394 INVESTIGATION BY CULTURE EXPERIMENTS 



mind, also, that soluble acid phosphate is almost immediately 

 converted into an insoluble form when brought in contact with 

 ordinary soil, and that alkali salts have more or less power to make 

 phosphates soluble. 



The yields harvested for the first and second lo-year periods 

 are comparable for most plots, and this is also true for the following 

 2o-year and lo-year periods; although the yields of first crops 

 only (1856 to 1875) cannot be compared with the yields of two 

 cuttings (1876 to 1905). The double comparisons plainly indicate 

 that the yield of hay is decreasing on all plots except those to which 

 minerals are applied without nitrogen (plots 5, 6, 7, and 15) orwith 

 organic matter (plot 13). The largest percentage decrease during 

 the last thirty years has occurred on the unfertilized land (plots 

 2, 3, and 12) and on plot i, where ammonium salts and heavy 

 applications of farm manure were used during the eight years, 

 1856 to 1863, and ammonium salts alone thereafter. Marked 

 decreases have also followed the use of acid phosphate and ammo- 

 nium salts, either separately or together; while the addition of 

 alkali salts with both nitrogen and phosphorus has lessened the 

 decrease, but not entirely prevented it. 



Plots 6 and 7 appear to have reached an equilibrium, having 

 produced about the same yield during the last lo-year period as 

 during the previous 2o-year period, and plot 15 appears to be in 

 the same class during the last lo-year period. 



A most striking fact is the controlling influence of the alkali 

 salts; but there is no plot receiving alkali salts alone, and the 

 question again arises whether the effect of the alkali salts is more 

 largely direct or indirect. Here, as on the Broadbalk field, the mag- 

 nesium and sodium salts have produced a marked effect, as will be 

 seen from plots 8 and 10 in comparison with plots 4-1 and 4-2. 

 Thus, as an average of the thirty years, 1876 to 1905, the addition 

 of 250 pounds of sodium sulfate and 100 pounds of magnesium sul- 

 fate increased the yield of plot 10 over that of plot 4-2 by 1243 

 pounds of hay per acre per annum; but increasing the application 

 of alkali salts from 350 pounds to 700 pounds, by substituting 500 

 pounds of potassium sulfate for 150 pounds of the sodium sulfate, 

 produced a further increase of only 1009 pounds of hay on plot 9; 

 while the further addition of 400 pounds of sodium silicate on plot 



