130 SOIL CONDITIONS AND PLANT GROWTH 



These curves resemble those obtained at Rothamsted for the 

 fluctuations in nitrate content of arable soils during the year. In this 

 case the figures do not measure nitrate production but nitrate accumu- 

 lation, i.e. , the difference between nitrate production and nitrate losses. 

 But they persistently show an increased nitrate content in spring, 

 a fall in summer and a rise in autumn. Leather (167$) and Jensen 

 (144) have obtained parallel results. The curves showing the per- 

 centage of carbon dioxide in the soil air are also of the same type 

 (Russell and Appleyard, 2402). This observation is highly significant 

 because it indicates that the actual production both of nitrates and 

 of CO 2 attains a maximum in spring and another in autumn but falls 

 off in summer and in winter. This conclusion is based on the fact that 

 both nitrates and carbon dioxide are formed by the same factor, the 

 micro-organisms acting on the organic matter of the soil, but they are 

 lost from the soil by wholly dissimilar processes; carbon dioxide 

 being lost most quickly in dry weather and nitrates most quickly in 

 wet weather. The general identity of the curves, therefore, shows that 

 the production factors dominate the situation and give the curves their 

 characteristic shape (Fig. 8). During the winter months the curves 

 follow the temperature curve closely, and during the summer they 

 rather resemble the moisture curves and still more closely the rainfall 

 curves. But not altogether; and it is very desirable that these 

 seasonal effects, and especially the remarkable spring and autumn 

 maxima, should be more closely studied. 



Reviewing the whole of the preceding results, it seems clear that in 

 normal soils we are dealing with something more than a bacterial 

 decomposition. The erratic results obtained with changes in tempera- 

 ture and moisture are difficult to explain unless one supposes that 

 other organisms are also present interfering with the activity of the 

 soil bacteria. This view arose out of the work on partial sterilisation 

 which revealed the presence 01 the second great group ot organisms, 

 and which we must now proceed to discuss. 



II. Organisms not Directly Affecting Plant Growth but Acting 



on Those that do. 



Investigation on the Partial Sterilisation of the Soil. 



The earliest observations that soil is altered by an apparently inert 

 antiseptic arose out of attempts to kill insect pests in the soil by means 

 of carbon disulphide. This substance, which for fifty years has been 



