THE CHEMICAL COMPOSITION OF SOILS 79 



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 

 in the soil and its economic supply as plant food are 

 important factors in crop production. 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. 



91. Calcium is in the soil in a variety of forms, as 

 calcium carbonate, calcium silicate, calcium sulphate, 

 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 of 

 lime will stand heavy cropping and remain in excellent 

 chemical and physical condition for crop growth. In a 

 good soil there is usually 0.2 per cent or more of lime, 

 mainly as calcium carbonate. 



92. Magnesium is found in all soils and is usually 

 associated with calcium, forming the mineral dolomite, 

 which is a double carbonate of calcium and magnesium. 

 Magnesium may also be present in the soil in the form 

 of magnesium sulphate or magnesium chloride. All 

 crops require a certain amount of magnesia in some 

 form, in order to reach maturity and produce fertile 

 seeds. There is generally in all soils an amount suffi- 

 cient for crop purposes, hence it is not necessary to 

 consider this element in connection with fertilizers. 



