522 AGRICULTURAL ANALYSIS 



In addition a third experimental plot was prepared, to which 

 no potash was added. All three beds were supplied with a 

 sufficient quantity of nitrogen for the purposes of crop growth 

 in the form of ammonium nitrate. 



After the harvesting of the crop the data were collected with 

 the following results : 



RESULTS OF GREENHOUSE EXPERIMENTS WITH TOBACCO PLANTS. 



Cushman calls attention to the fact that these yields are not 

 equal to those obtained in the field under good conditions, but 

 are satisfactory for a winter crop in a green house. 



441. The Effect of Fineness of Grinding. In the study of these 

 experiments the theoretical contention on which is based the idea 

 that the fineness of the grinding is to some extent the measure of 

 the availability of the feldspar in feldspathic rocks was thor- 

 oughly tested. The conclusion is that the potash in these rocks 

 is partially available even during the first season, but that the 

 availability in the coarse particles is not very great, hence fine- 

 ness of grinding is not only a necessary condition to the utiliza- 

 tion of the potash, but time is also an important factor. It 

 would be unwise in the present state of our knowledge to depend 

 upon the finely ground potash alone for the production of a 

 large crop. It is wiser to add during the first years at least a 

 sufficient quantity of more available potash to supply the needs 

 of the crop until the weathering of the finely ground rock has 

 somewhat progressed. 



The problem of cost is also an important factor in these in- 

 vestigations. Unless feldspar can be reduced to a fine powder, 

 transported and sold at a small price per ton, it is evident that at 

 least for the present it can not play any important role as a plant 

 food. 



442. Mills for Grinding. The mills which are used for grind- 



