268 CROP ROTATION 



best results when it is plowed under in the fall for corn. Under 

 certain conditions the application of manure to the clover fields 

 gives best results. 



Some beUeve that liming tobacco lands results in lowering the 

 quality of the leaf. Results do not seem to support this view, 

 however. Furthermore, experiments have shown that tobacco 

 responds readily to hming when grown on acid soils. "When tobacco 

 is grown on acid soil in a rotation containing clover, the Ume need 

 not be apphed directly to the tobacco crop, but for the clover 

 instead. In this way any possible injury caused by the direct 

 effect of lime on tobacco may be entirely prevented. In case of 

 potatoes, the lime may be applied in the rotation immediately 

 following the potato crop. Other similar problems concerning 

 hming and the use of fertihzers may be solved through rotation. 



Rotation Controls Weeds. — It has been demonstrated as well 

 as commonly observed that fields on which small grains are grown 

 continuously become very weedy, and crops diminish in yield and 

 even fail as a result. This is because most weeds go to seed in the 

 time required for the grain to mature. The growing of intertilled 

 crops is a most effective way to kill weeds, especially those such as 

 quack grass and Canada thistle. The growing of good clover and 

 alfalfa, or any other leafy forage crop, is another effective way to 

 kill the common weeds and keep them under control. A rotation 

 containing at least a cultivated crop and one good clover crop 

 gives the farmer at least two good chances in his system of cropping 

 to fight the weeds. 



Rotation Helps to Maintain and Increase Fertility. — In the 

 foregoing discussion it is plain that a proper rotation of crops 

 affects favorably, either directly or indirectly, practically all the 

 factors which determine soil fertility. When these factors are 

 thus affected, soils necessarily produce better crops. In this 

 manner, therefore, rotation aids in maintaining and increasing 

 fertility — ^that is to say, crop rotation helps to maintain and in- 

 crease the productive power of soils. The productive power of a 

 soil is reduced more rapidly when a crop is grown on it continu- 

 ously than when a rotation is practiced. Among the many rota- 

 tion experiments that may be cited to prove this, those in progress 

 at the University of Illinois may be selected as typical. The fol- 

 lowing results are available from the oldest rotation experiments 

 in America. These plots are located on brown silt loam, the typical 

 prairie land of the corn belt. (Illinois Station Circular No. 193.) 



