16 BULLETIN 901, TJ. S. DEPARTMENT OF AGRICULTURE. 



Possible differences in soil condition and natural yielding power 

 may be largely eliminated by comparing .the relative yielding power 

 of the crops in the several rotations in succeeding periods. The 12- 

 year period can be divided into three periods of four years each, cor- 

 responding to the length of the rotations. When so studied it is 

 found that the yields of all crops in rotations Xos. 14, 15, 16, and 17 

 have been decreasing instead of increasing, as compared with the 

 yields of the same crops in the corresponding fallow rotations, Nos. 

 18 and 19. The possibility that the later seasons may have been 

 relatively more favorable to bare fallow than the earlier ones might 

 be advanced in explanation of the behavior of the first crop follow- 

 ing fallow or green manure; but such an explanation could hardly 

 account for the behavior of the corn following this crop, and cer- 

 tainly not for the crop of wheat or oats which follows the corn and 

 has two crops intervening between it and the fallow. 



An exception has been mentioned above. This is noted in the 

 sweet-clover rotations, Xos. 31 and 32. In these rotations the total 

 yield of corn, which is the second crop after the sweet clover is 

 plowed under, has been increasing in comparison with the yield of 

 corn in the other rotations of this series. 



Unfortunately, there is no rotation to determine what the effect 

 would have been had the sweet clover been harvested for hay or seed 

 instead of being plowed under. Rotations to test this have been in- 

 corporated in the newer work on section 9, but are not yet advanced 

 enough to furnish the desired data. 



As to the relative values of rye and peas for green manure, the 

 evidence is somewhat contradictory. Rotation No. 14 with rye has 

 yielded heavier than Xo. 16 with peas. In these rotations wheat fol- 

 lows the green manure. The corn in rotation Xo. 15 with rye has 

 outyielded the corn in Xo. 17 with peas, but the other crops have 

 yielded more in Xo. 17. In these two rotations the green manure is 

 followed by oats. The differences are small and probably well within 

 the limits of experimental error. 



In view of the fact that in more humid sections increases are 

 usually expected from the use of legumes as green manure, it might 

 be fair to state that one of the most interesting results of these ex- 

 periments is the failure of peas as green manure to increase yields in 

 comparison with those obtained on either fallow or nonleguminous 

 green manures. 



A result from these experiments more important than the differ- 

 ences between green manures or fallow is that on disked corn ground 

 the wheat has averaged 1.3 bushels per acre more and the oats 

 4 bushels per acre more than the same crops on green manures and 

 fallows. The corn following wheat in fo,ur rotations has averaged 

 6.8 bushels of grain and 3,065 pounds of stover per acre, and follow- 

 ing oats in four similar rotations it has averaged 6.9 bushels of grain 

 and 3,407 pounds of stover per acre. 



