4 io INVESTIGATION BY CULTURE EXPERIMENTS 



nearly the total, but in different soils it may vary from the total 

 to as low as 75 per cent of the total; while only from 15 per cent 

 to 30 per cent of the total potassium is acid soluble, although in 

 some abnormal soils, as certain peaty soils, it may reach 60 per 

 cent or more of the total. Potassium varies greatly in this respect 

 at different depths in the same field. Thus, on the gray silt loam 

 prairie of the lower Illinoisan glaciation the percentage of the total 

 potassium that is soluble in hydrochloric acid (specific gravity 

 1.115), during ten hours' digestion at the temperature of boiling 

 water, varies from as low as 14 per cent in the surface soil to as 

 high as 38 per cent in the subsoil of the same field. 



Because of these facts the determinations of potassium reported 

 in Table 73 must not be considered as the basis for any final con- 

 clusions, but the phosphorus data must be approximately correct, 

 and the results for nitrogen are practically exact, except for pos- 

 sible variation (from the field average) of the samples of soil col- 

 lected. The data are all reported for 9-inch strata of soil, corre- 

 sponding to the depths to which the samples were taken. 



Table 73 contains much information, but it is self-explanatory. 

 Thus, plot 7, which has received both ammonia and the regular 

 minerals (as more fully explained in the previous pages), produced 

 an average yield of 32.8 bushels of wheat and 3668 pounds of straw, 

 and 2450 pounds of nitrogen, 482 pounds of phosphorus, and 2117 

 pounds of potassium were removed in the crops during the fifty 

 years; while there were applied 4300 pounds of nitrogen, 1336 

 pounds of phosphorus, and 4181 pounds of potassium. The appli- 

 cations have been nearly double or more than double the amounts 

 removed. 



If we compare plots 7 and 3, we find in the first 9 inches about 

 23 per cent more nitrogen, 71 per cent more phosphorus, and 19 

 per cent more potassium in plot 7 than in the unfertilized plot 3. 

 On the other hand, in the lower strata, plot 7 contains distinctly 

 less phosphorus than plot 3 or 4, but this difference is much less 

 marked if plots 12, 13, and 14 be considered. The variations in 

 the lower strata are too great to draw conclusions from any one 

 plot, and this is more especially true as regards potassium. 



In the lower part of Table 73 are recorded some average results 

 that should be more significant, at least for nitrogen and phos- 



