148 MASS. EXPERIMENT STATION BULLETIN 162. 



by the work of others, the more soluble phosphates exert special influences 

 which are highly important as a result of their relatively soluble con- 

 dition when apphed. Any advantage which may be connected with this 

 relatively high degree of solubihty is of course largely lost, in so far as 

 the residual phosphoric acid they contain is concerned, because of the 

 change in the soil which converts this phosphoric acid into a much less 

 soluble form. The true way to use dissolved phosphates, as is well under- 

 stood, is not to apply at any one time in great excess of the requirements 

 of the immediately succeeding crops, but to apply as a rule annually, in 

 the case of hoed crops at least, in quantities more nearly equal to the im- 

 mediate crop requirement. 



In this series of experiments the different phosphates under comparison 

 (dissolved bone black, basic slag meal. South Carohna rock phosphate, 

 Florida rock phosphate and Mona guano) were applied during only four 

 years. The experiment was continued twelve years. During this long 

 period of time the basic slag meal gave the greatest total crop yield; the 

 South Carohna rock phosphate ranked next, but was followed so closely 

 by the dissolved bone black that the difference was quite insignificant in 

 spite of the fact that the latter was used in a manner so absolutely irra- 

 tional, and applied in ciuantity furnisliing only about one-third as much 

 phosphoric acid as was apphed in the South Carohna rock phosphate. 

 The yields on the Mona guano and Florida rock phosphate, especially on 

 the latter, were materially below those obtained on the dissolved bone 

 black. 



It should, perhaps, be pointed out further that this experiment was 

 continued a number of years after the crop yield on all plots had sunk 

 below the profitable level, while there stiU remained in the soil of the 

 plots which had received the rock phosphates more than two-thirds of 

 the large amount of phosphoric acid which had been applied. At the 

 same time, the phosphoric acid which had been apphed in the dissolved 

 bone black had nearly all been carried away in the crops. 



Phosphates compared on the Basis of Equal Annual Applications 

 OF Phosphoric Acid. 



This series of experiments was begun in 1897 and is still continued. 

 We now have the results of eighteen years. The soil is a medium silt 

 loam which had been in grass a number of years previous to being plowed 

 for the experiment. The soil varies somewhat in physical character in 

 different plots, but as the variation is progressive from one end of the 

 field to the other, and the arrangement includes a no-phosphate plot at 

 either end and one in the middle, each phosphate being compared only 

 with the two no-phosphate plots between which it hes, and each of these 

 being given a weight inversely proportional to its distance, it is not be- 

 heved that any injustice is done to any of the phosphates in the results 

 as presented. The more soluble phosphate plots are at the end of the 

 field where the soil is the more heavy. 



The area of the plots is one-eighth acre, — thirteen in all. 



