FORMS OF PLANT FOOD 



85. Three Classes of Compounds. — For agricul- 

 tural purposes, compounds present in soils may be 

 divided into three classes : l6 The first class includes 

 silicates and other compounds of potash, soda, lime, 

 magnesia, phosphorus, etc., which are soluble in the 

 soil-water and in dilute organic acids. This class 



\\\\\\\\\\\\\\^1 



1 



\^^\^^^^^^^^^^\^^■^^■^■•;. , ■^:■■.■•''^:••.'■ 



2.3. 



o v y o a t 



L/me 



1 il 



h 



ml 



/&&/ f //7Jc/t/6/e /fe/&$- 



Fig. 



17. Average composition of soils. 

 Nitrogen ; 2. Potash ; 3. Phosphoric acid. 



represents the most soluble and the most active and 

 valuable forms of plant food. There is only a very 

 small amount in these forms. In 100 pounds of soil, 

 rarely more than a few hundredths of a pound of any 

 one of the important elements is soluble in the soil- 

 water or in dilute organic acids. 



