20 FIELD CROP PRODUCTION 



ing 8 tons of manure in five years with continuous culture 

 plots receiving 12| tons in the same length of time, we 

 have the following yields, the larger always in favor of 

 the rotation system : corn, 47 and 18 bushels, oats, 39 

 and 23, and wheat, 26 and 17. Certainly here is abundant 

 data to show that even with liberal fertilizing crops can- 

 not be grown to good advantage under a one crop system. 

 22. Why rotation gives better yields. — In studying 

 natural phenomena it is good practice first to make 

 observations and gather data and then try to discover the 

 principles which underlie the working of these phenomena. 

 Having obtained definite data to the effect that a rotation 

 system of cropping affords better yield than continuous 

 culture, we next ask the question, why? 



(1) Cultivated crops rapidly deplete the organic matter 

 and nitrogen of the soil. Cultivation favors rapid oxida- 

 tion and destruction of organic matter with the consequent 

 rapid liberation of nitrogen. Then, too, a cultivated crop 

 leaves very little in the way of roots and stubble to be 

 added to the stock of organic matter in the soil. Erosion, 

 both by wind and water, is much more rapid when a few 

 inches of the surface is kept loose by cultivation. As the 

 organic matter and nitrogen is found mainly in the surface 

 soil, it is rapidly lost when erosion is accelerated. Aside 

 from this loss, erosion is, of course, wasteful of the best 

 part of the soil. 



(2) Single cropping favors insects and weeds. Any crop 

 grown on the same ground year after year encourages the 

 presence of such insects and in some cases such weeds as 

 prey especially on that crop. This is one of the worst 

 troubles in continuous corn growing ; the corn root worm 

 develops badly under those conditions. In the continuous 

 culture of wheat at the Rothamsted station the ground at 



