68 



SOILS AND FERTILIZERS 



insoluble residue obtained after digesting a soil with 



strong hydrochloric acid, contains potash, soda, and 



limited amounts of magnesia, 



and phosphoric acid, with 



other elements which are of 



no value as plant food. 



When seed was planted in 



soil extracted with strong 



hydrochloric acid, it made 



no growth after the reserve 



food in the seed had been 



exhausted. A plant grown 



in such a soil is shown in 



the illustration, 1 ? Fig. 18. 



The acid-insoluble matter 

 of soils is capable of under- 

 going disintegration and in 

 time may be changed to the 

 second or zeolitic class of 

 silicates. This process, how- 

 ever, is too slow to be relied 

 upon as an immediate source 

 of plant food. 



In the following table the 

 percentage amounts of com- Fi S- l8 - Oat P lant S rown in 



soil extracted with hydro- 

 pounds soluble and insoluble chloric acid. 



in hydrochloric acid are given : 17 



