254 NATURE AND PROPERTIES OF SOILS 



Table LIV 



EFFECT OF FARM MANURE ON THE CARBON DIOXIDE CONTENT OF 

 SOIL AIR. ROTHAMSTED, ENGLAND 



Although the formation of carbon dioxide in the soil is in- 

 fluenced to a marked degree by the decomposition of organic 

 matter, the effect is by no means proportional to the quantity 

 of organic matter present. The rate of decomposition varies 

 greatly, and where this is depressed, as sometimes occurs in 

 muck or forest soils, the content of carbon dioxide is relatively 

 low. A high percentage of organic matter is in itself likely 

 to prevent a proportional formation of carbon dioxide, since 

 the accumulation of the gas may inhibit further activity of 

 the decomposing organisms. 



131. Oxidation and its effect on the composition of the 

 soil air. — Oxidative processes in the soil are of two general 

 types, those due to chemical reactions alone and those due 

 to biochemical transformations. The purely chemical oxida- 

 tion may be illustrated best by recalling the processes of soil 

 formation. 1 Here it was noted that certain minerals, espe- 

 cially those carrying iron, were susceptible to the influence of 

 oxygen. The following reactions show how olivine may as- 

 sume water and then produce ferric oxide through oxidation : 

 3MgFeSi0 4 + 2H 2 = H 4 Mg 3 Si 2 9 + Si0 2 + 3FeO 

 4FeO + 2 = 2Fe 2 3 



This is illustrative of the complex reactions which are con- 

 tinually taking place and which tend materially to decrease 

 the oxygen of the soil air. 



1 See Chapter II, par. 16, of this text. 



