156 



Decomposition of Soil Organic Matter 



the air; that absorption by soil solution with subsequent loss in drain- 

 age water carries away part of the CO2 produced. Total production 

 of COo is, therefore, much greater than the amount which escapes 

 into the atmosphere. Hence the amount of CO2 collectible over a 

 given area cannot be regarded as a measure of the COo produced in 

 the soil. 



175 



150- 



125 



5P100 



75 



50 



25 



6 ^ 



Total 

 material 



Cold-water- 

 soluble 



Hemi- 

 celluloses 



Celluloses Lignins 



Fig. 68. 



Crude 

 protein 



Decomposition of ^'arious chemical constituents of rye straw with addi- 

 tional nutrient salts ( from Tenney and Waksman ) . 



As shown previously, the humus in the soil is not absolutely re- 

 sistant to decomposition, but undergoes slow but continuous decom- 

 position. The rate of evolution of CO2, especially under aerobic con- 

 ditions, is the most accurate and simplest method for measuring 

 humus decomposition. This can be measured either (a) as total 

 COo arising from a given volume of soil during a certain period of 

 time, or (Z?) as the amount of COo found in the soil atmosphere. 

 The mineralized nitrogen liberated as a result of humus decomposi- 

 tion can be measured as ammonia or as nitrate, or both; in most soils 

 ammonia does not accumulate as such but is rapidly oxidized to ni- 

 trate. In addition to determining the products of humus decompo- 



