160 SOILS AND MANUEES 



or it may be supposed that they merely cause reduction 

 of nitrates to nitrites and ammonia, and that the libera- 

 tion of the gases is due to the interaction of these com- 

 pounds. Nitrous acid and ammonium nitrite are both very 

 unstable ; the former breaks up into nitric oxide, nitric 

 acid and water, and the latter into water and free nitrogen. 

 There are difficulties in connection with such a theory, and 

 the question is one for experiment rather than speculation. 



Whatever may be the true explanation, the facts point 

 to the importance of maintaining the soil in a state of 

 thorough oxidation. They show also that the use of ex- 

 cessive quantities of farmyard manure may be actually 

 injurious to the crops, and that it is inadvisable, in some 

 cases, to combine the use of large dressings of farmyard 

 manure and nitrates. 



Practical Importance. The capacity of soils to supply 

 crops with nitrogen depends upon the process of nitrifi- 

 cation, for it is by that means that the nitrogenous com- 

 pounds are rendered available to plants. Nitrification has, 

 therefore, a very close relation to productiveness. It takes 

 place naturally on all cultivated soils, but can be acceler- 

 ated by treatment which ensures suitable conditions. It 

 is chiefly influenced by the temperature, and the supply of 

 moisture, oxygen and lime. 



Good drainage has a favourable effect ; by removing 

 excess of water, the temperature of the soil is raised and 

 oxygen finds more ready access. 



Cultivation also favours nitrification by opening up the 

 soil and exposing it to oxygen, but if carried on so as to un- 

 duly promote evaporation it may have a contrary effect. The 

 penetration of the roots of plants helps to loosen the soil 

 and admit air, and nitrification generally proceeds faster 

 in soils under crop than in bare soils under like conditions. 

 Consolidation tends to exclude the oxygen, and on some 

 soils may retard the process. 



