68 SOILS AND FERTILIZERS 



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 of a per 

 cent, or more of lime mainly as calcium carbonate. 



82. 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 

 magnesia in some form, in order to reach maturity 

 and produce fertile seeds. There is generally in all 

 soils an amount sufficient for crop purposes, hence it is 

 not necessary to consider this element in connection 

 with soil fertility. 



83. Sodium is found in the soil mainly as sodium 

 silicate, and is present to about the same extent as 

 potassium which it resembles chemically in many ways; 

 it cannot, however, replace the element potassium. 

 Inasmuch as sodium takes such an indifferent part in 

 plant nutrition it is never used as a fertilizer except in 

 an indirect way. 



84. Iron is an element necessary for plant food and 

 is found in all soils to the extent of from 1 to 4 per 

 cent. Crops require only a small amount of iron, 

 hence there is always sufficient for crop purposes. 

 Iron is present in soils in the form of oxides, hy- 

 droxides, and silicates. 



