82 SOILS AND FERTILIZERS 



elements make up only a small part of this amount. In 

 200 samples of soil, the potash, nitrogen, lime, magnesia, 

 and phosphoric and sulphuric anhydrides amounted to 

 3.5 per cent ; in many fertile soils the sum of these is less 

 than 1.50 per cent. This means that in every 100 pounds 

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

 take any active part in the support of a crop, while 96 to 

 98.5 pounds are present simply as so much inert material, 

 and valuable only from a physical point of view. Not 

 all of the potash, for example, soluble in hydrochloric 

 acid is equally valuable. In fact, the acid represents 

 more than the limit of the crop's feeding power, when 

 there is not enough of more soluble forms to aid in the 

 first stages of growth. 



98. Acid-insoluble Matter of Soils. This class in- 

 cludes all of those compounds of the soil which require 

 the joint action of the highest heat and the strongest 

 chemicals in order to decompose them. The insoluble 

 residue obtained after digesting 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 immediate value as plant food. When 

 seed was planted in soil extracted with strong hydro- 

 chloric 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. 84 (Fig. 23.) 



The acid-insoluble matter of soils is capable of under- 

 going disintegration and in time may be changed to the 



