8O SOILS AND FERTILIZERS 



The term 'magnesia' (magnesium oxide, MgO) is used 

 when reference is made to the magnesium compounds 

 of the soil. 



93. Sodium is in the soil mainly as sodium silicate, 

 and to about the same extent as potassium, which it 

 resembles chemically in many ways. It cannot, how- 

 ever, replace potassium in plant growth. Inasmuch as 

 sodium takes an indifferent part in plant nutrition, it 

 is never used as a fertilizer except in an indirect way. 



94. Iron is an element necessary for plant food and is 

 found in all soils to the extent of from I to 4 per cent. 

 Crops require only a small amount of iron, hence there 

 is always sufficient for crop purposes. Iron in soils is 

 in the form of oxides, hydroxides, and silicates, 



FORMS OF PLANT FOOD 



95. Three Classes of Compounds. For agricultural 

 purposes, the compounds present in soils may be divided 

 into three classes : 9 (i) Compounds soluble in water and 

 dilute organic and mineral acids ; (2) compounds soluble 

 in more concentrated acids ; (3) insoluble compounds 

 decomposed by strong acids and fluxes. 



96. Water- and Dilute Acid-soluble Matter of Soils. 



This class includes silicates and other compounds of 

 potash, soda, lime, magnesia, phosphorus, etc., which are 



