier views. An account of the experimental work on 

 this subject is given by Hill (19 15), who made a thoro study of various 

 green manures on several types of soils under greenhouse conditions. 

 He found that organic matter, as bluegrass, clover, and alfalfa, passed 

 over into nitrates. The addition of the green manures increased the 

 rate of nitrate formation. Brown and Allison (1916) have shown that 

 the application of the common humus-forming materials in maximum 

 amounts for farm conditions increases ammonification and nitrification 

 to a considerable extent. Wright (191 5), working with vetch and rye, 

 found that plowing under green manures resulted in rapid decay accom- 

 panied by vigorous nitrification. The work of Hutchinson and Milligan 



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