64 SOILS AND FERTILIZERS 



81. Potassium is present in the soil mainly in the 

 form of silicates, and is one of the elements absolutely 

 necessary for plant growth. The term potash (potas- 

 sium oxide, K 2 O) is usually employed when the potas- 

 sium 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 Chap- 

 ter 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. 



82. 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 

 of lime will stand heavy cropping and remain in ex. 

 cellent 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. 



83. Magnesium is present in all soils and is usually 

 associated with calcium, forming the mineral dolo- 

 mite, 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 mag- 



