152 



Decomposition of Soil Organic Matter 



soil, 45 per cent of the increase in CO2 evolution due to the roots 

 was microbial in origin, which would reduce the contribution of the 

 oat roots to 16.5 per cent. One may thus conclude that about 85 per 

 cent of the total CO2 liberated from the soil is due to the activity of 

 microorganisms. 



Evolution of COo from Soil Humus 



The study of COo evolution from soil has followed several lines, 

 depending on the methods of determination. These are: 



1. Methods based upon the extraction of gas from soil samples 

 removed from the field and taken to the laboratory. Leather estab- 

 lished that soil gases contain varying percentages of COo ranging 

 from 3.84 per cent to 15.29 per cent. 



39 



290 



96 157 



Decomposition, days 



Fig. 66. Changes in composition of horse manure compost during different stages 

 of decomposition ( from Waksman and Diehm ) . 



2. Methods in which an apparatus is placed on or thrust into the 

 soil, and the gas present is extracted by suction, as measured by 

 Appleman, by Russell and Appleyard, by Potter and Snyder, and by 

 Lundegardh. 



3. Methods in which representatixe soil samples are taken and 

 tested in the laboratory for capacity to evolve COo under arbitrarily 

 prescribed conditions, as done by Waksman and Starkey and by 

 Marsh. 



