THE CHEMICAL COMPOSITION OF SOILS // 



her of plants and by them indirectly. For ordinary 

 agricultural crops, particularly the cereals, nitrogen 

 must be present in the soil as combined nitrogen. This 

 is the most expensive of any of the elements of plant 

 food, and is liable to be deficient. No other element 

 takes such an important part in agriculture or in life 

 processes as does nitrogen. 



87. Oxygen. Oxygen is combined with both the 

 acid- and base-forming elements and is found in nearly 

 all of the compounds of the soil. It has been estimated 

 that about one half of the crust of the earth is com- 

 posed of oxygen, which in large amounts is combined 

 with silicon, forming silica. That which is held in 

 chemical combination in the soil takes no part in the 

 formation of plant tissue. In addition to being present 

 in the soil, oxygen constitutes eight ninths of the weight 

 of water and about one fifth of the weight of air. It 

 also forms about 50 per cent of the compounds found 

 in plants and animals. Oxygen in the interstices of the 

 soil is an active agent in bringing about many chemical 

 changes, as oxidation of the organic matter, and disin- 

 tegration of the soil particles. 



88. Hydrogen. This element is never found in a 

 free state in the soil, but is combined with carbon and 

 oxygen in animal and vegetable matter, with oxygen to 

 form water, and in a few cases with some of the base 

 elements to form hydroxides. It is not in the soil in 



