1905] PUBLIC DOCUMENT — No. 33. 131 



out. Up to the present year the yields on silicate of potash 

 with tlie difierent crops grown have not been markedly 

 inferior to those obtained on otlier potash salts. It is im- 

 possible, in the light of our present knowledge of the con- 

 ditions, to offer an explanation of the facts, although it is, 

 of course, evident that the soil, which was originally in fair 

 condition as regards its stock of available potash, has previ- 

 ous to this year been in condition to furnish a larger share 

 of the potash needed by the crop than at present. With 

 increasing exhaustion of natural stores of potash, the ditfer- 

 ences due to the several fertilizers used may naturally be 

 expected to increase. 



IV. — Comparison of Phosphates on the Basis of Equal 

 Application of Phosphoric Acid. 

 The present is the eighth season of this experiment, which 

 has for its object the determination as measured by crop 

 production, of the relative availability of different materials 

 which may be used as sources of phosphoric acid when used 

 in such quantities as to furnish equal amounts of actual phos- 

 phoric acid to each plot, and in connection with materials 

 which supply the other elements of plant food, especially the 

 nitrogen and the potash, in abundance and in the same forms 

 and in equal amounts on each of the plots. The field in 

 which these experiments are carried on is divided into thir- 

 teen plots, of about one-eighth of an acre each. Three plots 

 in the field, one at each end and one in the middle, have re- 

 ceived no phosphoric acid since the experiment began. The 

 phosphates which are employed on the other plots are as fol- 

 lows : apatite, South Carolina rock phosphate (fine ground), 

 Florida soft phosphate, basic slag meal, Tennessee rock phos- 

 phate (fine ground), dissolved bone-black, raw bone meal, 

 dissolved bone meal, steamed bone meal and acid phosphate. 

 These phosphates are used in such quantities as to furnish 

 actual phosphoric acid, at the rate of 96 pounds per acre. 

 The nitrogen and potash fertilizers used supply nitrogen at 

 the rate of 52 pounds and potash at the rate of 152 pounds 

 per acre. With some of the crops grown (onions and cab- 

 bages) a supplementary application of quick-acting nitrogen 



