20 CIRCULAR 421, U. S. DEPARTMENT OF AGRICULTURE 



high yields in favorable years. In years when the moisture supply 

 was insufficient to maintain this growth, these fields suffered more 

 severely from drought than fields in which no manure was used. 



COMPARISON OF YIELDS ON FALLOW WITH YIELDS FOLLOWING A GREEN-MANUEE CROP 



Yields of wheat obtained from plowing under rye, peas, and sweet- 

 clover for green manure may be compared to those obtained on sum- 

 mer-fallowed land in comparable rotations during the 20-year period. 

 The yields following both rye and peas plowed under approached 

 but did not quite equal those on fallowed land. The yield after sweet- 

 clover plowed under was materially lower than that on fallowed land. 

 It is apparent that any increase in fertility brought about through 

 plowing under a green-manure crop was counterbalanced by the loss 

 of moisture used in producing the green manure. 



The lower yield after sweetclover was undoubtedly due to a defi- 

 ciency in moisture. Sweetclover is plowed under later than other 

 green-manure crops, and when opportunities for moisture storage 

 are low after the sweetclover has been plowed under, yields the fol- 

 lowing year are likely to be lowered, particularly in dry years. 

 Yields following sweetclover were fully as high as following other 

 green-manure crops in years when the lower moisture storage was 

 not a limiting factor. 



The results of the 20 years indicate that no appreciable benefit 

 had been derived from plowing under green-manure crops. Fer- 

 tility in the plots not receiving manure was still high enough to 

 produce good yields under favorable conditions. Moisture rather 

 than fertility was the limiting factor in crop production, and the 

 limits set by the relatively low precipitation were such that other fac- 

 tors had little chance to show their effect. 



The difference in favor of bare fallow over green-manure crops 

 was more pronounced with oats than with wheat. As with wheat 

 the number of plots that could be compared was small, and experi- 

 mental error caused by original differences in productivity of the 

 plots may account for much of the difference. A study of the 20- 

 years' record makes it appear certain that no measurable increases in 

 yields on green-manured land over those on fallowed land took place 

 during the progress of the experiment. 



Summing up the results of the 20-y ears' experiment, it may be 

 stated that cumulative benefits from adding humus to the soil were 

 not a factor during the course of these experiments. Any beneficial 

 effect was upon the crop immediately following. Factors other than 

 fertility played a more important part in determining the yield. 



Reduction in yield from practices that tend to deplete the fertility 

 of the soil were not noticeable where weeds and similar factors not 

 associated with fertility were kept under control. It may be said 

 that the effect of any particular crop or method of production did 

 not make itself evident for a period of more than 3 years, and that 

 in most cases little effect was noted beyond the first year. 



APPLICATION OF RESULTS FROM ROTATIONS 



Average results from rotation experiments indicate what may be 

 expected from the use of certain crop combinations in field practice. 

 Average yields may not represent the utmost that may be obtained 



