PHOSPHATES IN MASSACHUSETTS AGRICULTURE. 159 



early growth — has already been referred to.^ There is considerable 

 evidence which tends to show that there are several other indirect or 

 secondary effects of importance. 



Effect on Soil Acidity. 



Most of the secondary effects are believed to be beneficial, but the 

 question is frequently asked whether the use of dissolved (acid) phos- 

 phates will not exercise an injurious secondary effect through making 

 soils sour. Those advocating the use of natural rock phosphates usually 

 call especial attention to this effect. Thus, Hopkins says: "A third point 

 in favor of raw phosphate, in common with bone and slag, is that it is 

 free from acidity and has no tendency to injure the soil."^ In the follow- 

 ing sentence he asserts that acidity does develop from continued use of 

 acid phosphate, but adds that it can be corrected at small expense by 

 the use of lime. 



The writer cannot point to results of chemical investigations in con- 

 nection with his work which either prove or disprove the correctness of 

 tliis assertion, that continued use of acid phosphate increases soil acidity. 

 No such investigations have been undertaken. In some of his experiments, 

 however, lime has been so applied as to afford opportunity to note the 

 relative benefit as indicated by crop yield under otherwise similar condi- 

 tions on plots over a long series of years, respectiveh^ without and with 

 apphcation of an acid phosphate (dissolved bone black). If the dissolved 

 bone black used continuously had increased acidity in any marked degree, 

 it would follow that crops to which acid is toxic would show greater bene- 

 fit from liming on the plots to which cUssolved bone black was aimually 

 aj)plied than on those plots not receiving it. In a long-continued series 

 of soil test experiments, 3 where one-half of all plots has been hmed, this 



1 See p. 151. 



2 "Soil Fertility and Permanent Agriculture," p. 242. 



3 This series of experiments was begun in 1890 and has continued to date. Nitrate of soda, dis- 

 solved bone black and muriate of potash have each been applied annually, as shown by the table. 



inA^^ one-half of all piots received an application of lime at the rate of one ton per acre- in 



1904 a second application was made to the same halves at the rate of 2,300 pounds per acre; and 

 in 1907 a third application at the rate of 1,000 pounds per acre. 



