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, 

 and 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 be 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 phosphates, but 

 continuing to apply the more soluble phosphates. 



From 1902, up to the close of 1913, 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, 



