1)E0X1DATI0N In soil 41 



subsoil at Kothamsted the organisms did not always 

 occur in small quantities of soil taken at more than 

 three feet below the surface. Nitrification only takes 

 place in a moist soil sufficiently porous to admit air. 

 It is also necessary that some base should be present 

 with which the nitric acid may combine ; this con- 

 dition is usually fulfilled by the presence of carbonate 

 of calcium, nitrate of calcium being produced. Nitri- 

 fication is most active at summer temperatures ; it 

 ceases apparently near the freezing point. The nitrify- 

 ing organisms may be killed by severe drought. 



The oxidation of humus not only makes the nitrogen 

 which it contains available as plant food, it also libe- 

 rates the ash constituents combined with the humus, 

 and enables them to take part again in the nourish- 

 ment of plants. 



Oxidation is most active in soils under tillage. Thus 

 in arable land the production of available plant food is 

 at its maximum, and so is also the waste by drainage. 

 The nitrogenous humic matter of arable land is main- 

 tained only when the new supply from crop residues 

 and organic manures is equal to the amount annually 

 oxidised. In an untilled pasture, or forest soil, on the 

 other hand, a considerable accumulation of organic 

 matter may take place, the annual residue of dead 

 roots and leaves being often in excess of the means of 

 oxidation. In a peat bog oxidation is further checked 

 by a high water-level, which excludes air from the 

 soil : under such conditions an unlimited accumula- 

 tion of organic matter may take place if plants capable 

 of growing in these circumstances are present. 



9. Deoxidation in Soil. — When a soil is not in an 

 aerated condition, but has the spaces between its 



