ALUMINUM 63 



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. 



79. 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 conies 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 



80. Aluminum is present in the soil in the largest 

 quantity of any of the base elements. It is calculated 

 that it forms 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. Physically, however, the aluminum 

 compounds take an important 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 alumi- 

 num silicates are the most numerous compounds pres- 

 ent in soils. 



