EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 25 



amount of money will frequently purchase four times as much 

 insoluble as soluble phosphoric acid. 



Many experiments conducted according to this third plan, how- 

 ever, are not comparable since not only do the prices of the two 

 forms of phosphoric acid vary considerably in different localities, 

 depending on the distance to the mines and fertilizer factories, but 

 the margin of difference also fluctuates from year to year, depending 

 on market and labor conditions. ." - 



While this third method of comparison appears m^i^fl fairer than 

 the first and second, it is open to objections from a sirictly business 

 standpoint. 



Practically all of the more ardent supporters of ground raw rock 

 phosphate as a fertilizer concede that full benefit can not be gained 

 from applications of such material until it has been allowed to 

 remain in the soil for a year or more, and that it becomes increas- 

 ingly effective as it becomes more thoroughly distributed through 

 cultivation and is exposed to the action of certain soil solvents. 

 This means the investment of capital which does not pay its full 

 interest for some years, while an equal amount of money invested 

 in acid phosphate may pay good interest the first year. The fol- 

 lowing plan, which does not yet seem to have been tried, appears 

 to be a more logical method of comparing the two classes of phos- 

 phates: 



Apply the first year the several phosphates in quantities repre- 

 senting equal money values. When the crops are harvested, any 

 increase from the acid phosphate plots over and above that from the 

 raw rock plots reinvest in acid phosphate to be applied to the next 

 crop of the acid phosphate plot, thus keeping the net profit from 

 the two plots constantly equal for a number of years until sufficient 

 time has elapsed for the raw rock to have reached its maximum 

 effectiveness. 



Hopkins has proposed and followed a scheme somewhat similar to 

 the above in the addition of manure or crop residues to variously 

 treated plots. His plan consists in adding to each treated plot after 

 the first year these materials in quantities equivalent to the amounts 

 which would be produced from the crop grown on that particular 

 plot. 



In considering the field work of the experiment stations, discussion 

 of the profits obtained from various fertilizer treatments have in 

 most instances been omitted since the cost of fertilizer materials as 

 well as most crops vary from year to year and place to place. It was 

 thought best, therefore, to allow those sufficiently interested in the 

 subject to figure the financial returns for any particular time and 

 locality. 



