CROP PRODUCTION 61 



difficulty of controlling plant diseases, weeds, and injurious 

 insects is increased. A liberal application of organic matter 

 may reduce the evils of continuous cropping in a measure, 

 but not enough to justify following such a system. An 

 experiment with a 1 2-year continuous crop of corn, where 

 clover and rye were used as green manure and where fertil- 

 izers were applied, showed but slight profit compared with 

 the yield on the same kind of soil, where rotation of crops 

 was employed. 



It has been found that where wheat is grown continuously, 

 the loss of humus from the soil amounts to 1800 pounds an 

 acre, but on the same kind of soil where rotation is practiced, 

 there is .a gain of from 1500 to 2000 pounds. The various 

 advantages of crop rotation may be summed up as follows: 

 It increases the amount of humus in the soil, thereby provid- 

 ing for greater water-holding capacity and better general 

 condition for plant growth; it corrects the injurious effects 

 of soil toxins or poisons; it makes possible an easier control 

 of plant diseases, weeds and injurious insects. 



The two chief means of keeping up the fertility of soil are 

 first, the use of barnyard and stable manure; second, the 

 rotation of crops. The latter, is of especial importance in 

 making a choice of crops and needs to be considered from this 

 point of view, somewhat in detail. There are two essential 

 features of a good crop rotation. One is to include some 

 legume in the rotation; the other, to include some cultivated 

 crop so as to control weeds. The following general rule for 

 rotation of crops has been suggested : cultivated crops prepare 

 conditions favorable for grains; grains prepare for legumes 

 like clover, and grasses; and legumes and grasses, in turn, 

 prepare the land for cultivated crops. 



The kind of legume to be used depends upon soil and 



