OXIDATION IN SOIL. 23 



by the chemical action of water, and their contents thus 

 gradually made available to the plant. The solvent power 

 of the water in a soil is greatly increased by the carbonic 

 acid, and perhaps also by the humic acids it holds in 

 solution. Water containing carbonate of calcium in 

 solution is especially capable of attacking silicates. 



The chemical analysis of a soil, though always of great 

 value, does not enable us to fix its degree of fertility 

 owing to the imperfect information it affords as to the 

 condition of the plant food. 



5. Oxidation in Soil. The materials from which the 

 nitrogen of soils is originally derived contain generally a 

 large proportion of carbon. In the roots and stubble of 

 cereal crops the relation of nitrogen to carbon is 1 : 43 ; in 

 those of leguminous crops 1 : 23 ; in moderately-rotted 

 farmyard manure about 1:18. In the soil these materials 

 are oxidised, chiefly by the action of various living organ- 

 isms (insects, worms, fungi, bacteria), large quantities of 

 carbonic acid being produced. As a result of this loss 

 of carbon, we find that the surface soil of a pasture 

 (roots removed) will contain about 1 of nitrogen to 13 

 of carbon ; the surface soil of an arable field about 1:10; 

 and a clay subsoil 1 : 6. 



The nitrogen contained in humus is not in a condition 

 to serve as a general plant food; cereal crops are apparently 

 unable to appropriate it ; some crops may, however, pos- 

 sibly assimilate some humic matters. For the nitrogenous 

 matter of humus to become available to crops it must be 

 farther oxidised ; this is accomplished by certain bacteria 

 in the soil, carbonic acid, ammonia, and finally nitric acid 

 being produced. The nitrifying bacterium occurs abun- 



