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



low level is disease, of which the most important at Edgeley is rust. 

 This generally occurs in wet years when yields would otherwise be 

 relatively high. It was the cause of the low yields in 1916 and 1919. 



Aside from disease, the most important factor controlling yields 

 at this station is the rainfall during: the OTowino; season. 



The precipitation for the months of April, May, June, and July 

 for the several years was as follows : 1906, 14.03 ; 1907, 6.44; 1908, 9.18 ; 

 1909, 10.50; 1910, 5.08; 1911,7.09; 1912, 15.63; 1913, 9.53; 1914, 13.23; 

 1915, 13.81; 1916, 12.48; 1917, 7.54; 1918, 8.58; 1919, 13.45; average, 

 10.47 inches. 



There is a close relation between these figures and those of yields 

 shown in Table IT, except in those cases where disease interferes with 

 the production of a good crop by a sufficient rainfall. 



It will be noted that, generally speaking, a rainfall of over 9 inches 

 during the growing season is necessary to the production of a good 

 crop. 



CONTINUOUS CROPPING COMPARED WITH ROTATION. 



Considerable study has been devoted to the subject of changes that 

 may be taking place in the relative yields of crops grown continu- 

 ously on the same plats and those grown in rotation with other crops. 

 The great fluctuations due to seasons and the relative response to 

 methods of cultivation in different seasons tend to obscure results 

 in even as long a series of years as that under study. There are, 

 however, rather marked indications of comparatively decreasing 

 yields under continuous cropping to any one small grain. This 

 observation is not confined to this station alone, but is more or less 

 general. After the first few years, from four to seven, on new land 

 there appears to come a break in the relative yields from land con- 

 tinuously cropped to one grain. The most obvious reason for this, 

 and one that in some cases clearly accounts for it, is the development 

 of weeds. Diseases that are propagated in the soil are probably an- 

 other reason. It is not believed that it is due to any impairment of 

 the soil. Another bumper crop year such as 191.5 was will be very 

 interesting on account of the evidence it will furnish on this subject. 



CONCLUSIONS. 



The results attending the use of barnyard manure, various green 

 manures (leguminous and nonleguminous), sod crops, and a con- 

 tinued and rather extensive test of commercial fertilizers which 

 has been conducted at the station but is not considered in the present 

 paper, all show that soil fertility is not a limiting or controlling 

 factor of major importance in crop production at Edgeley. On the 

 other hand, the seasonal variation in yields shows that, the chief 

 controlling factor is the seasonal rainfall. The full operation of its 

 control is interfered with by plant diseases, of which the chief one 

 not under control is rust. The nature of the soil and the amount and 



