FORMS OF PLANT FOOD 



71 



pounds of soil there are only from 1.5 to 3.5 pounds of 

 matter which can take any active part in the support 

 of a- crop, and 96 to 98.5 pounds are present simply as 

 so much inert material. 



86. Acid-insoluble Matter of Soils. — The insoluble 

 residue obtained after digest- 

 ing a soil with strong hydro- 

 chloric acid, contains potash, 

 soda, and limited amounts of 

 magnesia, and phosphoric 

 acid, with other elements 

 which are of no value as 

 plant food. If a seed were 

 planted in soil extracted with 

 strong hydrochloric acid, it 

 would make no growth after 

 the reserve food in the seed 

 had been exhausted. A plant 

 grown in such a soil is shown 

 in the illustration (Fig. 18). 



On the other hand it can- 

 not be said that all of the 

 plant food soluble in hydro- 

 chloric acid is equally valu- 

 able. In fact, the acid repre- 

 sents more than the limit of 



the crop's feeding power, Fig. iS. Oat plant grown in 

 , r, . v _ . 1 r soil extracted with hydro- 



when there is not enough of chloric acicL - 



