ORIGIN AND COMPOSITION OF SOILS 19 



bined with oxygen it forms silica, which consti- 

 tutes free or in combination, more than one- 

 half of the earth's crust. The oxide of silicon 

 occurs in the very common form of quartz, and 

 as silicate of alumina, lime or magnesia silicon 

 forms an essential part of many minerals, such as 

 the feldspars, amphiboles, pyroxenes and the 

 micas, besides being an essential ingredient of 

 many other minerals. Silica is relatively very 

 slightly affected by the ordinary forces concerned 

 in the decay of rocks, and even after the crystals 

 of feldspars, micas and other common minerals 

 occurring in rocks have been disintegrated it 

 remains as hard grains of sand, forming the 

 bulk of most soils. By far the largest part of 

 silicon in soils is in the form of grains of quartz 

 more or less modified. This form of silica is 

 probably chemically inert in regard to plant growth 

 but it plays a very important part in the physical 

 structure of soils in its relations to plant nutri- 

 tion. 



Carbon as an elementary substance occurs as 

 diamond and graphite and in an impure form as 

 anthracite and bituminous coals. In peat and 

 mucks carbon is the chief constituent. This sub- 

 stance is also contained in the organic matters of 

 the soil known as humus, and the relation of the 

 carbon to nitrogen often throws important light 

 upon the amount and character of the nitrogenous 

 matters. The humus content of the soil also 

 affects its relation to water and to the absorption, 



