CHAPTER V. 



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 continuously absorbed, and an almost equal 

 volume of carbon dioxide is evolved, indicating that the main 

 change is of the type — 



CnH2„,0„ + nO^ = nCO^ + mYifi. 



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



C 

 nitrogen, and the ratio ir which, in the original plant material, 



e.g. the stubble, is about 40,^ becomes reduced in the soil to 

 10 (Table XLVIL). Other products are formed as well, 

 including ammonia and the dark-coloured humus bodies 

 already described, but the details of these changes are un- 

 known. Investigations with the individual plant constituents, 

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

 brought out little beyond the fact that they all oxidise to CO2 

 in the soil, while the calcium salts of organic acids change to 

 CaCOa. 



The absorption of oxygen by soil was demonstrated by 

 de Saussure about 1800, studied by Boussingault (48), and 

 definitely attributed by Schloesing to micro-organisms in 

 1883.^ Wollny developed this thesis in 1884 (317) and 

 showed that the rate of oxidation is much diminished by 



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



^Leqons de chimie Agricole, 1883, p. 277 : " C'etait 1^, pensait on alors, un fait 

 purement chimique. On sait aujourd'hui que c'est principalement un fait biolo- 

 gique, c'est- a-dire que la combustion observ6e est le resultat de la vie de nom- 

 breux organismes, tel par exemple que le ferment nitrique, lequel est charge de 

 transporter I'oxyg^ne sur I'azote ". 



175 



