ACID-SOLUBLE MATTER OP SOILS 65 



nesia 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 

 fertilizers. 



84. 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 in plant growth 

 the element potassium. Inasmuch as sodium takes 

 an indifferent part in plant nutrition it is never used 

 as a fertilizer except in an indirect way. 



85. 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 is present in soils in the form of oxides, hydrox- 

 ides, and silicates. 



FORMS OF PLANT FOOD 



86. Three Classes of Compounds. For agricul- 

 tural purposes, the compounds present in soils may be 

 divided into three classes : 17 (i) Compounds soluble 

 in water and dilute organic and mineral acids ; (3) 

 compounds soluble in more concentrated acids ; (3) in- 

 soluble compounds decomposed by strongest acids and 

 fluxes. 



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



This class includes silicates and other compounds of 

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

 are soluble in the soil water and in very dilute organic 



(5) 



