150 



Decomposition of Soil Organic Matter 



in a given time. The ratio of COo liberated to the organic matter 

 decomposed is similar, however, and depends on the organisms and 

 conditions of decomposition. In nitrogen-rich materials, liberation 

 of COo is accompanied by production and accumulation of ammonia, 

 which is soon changed in field and garden soils to nitrate; the ratio 

 between the carbon and nitrogen liberated depends on the nitrogen 



4 8 12 16 20 24 28 4 8 12 



Time, (days) Time, (weeks) 



Fig. 64. Influence of age of plant (stems and leaves) on its decomposition in 

 sand medium as indicated by the evolution of carbon dioxide (from Waksman 



and Tenney). 



content of the material undergoing decomposition and on the rate 

 of decomposition. 



Decomposition of plant residues low in nitrogen is controlled by 

 the amount of available nitrogen present. Ammonia is not liberated 

 and may actually be consumed. The wider the carbon-nitrogen ratio 

 in the plant and animal residues, the greater is the proportion of COo 

 to ammonia liberated, until the latter becomes a negative figure; 

 then nitrogen must be added for active decomposition. 



Figures 65-67 illustrate the course of COo evolution in the de- 

 composition of different plant materials under different conditions, 

 both in soils and in composts. 



Although some COo may be formed by purely chemical processes 

 in the soil and although considerable COo is evolved by the roots of 



