960 



Journal of Agricultural Research 



Vol. VI, No. 24 



possible that some other factor was influencing plot 20. The plots re- 

 ceiving chemical fertilizers ranked a little lower than the lowest receiving 

 manure but materially higher than those receiving nothing. 



Table III. — Number of bacteria per gram of soil a 

 [,000 omitted] 



a The soil was dried at no° C for two hours. 



If the effect of the various cropping systems in the absence of manure 

 is considered, the various systems rank in the following order in 1914: 

 (1) Timothy, (2) rotation, (3) corn, and (4) wheat. Wheat and corn 

 are about equal, with a marked increase in the rotated and the timothy 

 soils. In the presence of manure the rank is almost the reverse: (1) 

 Corn, (2) wheat, (3) timothy, and (4) rotation. Here, again, wheat and 

 corn are approximately equal, with a marked falling off to timothy and 

 a somewhat less decrease to the rotation. 



Just why manure should have the effect of raising the bacterial content 

 of wheat and corn from the lowest to the first rank is not known. On 

 the other hand, just why it should have less effect upon plots with a nor- 

 mally higher count is equally not understood. The peculiar behavior 

 of the rotation plot receiving manure is very striking, the bacterial 

 numbers being only slightly affected. Particularly is this so when we 

 remember that this rotation is composed of corn, oats, wheat, clover, 

 and timothy, since corn, wheat, and timothy bring about conditions which 

 readily respond to manure. It should be remembered that if samples 

 had been taken from rotation plots when some crop other than clover 

 or timothy was growing, the results might have been different. A 

 possible explanation of the lack of effect of manure upon the rotation 

 plots and a less-marked effect upon timothy may lie in the amount of 

 organic matter that these plots themselves return to the soil. The less 

 the quantity of organic matter returned to the soil apparently the more 

 marked is the result from manure. 



The results secured from eight analyses of plots 29 and 30 (not given 

 in Table III) indicate that the effect of manure is in part accumulative. 

 Plot 29 had received only 6 applications of manure against 25 for plot 30, 



