160 Decomposition of Soil Organic Matter 



tive salts exerted a stimulating effect upon COo production; moisture 

 and organic matter content of the soil are among the most impor- 

 tant factors. 



On comparing the curves for bacterial numbers, nitrate accumula- 

 tion, and COo content of the soil air, Russell and Appleyard found 

 them to be sufficiently similar to justify the view that all these phe- 

 nomena are related. A rise in bacterial numbers was accompanied 

 by a rise in the CO2 content of the soil air and somewhat later by a 

 rise of nitrate in the soil. The rate of decomposition of organic mat- 

 ter in the soil, as measured by CO2 evolution, was, therefore, looked 

 upon as a function of bacterial activities. The rate of these activities 

 in the soil attained maxima in late spring and autumn, and minima 

 in summer and winter. In autumn the bacteria increased first, then 

 the CO2 content rose, and finally the nitrate increased. 



Neller measined the C02-producing capacity of the soil by adding 

 plant material to 200-gm portions of soil placed in tumblers under 

 bell jars, and passing C02-free air for 16 days. On comparing two 

 limed and two unlimed soils, he obtained distinct correlation between 

 crop yield, nitrate accumulation, and numbers of bacteria, but these 

 did not correlate with ammonia accumulation. 



The evolution of COo from soil resulting from the decomposition 

 of humus was thus found to be the best index of the rate of decom- 

 position of this humus. This is further brought out in Table 31, 



Table .SI. Effect of Liming upon Microbiological Activities in Soil (from Xeller) 



* Per 200 gra soil in 16 days. 



t Per gram soil, as determined by plate method. 



where the respiratory power of soils has been measured for a group 

 of plots variously treated and of varying degrees of fertility. The 

 curves for bacterial numbers, accumulation of nitrate, and evolution 

 of CO2 were found to be sufficiently similar to justify the view that 

 they are closely related. A rise in bacterial numbers was found to 



