14 BULLETIN 991, U. S. DEPARTMENT OF AGRICULTURE. 



of wheat has been four-tenths of a bushel less on fallow than on disked 

 corn ground. The yield of corn in rotation No. 19, which shows the 

 greater yield of both oats and wheat, is also slightly higher than in 

 rotation No. 18. 



In section 9 the oats on fallow have averaged 2.7 bushels more than 

 on corn ground, and the wheat 3.4 bushels more. 



MANURED COMPARED WITH UNMANURED FALLOW. 



Nos. 18, 19, 71, and 72 are 4-year rotations. The first two were 

 started in 1907 and the others in 1908. Nos. 18 and 72 are fallow, 

 wheat, corn, and oats. Nos. 19 and 71 are fallow, oats, corn, and 

 wheat. The fallow in Nos. 71 and 72 receives 10 tons of rotted barn- 

 yard manure per acre before plowing. In the 11 years from 1909 

 to 1919 each crop in the manured rotations has averaged higher yields 

 of both grain and straw than the corresponding crop in the un- 

 manured rotations. The average increases, however, have been small, 

 the highest being 1^ bushels for wheat on fallow. 



What appears to be the true significance of the value of manure in 

 a rotation is shown when the results are studied in another way. 



The crops are now being grown on land that has been manured the 

 third time. When the results are studied in detail from year to year 

 or grouped and studied in periods of no manure in the first years, 

 manured once, manured twice, and manured three times, it is shown 

 rather clearly that the use of manure on fallow once in four years 

 not only increases the yields of the three crops in the rotation but has 

 a cumulative effect, the increase becoming greater with each round of 

 the rotation. Before the corn came on the manured land in rotations 

 Nos. 71 and 72 the total weight of corn from these rotations averaged 

 only 151 pounds per acre more than in rotations Nos. 18 and 19. 

 When the land had been manured once the increase was 750 pounds ; 

 manured twice, 983 pounds; and manured three times, 1,438 pounds 

 per acre. 



The }delds of wheat and oats are affected by the fact that in very 

 favorable seasons the manure increases the tendency to lodge and 

 to rust, but in the second and third rounds of manuring these crops 

 show decided increases on the manured land. 



It is a difficult question to study, but all evidence points to the 

 belief that the observed differences are due to an increase in the 

 manured rotations rather than to any deterioration or reduction in 

 the original yielding power of the unmanured rotations. 



These rotations are duplicated on section 9, but the rotations have 

 only been one round in this location. The differences in any exhibi- 

 tion of yields are not as yet great enough to be distinguished from or 

 among the natural differences due to soil variation. 



GREEN MANURE COMPARED WITH BARE FALLOW. 



At the time these experiments began it was thought that green 

 manures might possibly offer a means of increasing or maintaining 



