114 FARM DEVELOPMENT 



ciples and facts concerning the rotation of crops may 

 properly be stated here. (See Figures 41 and 43.) It 

 should be observed that the following statements apply 

 somewhat locally to the farm conditions of the middle 

 Northwest. 



The average yearly value of the series of crops in rota- 

 tion must considerably exceed the average cost of pro- 

 duction, that there may be a large net annual profit per 

 acre and per worker. 



Each crop chosen must do its share toward producing 

 the average net profit by its direct net profit, taking into 

 account the reduction of the productivity of the soil, or 

 its improvement of the soil for succeeding crops. 



Soil-reducing crops include most of the grains and cul- 

 tivated crops. Soil-improving crops include most of the 

 grasses, clovers and such other leguminous crops as 

 peas, beans and lupins. 



Some crops reduce the productivity of the soil for the 

 same and certain other crops, while some crops increase 

 the productivity of the soil for certain crops. Thus 

 wheat, oats and barley reduce the productivity of the soil 

 for wheat, oats and barley. Corn, on the other hand, 

 leaves the soil in peculiarly favorable condition for these 

 small grains, and for grasses and clovers seeded with 

 them. Clovers, and the legumes generally, leave the soil 

 peculiarly improved for nearly all crops. 



Crops which reduce the productivity of the soil may 

 do so in various ways, as, by allowing to multiply those 

 kinds of weeds which are peculiarly harmful to the 

 succeeding crops; by introducing plant diseases; pos- 

 sibly by introducing substances poisonous to the soil; 

 by leaving the soil in poor mechanical condition ; and by 

 leaving it lean of certain compounds needed for plant food. 



Crops which increase the productivity of the soil may 

 accomplish this in numerous ways, as by adding organic 

 matter which support bacterial and other activities; by 



