CROPPING SYSTEMS 403 



of deep and shallow-rooted crops ; (7) may provide for a 

 balanced removal of plant food ; (8) may control toxic 

 substances ; (9) systematizes farming. 



(1) Nearly every crop is accompanied by certain weeds 

 that are able to grow with it, but that do not bother other 

 crops. The weeds that bother tilled crops are different 

 from those that interfere with hay. 



If small grain is grown continuously, the land may 

 become very weedy. These particular weeds are usually 

 easily killed by cultivation. Wild oats are a serious pest 

 in various parts of Minnesota and Dakota. If tilled 

 crops are grown, they are readily controlled. Wild mus- 

 tard is a very serious weed in small grain in some regions. 

 Daisies and wild carrot are weeds in some regions where 

 hay is left continuously, but are not bad weeds in corn. 

 The opposite is true of pig weeds and foxtail. 



Similarly, there are many diseases that injure one crop, 

 but that are not harmful to some other crop. Flax-sick 

 soils are merely soils that are infested with a flax disease ; 

 rotation can control it. Potato scab may become serious 

 if this crop is grown on the same land year after year. 



The same principle holds for insect pests. The corn 

 root worm and root louse often force rotation. Nearly 

 all of the insect enemies of crops are checked to some 

 extent by crop rotation. Many of them are controlled. 



(2) If crops are not rotated, the fields that are con- 

 stantly in tilled crops will soon have their humus supply 

 seriously decreased. Small grain crops bring the same 

 result, but less rapidly. All the serious results that 

 follow the exhaustion of the humus then follow. This is 

 most disastrous in the Southern States, where cotton is 

 the chief crop. The land is usually cultivated for a long 

 season. This, together with the hot weather, favors the 



