Soil Nitrate Studies 279 



Effect of Crop Rotation on Nitrifying Power of Soil. 



Table VII gives the average nitrate content during the whole season 

 for all the plots. The same crops are grouped together so that direct 

 comparisons can be made. 



The graphical comparison, figure 2, shows more clearly the com- 

 parison between the same crops, as well as the effect of the various 

 rotations on the nitrifying power of the soil. 



Because of the fact that the fresh nitrate content of a soil is only 

 that part of the nitrate above that which is leached out and used by 

 the crop, the nitrate content of incubated samples is more indicative of 

 the effect of a crop or cropping system on the nitrifying power of the 

 soil. 



To illustrate this point it will be noticed that the average fresh 

 nitrate contents of plots 13 and 14 (continuous wheat) are higher than 

 any of the other wheat plots. Turning back to the previous tables, the 

 fact can easily be seen that this high average was a result of the 

 stimulus given to nitrification in late August and September following 

 the plowing of these plots for wheat. 



It was primarily for the purpose of determining the effect of the 

 various crop rotations on the nitrifying power that the determinations 

 of the nitrate content of incubated samples No. 1 (those without am- 

 monium sulphate) were made. The purpose of the incubated samples 

 with ammonium sulphate has already been implied in the previous dis- 

 cussions, i. e., to determine whether easily available nitrogenous ma- 

 terials were or were not a limiting factor in the nitrification processes 

 in the various plots. 



In table VII the averages for all three sets of samples are given, 

 but only the averages for incubated samples I will be discussed for the 

 reasons already given. 



Plot II showed the highest average of all the corn plots with an 

 average nitrate content of 31.56 parts per million; plot 12, wheat, showed 

 the highest average of all the wheat plots with 16.94; and plot 10, clover 

 and timothy, showed the highest nitrate content of the two clover and 

 timothy plots with 17.25 parts per million. It would seem then that the 

 corn, wheat and clover and timothy rotation was the most efficient in 

 nitrifying power. 



Plot 4, corn, was second in nitrate content to plot II with an average 

 of 29.99 parts per million; plot 6, wheat, was second of all the wheat 

 plots with 16.26 parts per million; and plot 5, oats, with 13.20 had 

 the highest nitrifying power of the two oats plots. The above facts 

 seem to indicate that the corn, oats, wheat and clover and timothy rota- 

 tion had the second greatest effect on nitrification. 



Plot 2, corn, with 27.52 parts nitrate per million, was third high- 

 est in nitrate content of the corn plots, and plot 1, wheat, was the third 

 highest of the wheat plots with 15.97 parts nitrate per million. 



Plot 8, corn, was fourth in average nitrate content with 23.12 among 

 the corn plots; plot 7, wheat, was fourth among the wheat plots with 

 15.54; and plot 9, oats, was lowest of the two oats plots with 12.16 

 parts of nitrate per million. 



