EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 99 
to somewhat greater advaniage than the raw rock when the apparent 
_ natural fertility of the various plots is considered, but the difference 
_ between the acid phosphate plots and those treated with raw rock 
was so slight as to be well within experimental error. All the phos- 
phate plots gave considerably greater yields of hay than those re- 
ceiving no phosphate. 
The corn crops (1886, 1890, 1894) showed their best yields on the 
plots treated with finely ground bone and reverted phosphate. When 
the natural (apparent) fertility of the plots is taken into considera- 
tion, the acidulated phosphate plots show better yields than the raw 
rock plots. All the plots treated with phosphates, however, showed 
considerably greater yields of corn than those receiving no phos- 
phates. ~ 
With oats, the ground-bone plots gave on the whole better yields 
than any of the other phosphate plots, but the raw-rock plots were 
not far behind and showed to considerably greater advantage than 
those treated with soluble phosphate. 
In summing up the results of this experiment, Hess? of the 
Pennsylvania station, concludes that, considering the cost of practi- 
cally equal quantities of phosphoric acid in its various forms, ground 
bone and raw rock phosphate gave the largest returns for the money 
invested, “thus indicating that the insoluble phosphoric acid is of 
more value as a manure than is often supposed * * *,” 
While the results do seem to show that raw rock phosphate had a 
distinctly beneficial effect on this field, the increases in yield appear 
to the writers out of proportion to the six light applications of this 
material employed, since the total quantity of phosphoric acid thus 
applied (provided none was removed or lost) would not be sufficient 
to add 0.01 per cent to the upper 9 inches of an average soil. In 
other words, raw rock phosphate was applied at the rate of about 54 
pounds per acre per annum. 
The only other work on raw rock phosphate reported by the Penn- 
sylvania station is a laboratory experiment conducted by McDowell? 
in 1908, to test the effect of rotting manure on the solubility of a 
natural phosphate. This experiment is mentioned elsewhere in this 
bulletin. — 
While the field work of the Pennsylvania station seems to indicate 
that raw rock phosphate was effective in increasing crop yields, the 
increases obtained seem so out of proportion to the light applications 
made that the writers hesitate to attach great importance to the re- 
sults. As far as they go, however, the data show as great a response 
from the less soluble as from the more soluble forms of phosphoric 
acid. 
1Pa. Agr. Expt. Sta., Ann. Rept. for 1895, p. 210 (1896). 
2Pa, Agr. Expt. Sta., Ann. Rept for 1907-8, pp. 175-178 (1908). 
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