30 USE OF FELDSPATHIC EOCKS AS FERTILIZERS. 



cents per pound. At $5 per ton the cost per pound would fall to 28 

 mills. The figures are of course only applicable provided the potash 

 in the ground material can be proved available as a plant food. 



It must be remembered that the only real measure of available 

 potash is that which is made use of by the crop. It is not likely that 

 all the potash added, even in the form of soluble j^otash salts, is 

 actually used, and the amount that can be supplied by ground rock is 

 still an unknown quantity. 



POSSIBLE HARMFUL EFFECTS OF GROUND FELDSPAR. 



The question is frequently asked whether there is possible danger 

 to the land in experimenting with the use of ground feldspathic rock. 

 It is Avell known that in some cases, notably Avith tobacco, injurious 

 etfects are produced by the continued use of the soluble potash salts, 

 particularly the sulphate and muriate. Feldspar grains of various 

 sizes are normally present in many soils; it does not, therefore, seem 

 possible that any harmful effect could follow the application of 

 ground rock. As has been pointed out in an earlier portion of this 

 paper, feldspar consists of the alkaline elements, soda, potash, and 

 lime, combined with alumina and silica. After decomposition, 

 hj^drated aluminum silicate, the essential base of all clavs, is left 

 behind, the alkalis and the silica being set free in a condition in 

 which they can be absorbed by the root action of plants. It would 

 seem, therefore, that whatever the value of the results obtained no 

 possible harm can follow the experimental use of ground feldspar in 

 reasonable quantities. 



EXTRACTION OF POTASH FROM GROUND ROCK. 



The discussion of the use of ground rock as a source of potash is 

 not complete unless it includes the extraction of potash by chemical 

 and electrical processes. If future experiments should demonstrate 

 that fast-growing crops are dependent on very soluble forms of potash 

 the question of the extraction of this element from ground feldspar 

 becomes a matter of importance. 



The extraction of potash from rock has not as yet been accom- 

 plished on a commercial basis, but it has been done in the laboratory, 

 and the method has been published in a recent bulletin." The full 

 details of the investigation are too technical for insertion here, but if 

 the i^rocesses described coidd be carried on at a cost low enough, the 

 potash in ground rock could be rendered sufficiently soluble for all 

 practical purposes. Briefly, the method consists in sliming the 

 ground feldspar with Avater to which a small quantity of hydrofluoric 



o Bui. 28, U. S. Dept. of Agriculture, Office of Public Roads. 

 104 



