BASE-FORMING ELEMENTS 67 



the constituents of clay, and furnishes nothing 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 

 present in soils. 



80. Potassium is present in the soil mainly in 

 the form of silicates, and is one of the elements abso- 

 lutely necessary for plant growth. The term potash 

 (potassium oxide, K 2 0) is usually employed when the 

 potassium compounds are referred to. The amount 

 and form of the soil potash have an important bearing 

 upon fertility. Potassium is one of the three elements 

 of plant food usually supplied in fertilizers. The 

 form in which it is present in the soil and its economic 

 supply as plant food, are important factors of crop 

 growth, and are considered in detail in Chapter VIII. 

 The amount of potash in soils ranges from 0.02 to 0.8 

 per cent. In a fertile soil it rarely falls below 0.2 per 

 cent. 



81. Calcium is present in the soil in a variety of 

 forms, as calcium carbonate, calcium silicate, and 

 calcium phosphate. The calcium oxide (CaO) of 

 the soil is generally spoken of as the lime content. 

 Calcium carbonate and sulphate are important factors 

 in imparting fertility. A subsoil with a good supply 



