THE CHEMICAL COMPOSITION OF SOILS 



8l 



soluble in the soil water and in very dilute organic and 

 mineral acids, and 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 arable soil, rarely 

 more than 0.005 pound of 

 any one of the important 

 elements is soluble in the 

 soil water or more than 0.05 

 pound in dilute organic acids. 



97. Acid -soluble Matter of 

 Soils. The plant food of 

 the second class is in a some- 

 what more insoluble form, 

 and consists of compounds, 

 principally the zeolites, sol- 

 uble in hydrochloric acid of 

 23 per cent strength, sp. gr. 

 1.115. This represents the 

 limit of the solvent action 

 of the roots of plants. 9 In 

 this class are included also 

 all the mineral elements 

 combined with the humus and soluble in dilute alkalies. 

 As a rule, not over 10 to 20 per cent of the total soil is 

 soluble in hydrochloric acid ; and the more important 



FlG. 23. Oat Plant grown in soil 

 extracted with hydrochloric acid. 



