CHAPTER IV. 



THE CARBON AND NITROGEN CYCLES IN THE SOIL. 



THE organic matter added to the soil by plants, etc., rapidly undergoes 

 a number of changes in presence of air. Oxygen is slowly but con- 

 tinuously absorbed, and an almost equal volume of carbon dioxide is 

 evolved, indicating that the main change is of the type 

 C n H 2m Om + nO 2 = nCO 2 + mH 2 O. 



Thus the carbon in the soil tends to fall off relatively to the nitrogen, 



rj 

 and the ratio which in the original plant material, e.g. the stubble, 



is about 40, 1 becomes reduced in the soil to 10 (Table XXXIII.). 

 Other products are formed as well, including ammonia and the dark- 

 coloured humus bodies already described, but the details of these 

 changes are unknown. Investigations with the individual plant con- 

 stituents, cellulose, fats, various organic acids, proteins have so far 

 brought out little beyond the fact that they all oxidise to CO 2 in the 

 soil, while the calcium salts of organic acids change to CaCO 3 . 



The rate of oxidation, as Wollny pointed out in 1884 (317), is 

 very much diminished by traces of antiseptics, and the process is 

 therefore apparently affected chiefly by micro-organisms. It shows a 

 general increase up to a certain point with the amounts of moisture, 

 organic matter and calcium carbonate present, although no sharp 

 proportionality exists. It is closely related to productiveness ; in a 

 series of soils where the climatic and other external circumstances 

 were similar the respective rates of oxidation were found to vary in the 

 same way as the values for productiveness (Table XXXIII.). 



The reasons for the connection between oxidation and fertility will 

 become more evident as we proceed ; the immediate connection is 

 between oxidation and the activity of micro-organisms. In so far as 

 oxidation is due to micro-organisms, its velocity obviously affords a 

 measure of their activity. But there is a more fundamental relation- 

 ship. Oxidation affords, so far as is known, the chief source of energy 

 for the numerous micro-organisms of the soil. These organisms live in 



1 For leguminous crops, however, it is about 25. 

 80 



