8 USE OF FELDSPATHIC EOCKS AS FERTILIZERS. 



Table I, — Imports of potasli salts for use as fertilisers^ 



<■ Compiled from data furnished by the Bureau of Statistics, U. S. Department of Agri- 

 culture. 



All of the above salts of potash are readily soluble in water and 

 therefore become quickly available as plant food when applied as 

 fertilizers. On the other hand, the acids with which the potash is 

 combined in these salts, especially in the cases of the sulphate and 

 chlorid, are absorbed only in extremely minute quantities by grow- 

 ing plants. The natural consequence of this is that as the potash is 

 taken away year after year the acid elements collect, in some cases to 

 the great detriment of the soil. 



Potash in some form is absolutely necessary to the successful growth 

 of all crops, and is found in varying degrees in the ashes of wood 

 and of vegetable growths of all kinds. This is illustrated in the 

 following table, in which analyses of ashes of a number of vegetables 

 are given. It must be remembered, however, that the amounts of 

 this element found in the ashes of plants vary greatly under different 

 conditions and that the sap of plants frequently absorbs much more 

 of a given element than is actually necessary for its growth. The 

 important point is that potash is an essential plant food of all crops, 

 and particularly of those species which are starch builders. 



Table II.^ — Anahj.^e.^ of vefjetahlr a.^li. 



We can trace the source of potash in all its forms and whei^er 

 found to the igneous rocks which constituted the original crust of the 

 earth. From the averages obtained from the analyses of a large 

 number of crystalline rocks of common occurrence found i-n this coun- 



104 



