CUSHMAN'S LATER INVESTIGATIONS 521 



"The question whether fine-ground feldspar can be used as 

 a potash fertilizer has been a matter of controversy for many 

 years. There is a large and widely scattered literature on the 

 subject, an examination of which shows that the matter has 

 been debated with much vigor and sometimes with prejudice and 

 intolerance on both sides. It is easy to find the published re- 

 cords of a number of experiments, made by trained and thor- 

 oughly competent agriculturists, which tend to show that ground 

 feldspar is an efficient potash fertilizer. On the other hand, a 

 number of experiments seem to indicate that the potash is only 

 slightly available, while others would appear to show that the 

 ground rock is entirely useless. On account of the large inter- 

 ests involved in the settlement of this question it is not difficult 

 to see why vigorous differences of opinion, and even unjust 

 prejudice should have arisen. When, however, trained investi- 

 gators reach opposite conclusions, based upon experimental evi- 

 dence, we are forced to the opinion that while ground feldspar 

 may be a useful fertilizer under certain conditions it is not so 

 under others." 



Very full reference to the literature of the availability of rock 

 potash is given by Cushman in the work above cited, and in 

 addition to these data experiments were undertaken by him to 

 test anew the value of the fine-ground rocks for this purpose. 

 Tobacco was selected as the plant most suitable for the experi- 

 ment, because it is one of those plants which requires a very 

 large quantity of potash for its proper nourishment. Artificial 

 soils were prepared having for a base coarse grained white sand 

 and finely ground feldspar containing about 8 per cent, of potash. 

 Seedlings were set out in this mixture and moistened from time 

 to time with solutions of ammonium nitrate and ammonium 

 phosphate in order to supply the necessary amount of nitrogen 

 and phosphoric acid. 



In addition to these field experiments others were made in a 

 green house, in which the effect of potassium in the form of 

 carbonate containing about 67 per cent, of potash was compared 

 with the potash in fine-ground feldspar, containing about 8.3 

 per cent of potash. 



