58 Agricultural Chemistry. 



will contain about 1 part of nitrogen to 13 of carbon ; the surface 

 soil of an arable field 1:10, and a clay soil 1:6. These figures 

 represent the proportion of nitrogen to carbon in the commonest 

 forms of humus matter. Humus represents merely a stage in 

 the decomposition of organic matter; in the end the whole of 

 the carbon, hydrogen and nitrogen appear as carbon dioxide, 

 water and ammonia or nitrates. 



The nitrogen contained in humus is not in a condition to serve 

 as food for ordinary crops. The gradual decomposition of soil 

 humus is consequently generally essential to fertility. This 

 change in the humus is brought about by fungi and bacteria, 

 which convert the nitrogen of organic matter into ammonia and 

 nitrates, forms which are soluble in water and available to the 

 plant. The final nitrification of ammonia is performed by two 

 species of bacteria, one of which produces nitrites, which the 

 other changes into nitrates. Fresh plant residues are more easily 

 nitrified than old humus matter, but nitrification does not begin 

 until the earlier stages of decomposition have occurred. 



The nitrifying organisms occur most abundantly in the surface 

 soil; the depth to which their action extends depends on the 

 porosity of the soil. In experiments at Rothamsted, England, 

 on a clay subsoil, it was found that the organisms did not always 

 occur in samples of the soil taken at more than 3 feet below the 

 surface. 



Nitrification only takes place in a moist soil and one sufficiently 

 porous to admit air. It is always necessary that some base 

 should be present with which the nitric acid formed may com- 

 bine. This condition is usually fulfilled by the presence of car- 

 bonate of lime. Lack of oxygen and an acid condition of the 

 soil are both unfavorable to the growth of nitrifying organisms. 

 This gives us a rational explanation of the advantages of thor- 

 ough tillage which aerates the soil and of the maintenance of 

 non-acid soils by the application of lime. Nitrification is most 

 active in the summer season; it ceases near the freezing point. 

 The nitrifying organisms may be killed by severe drought. 



