3 
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, 
cepending 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 much fairer than 
the first and second, it is open to objections from a strictly business 
standpoint. 
Practically all of the more ardent supporters of ground raw reck 
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 
im 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 
ierease 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 
loeality.- ~~. 
