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EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 101 
In Table LXII showing the yields of crops obtained during the 
first period (five years) of the experiment it has been assumed 
that the preliminary corn crop of 1893 is an index of the natural fer- 
tility of the experiment field. This assumption may not be justified, 
but it serves to show what different conclusions may be drawn, de- 
® pending on the method employed in comparing field results. 
The actual yields therefore have been recalculated on the basis of 
the apparent natural fertility of the plots as determined by the 1893 
corn crop. By subtracting this value from the actual yield the gain 
due to the fertilizer treatment has been determined and recorded 
under the heading “ Calculated Increase.” 
This method of comparison is not used in the second and third 
periods of the experiment, since any initial inequalities in the field 
were probably modified by five years of fertilization. 
The application of fertilizers was begun in 1894. At this time 
one-half of the plots (odd numbers) were limed at the rate of 1 ton 
per acre. Similar applications were made to the same plots in 1903, 
aid again in 1911. The plots having even numbers have received 
no lime whatever during the experiment. Ample amounts of soluble 
potash and nitrogen carriers were applied to all plots (limed and un- 
limed) each year. For the first five years, 1894 to 1898, inclusive, the 
various phosphate carriers were applied in quantities representing at 
that time equal money values. In 1899, however, it was decided that 
in view of the fluctuations in the price of the various phosphates it 
would he wiser to equalize the amount of phosphoric acid on each 
plot. This equalization, though not completed till 1902, was brought 
about by ceasing the applications of the insoluble piecenans but 
continuing to pes the more soluble phosphates. 
From 1902, up to the close of 1918, no phosphate whatever was 
applied to any of the plots. 
The experiment, therefore, may be divided into the three following 
periods: First, from 1894 to 1898, inclusive, when equal money values — 
of the several phosphates were applied. During this period phos- 
phoric acid was added in the form of raw rock phosphate in consid- 
erably greater quantities than in the form of the readily soluble or 
available phosphates. Second, from 1899 to 1902, inclusive, when ~ 
the phosphoric acid applications were being equalized on all the 
plots; and, third, from 1903 to 1913, inclusive, when the residual 
effects of equal amounts of phosphoric acid in the different forms 
were being studied. 
In the following table, which is taken from Bulletin No. 163 of the 
Rhode Island station, the rate of application (per one-tenth acre) 
of the various phosphates, up to the close of 1898, are given. The ap- 
plications of all the phosphate carriers except raw rock were then 
continued until the amount of phosphoric added to each plot was 
the same. This equalization was completed in 1902. 
