E. J. liUSSELL AND A. APPLEYAKD 387 



dominating the situation, when they are falhng loss is the dominating 

 factor. 



The curves for bacterial numbers, nitrate content, and COg in soil 

 air, are sufficiently similar to justify the view that all the phenomena 

 are related. Thus, with some exceptions during periods of active plant 

 growth, a rise in bacterial numbers is accompanied by a rise in CO2 in 

 the soil air, and somewhat later by a rise in nitrate in the soil. Con- 

 versely a fall in bacterial numbers is accompanied by a drop in COg 

 evolution and in nitrate accumulation. This occurs so frequently that 

 it cannot be accidental, and we are forced to conclude that the pro- 

 duction of CO2 and of nitrate in the soil is definitely connected with 

 the rises and falls in bacterial numbers. The curves thus afford a 

 demonstration of the fundamental proposition in soil bacteriology, that 

 the rate of decomposition of organic matter in the soil is a function of 

 the bacterial activity. 



The curves have further enabled us to ascertain the more important 

 factors at work in determining the amount of change in the soil in natural 

 conditions, and to form some estimate of their relative importance. In 

 order to do this we have made systematic determinations of the soil 

 moisture and soil temperature, and these, as well as the other meteoro- 

 logical data, have been plotted. In so far as the curves for nitrate, 

 COg and bacterial numbers agree with the curve of any one of these 

 factors over a sufficient period to eliminate accidental coincidences, that 

 factor is taken as dominating the situation. When no agreement exists, 

 we suppose that some other factor is operating for which therefore search 

 is made. 



The field observations thus show the dominating factors under 

 given conditions, or, alternatively, they show that the dominating factor 

 is not yet discovered, and that further investigation is necessary. They 

 do not, however, indicate very distinctly the nature of the relationship 

 between the various factors and the biochemical reactions. This 

 requires a separate set of investigations to be made in the laboratory 

 under rigidly controlled conditions where all the factors except the 

 one under investigation are kept constant. 



The apphcation of this method has led to the following conclusions : 



Temperature is the chief factor in determining the extent of the 

 biochemical changes in the soil. Until the temperature exceeds 5° C, 

 change is only very slow ; from November to March the reactions we 

 are considering seem to be almost at a standstill. 



As soon as the temperature begins to rise above 5° C. biochemical 



