40 THE CHEMISTRY OF THE PAEM 



8. Oxidation in Soil. — The materials from which the 

 nitrogenous matter of soils is derived contain always 

 a large proportion of carbon. In the roots and stubble 

 of cereal crops the relation of nitrogen to carbon is 

 about 1 : 43 ; in those of leguminous crops 1 : 23 ; in 

 moderately rotted farmyard manure 1 : 18. In an 

 aerated soil these materials are oxidised by the action 

 of various living organisms (insects, worms, fungi, 

 bacteria), large quantities of carbonic acid being pro- 

 duced. 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 1 : 10 ; and a clay sub- 

 soil 1 : 6. These figures represent the proportion of 

 nitrogen to carbon in the commonest forms of humic 

 matter. Humus represents merely a stage in the 

 decomposition of organic matter ; in the end, the 

 whole of the carbon, hydrogan and nitrogen appear 

 as carbonic acid, water, and ammonia or nitrates. 



The nitrogen contained in humus is not in a con- 

 dition to serve as a food for ordinary crops, the gradual 

 decomposition of soil humus is thus generally essential 

 to fertility. Many kinds of fungi and bacteria are 

 capable of converting the nitrogen of organic matter 

 into ammonia; the final nitrification of ammonia is 

 performed by two species of bacteria, one of w^hich 

 produces nitrites, which the other changes into nitrates. 

 Fresh vegetable residues are more easily nitrified than 

 old humic matter. Nitrification does not begin till the 

 earlier stages of decomposition are past. 



The nitrifying bacteria occur most abundantly in the 

 surface soil ; the depth to which their action extends 

 depends on the porosity of the subsoil. In the clay 



