26 USE OF FELDSPATHIC EOCKS AS FERTILIZEKS. 



we assume that A'On Feilitzen and others are right and that the feld- 

 spar is of little or no value, we must conclude that the practice of 

 adding potassium carbonate each year to the tobacco fields is 

 unnecessary. 



Many attempts have been made by the writer to devise crucial tests 

 which would show decisively whether or not the ground feldspar is 

 actually capable of giving up potash to growing plants. One of these 

 experiments, which was undertaken in cooperation with Dr. L. J. 

 Briggs, of the Bureau of Plant Industry, is interesting on account 

 of its direct bearing on this point. A wooden trough of 1 square foot 

 cross-sectional area and 30 feet in length was filled with a poor 

 meadow soil, containing no aj^preciable quantity of water-soluble 

 j)otash. About 5 pounds of fine-ground feldspar, containing 9.3 per 

 cent of total potash, was then evenly distributed over the surface of 

 the trough and well worked into the soil. Carbon electrodes of 

 about 1 square foot area were inserted at the extreme ends and 

 tobacco seedlings set out at 1-foot intervals along the trough. The 

 carbon electrodes were connected to a 110- volt direct-current lighting 

 circuit and a current of electricity was passed continuously in the 

 same direction through the soil while the growth of the plants was 

 in i:)rogress. The amount of current varied with the moisture content 

 of the soil — between the limits of 5 and 15 milli-amperes. The plants 

 made a flourishing growth, and at the end of several wrecks it was 

 apparent that the best growth was being made in the neighborhood of 

 the center of the trough, the plants at the extreme positive and nega- 

 tive ends being more or less stunted. The soil at the beginning of 

 the experiment was neutral to litmus paper. At the end of the experi- 

 ment, when the plants had attained a maximum height of 2 feet, a 

 sample taken from the- positive end of the trough reddened litmus 

 paper, showing it to be decidedly acid. The middle section by the 

 same test was shown to be faintly and the negative end strongly 

 alkaline. While no sweeping deductions can be drawn from the 

 single experiment, it is quite certain that a partial transference of the 

 alkaline bases from the positive to the negative end of the trough had 

 occurred. This effect could only take place with that portion of the 

 bases which had actually passed into solution. The results of this 

 experiment appear sufficiently interesting to warrant repetition under 

 still more careful conditions. 



THE EFFECT OF FINENESS OF GRINDING. 



From the evidence which has been submitted in the preceding pages 

 it may be safelf" concluded that the potash contained in ground feld- 

 spar is at least in some part available as plant food. The question 

 whether it can be made sufficiently available under certain conditions 



104 



