66 SOILS AND FERTILIZERS 



portions 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 ad- 

 dition 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. Oxy- 

 gen in the interstices of the soil is an active agent in 

 bringing about many chemical changes, as oxidation 

 of the organic matter, and disintegration of the soil 

 particles. 



78. Hydrogen. — This element is never found in a 

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

 oxygen as in animal and vegetable matter, with oxy- 

 gen to form water, and in a few cases with some of 

 the base elements to form hydroxides. It is not found 

 in large amounts in the soil, and that which forms a 

 part of the tissues of plants and animals comes from 

 the hydrogen in water. Hydrogen in the organic 

 matter of soils takes no .part directly in producing the 

 hydrogen compounds of plants. On account of its 

 lightness, hydrogen never makes up a very large pro- 

 portion, by weight, of the composition of bodies. 



BASE-FORMING ELEMENTS 



79. Aluminum is present in the soil in the largest 

 quantity of any of the base elements. It is calculated 

 that from 6 to 10 per cent, of the solid crust of the 

 earth is aluminum. As previously stated it is one of 



