19 



COMPARISON OF YIELDS ON MANURED AND UNMANUKED FALLOW 



One of the major fields of rotation studies is the cumulative effect 

 of continuing cultural practices over a long period of years. In- 

 volved in this is the possibility that rotations in which no fertiliz- 

 ing material is added to the soil will eventually decrease in yield. 

 For this reason rotations receiving barnyard manure and rotations 

 including winter rye, peas, and sweetclover to be plowed under for 

 green manure were grown in comparison with rotations in which 

 no such material was added. In the rotations where manure was 

 used, it was applied at the beginning of the fallow period to the plot 

 to be fallowed. This was for the purpose of permitting the manure 

 to rot and become thoroughly incorporated with the soil before a 

 crop was grown. 



The 20-year average yield of wheat grown on manured fallow was 

 1.3 bushels per acre more than that on fallow not manured. The 

 number of plots entering into the comparison was small, and origi- 

 nal differences in soil may account for much of the difference in 

 yield. The principal value in manure should be in maintaining or 

 increasing the fertility of the soil. A study was made of the yields 

 during the whole period to determine wmether the difference in 

 favor of the manured rotations was greater during the last few 

 years than it was earlier in the course of the experiments. Results 

 showed no cumulative effect in favor of the manured rotations. 

 There were differences from year to year, but as a whole there was 

 no measurable increase in the differences between the manured and 

 the unmanured wheat plots during the 20-year period. Results 

 from individual years indicated that the greatest returns from 

 manure were obtained in years of good production. In the 11 years 

 when the yields of wheat were above 20 bushels per acre, the wheat 

 on manured fallow produced 2.5 bushels more per acre than wheat 

 on unmanured fallow. In the years when yields were below 20 

 bushels per acre, the yields on manured fallow were 0.2 bushel lower 

 than on unmanured fallow. It appears that the increase in vegetative 

 growth brought about through the use of manure may be detrimental 

 in years of low precipitation. 



Oats on manured fallow yielded 2.3 bushels per acre more than 

 on unmanured fallow. No increase in the difference appeared dur- 

 ing the 20 years. In fact, there appeared to be a tendency for the 

 difference to become less rather than greater. It is safe to say that 

 no cumulative benefit from manure appeared during the 20 years of 

 the experiment. 



In years when oats produced more than 40 bushels per acre the 

 yield was 5 bushels higher on manured than on unmanured fallow. 

 In years when production was less than 40 bushels per acre, oats 

 on manured fallow yielded 0.4 bushel per acre less than on un- 

 manured fallow. These differences are almost exactly the same in 

 pounds per acre as those for wheat. They show that the benefit 

 from manure is obtained largely in years of good production, and 

 that manure may lower the yields in dry years. 



This conclusion is borne out by observations of other fields where 

 applications of manure were much heavier than in the rotations. 

 In these fields there was a stimulation of growth that resulted in 



