MANAGEMENT 237 



theory, in addition to the known enemies of the different crops, 

 such 'as insect pests and the like, there is a tendency of plants 

 as well as of animals to throw off excreta which are poisonous 

 to themselves. Therefore, after dense crops of the same plant 

 have been grown continuously for several years, the soil becomes 

 unhealthy for that plant, just as the conditions become unwhole- 

 some for animals which live in crowded quarters for a long time. 



Another reason for diversification is that different crops 

 extract the different elements of plant food from the soil in 

 different proportions. A wise diversification of crops will tend, 

 therefore, to exhaust the soil less rapidly, or rather, to retain 

 for a longer period a proper balance of the various elements 

 which go to make up the soil fertility. A third reason for 

 diversification, and one which, though no more important in 

 itself, appeals to the average farmer much more strongly, is that 

 different crops require labor and attention at different times of 

 the year. Suppose a farmer is located in a corn country. With 

 a given labor force there is a pretty definite limit to the amount 

 of corn which he can plant, cultivate, and harvest. But if he 

 grows nothing but corn, there will be times and seasons when 

 there is no work to do, for example, between corn-plowing and 

 corn-husking time. His labor power is going to waste at these 

 times. If he can find another crop which will occupy his time 

 during these intervals, he can grow it without adding to his 

 labor force and without subtracting from his corn crop. Some 

 form of small grain, preferably spring grain such as oats, or 

 spring wheat where the climate is suitable both for spring wheat 

 and corn, will fit in admirably with his plans. This grain can be 

 sown in the early spring, before it is safe to plant corn, and the 

 harvesting and threshing will come between corn plowing and 

 corn husking. 



Competing and noncompeting crops. This situation gives rise 

 to the distinction between competing and noncompeting crops, 



