386 DRY LAND FARMING 



fungous diseases, and (4) to prevent the shifting of 

 soils by water and winds. Other reasons may be added 

 to these but not of equal importance. 



Where but one crop is grown through successive years, 

 there is an increase in the growth of many kinds of 

 weeds, especially those which mature their seeds before 

 this can be prevented by any of the ordinary processes 

 followed in the cultivation of the land. Especially an- 

 nuals and perennials may be thus allowed to multiply 

 until their number becomes a menace to the growth of 

 any kind of crop that may be sown. Summer-fallowing 

 the land every few years may bring some relief, but in 

 earch instance where it is practised no crop is reaped 

 that year from the land. An interchange in the suc- 

 cession of cereals may bring a small measure of relief, 

 since some of these occupy the land at a somewhat dif- 

 ferent season from others. The most effective remedy 

 for such weed intrusion is a rotation that includes cul- 

 tivated crops, grass crops, and in some instances an 

 occasional summer-fallow. The cultivated crops are to 

 be relied on chiefly for cleaning the land. The grass 

 crops, especially if of long duration, as in the case of al- 

 falfa, favor the decay of weed seeds of many kinds in the 

 soil, and the summer-fallow aids effectively in destroy- 

 ing perennials that might not otherwise yield to less 

 heroic methods of eradication. 



Insect life peculiar to certain crops cannot be easily 

 mastered, in some instances at least, without an inter- 

 change of crops. Such interchange leads to their de- 

 struction by starving them through cutting off their 

 food supply. The Hessian fly, for instance, which preys 

 chiefly on winter wheat, may be banished, if not ex- 

 terminated in the locality, by dropping that crop out 

 of the rotation for a time. Chinch bugs, which prey 

 ravenously on spring wheat, may be kept at bay, at least 

 in a considerable degree, by ceasing to grow spring 



