64 THE REACTIONS OCCURHING IN SOILS. IV. 



in the upper layers may be more concentrated than that in the 

 subsoil. The roots of plants are probably, therefore, often 

 bathed in a solution of plant food much richer in dissolved 

 matters than is the drainage water from the same soil. For 

 description of the methods by which plants obtain their food 

 from the soil see Chap. IX. 



Nitrification. Important as the complex and little-known 

 changes which affect the inorganic portions of soils un- 

 doubtedly are, quite as much interest and perhaps more 

 fruitful labour have been directed to the study of the changes 

 undergone by the carbonaceous and nitrogenous constituents. 

 The organic matter in a soil is continually undergoing altera- 

 tion, attended by the absorption of oxygen, and the consequent 

 evolution of heat. This heat emission becomes greater when 

 farm-yard manure is applied to land, and in some cases has 

 been sufficient to raise the temperature of the soil 1 or 2 C. 

 (Wagner). The air in the interstices of a soil is always poorer 

 in oxygen and much richer in carbon dioxide than the air 

 above it. 



It is by changes of this kind that humus is produced from 

 vegetable fibre (v. p. 54). Other organic acids are also formed 

 by oxidation of vegetable matter, and if the soil be deficient in 

 basic materials these acids may exert a baneful influence (as in 

 so-called " sour" land). 



Humus itself is not a permanent substance, but is continually 

 being oxidised and broken down in a soil, a portion of its car- 

 bon being evolved as carbon dioxide, while its nitrogen passes 

 eventually into the form of nitrates. 



This latter change, the conversion of nitrogen existing as 

 organic matter or ammonium compounds into nitric acid, is a 

 most important one and has received an immense amount of 

 attention of late years. It is known as nitrification and is 

 effected, as indeed are probably all the changes of organic 

 matter, through the action of micro-organisms. 



The process of nitrification is an extremely important one, 

 since in the case of the majority of plants it is only in the form 

 of nitrates that nitrogen is assimilated. The organic nitro- 

 genous matter existing in the soil is present in various forms, 



