PHOSPHATES IN MASSACHUSETTS AGRICULTURE. 157 



Cumulative Effect. 

 The advocate of the use of the rock phosphates may at this point urge 

 that while such phosphates are at first less effective than the more sol- 

 uble and quickly available materials they will ultimately fully equal the 

 latter. This series of experiments has now continued eighteen years, 

 and it would seem that this result should have been already realized. 

 Tlds has not been the case. The more soluble phosphates, bone meal 

 and slag, still annually exceed the rock phosphates greatly in their effect 

 on crop yield. Such excess, so far as can be judged, is still as great as at 

 any earlier period. The corn crop affords the best chance of comparison, 

 ha\'ing been grown in 1899 and in 1914. The increases in crop per acre 

 in the two years are shown below : — 



Corn Crop — Increases per Acre, ivith Different Phosphates. 



These figures show greater increases in the corn crop on all classes of 

 phosphates in 1914 than in 1899. Such increases, however, are insignifi- 

 cant for the natural rock phosphates, while for the slag, bone meals and 

 soluble phosphates they are large. The latter excel the rock phosphates 

 in 1914 in much greater degree than in 1899. The conclusion, therefore, 

 seems justified that the natural agencies at work in this soil are not in 

 any marked degree increasing the availability of the natural mineral 

 phosphates. In the eighteen years during which this series of experiments 

 has continued we have suppUed 1,728 pounds of phosphoric acid per 

 acre to the soil of these phosphate plots. ^ In the crops harvested from 

 the rock phosphate plots, supposing them to have been of average com- 

 position, we have removed about 450 pounds. There has therefore been 

 a large excess of phosphoric acid applied (about 1,275 pounds per acre), 

 and stiU the amount available is insufficient to give maximum crops. 

 The yields are far below those obtained on the slag, bone meals and dis- 

 solved phosphates. - 



It is well understood that a large proportion of organic matter in the 

 soil is favorable to the activity of the raw phosphates. In commenting 

 on the results obtained in these experiments in his book "Soil Fertility 



1 Two exceptions have been noted, p. 150: plot 2, on which the shortage is 192 pounds, and plot 

 12 on which it is about 240 pounds. 



- These also must have furnished phosphoric acid in much larger quantities than have been 

 removed in the crops. 



