78 SOILS AND FERTILIZERS 



large amounts, 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 proportion, by weight, of the 

 composition of bodies. 



BASE-FORMING ELEMENTS 



89. Aluminum is present in the soil in the largest 

 amount of any of the base elements. It forms probably 

 from 6 to 10 per cent of the solid crust of the earth. 

 As previously stated aluminum is one of the constituents 

 of clay, and is not necessary for plant growth. Physi- 

 cally, however, the aluminum compounds take an im- 

 portant part in soil fertility. Aluminum is usually in 

 combination with silica or with silica and some base- 

 forming element, as iron, potassium, or sodium. The 

 various forms of aluminum silicate are the most numer- 

 ous compounds found in soils. Alumina is the oxide of 

 aluminum, A1 2 O 3 , and is the usual form in which this 

 element is reported in soil and rock analyses. 



90. Potassium is in the soil mainly in the form of 

 silicates, and is one of the elements absolutely necessary 

 for plant growth. The term 'potash' (potassium oxide, 

 K 2 O) is usually employed when reference is made to 

 the potassium compounds. The amount and form of 



