Tillage, Fertilizing and Rotation 113 



may be present in abundance ; the seasons may be ideal ; 

 and still a complete failure or a greatly reduced yield 

 may be the reward because each year the field has had 

 to nourish the same kind of crop, supplying it with the 

 same amount of the same food elements and receiving 

 in return the particular toxin or poison which is given 

 off by the plant in question. Each plant takes from 

 the soil its special kind of food and throws off certain 

 root excretions, corresponding to perspiration in ani- 

 mals, which are poisonous "to that particular plant but 

 which perhaps would be unharmful to any other species. 

 Each plant exerts a certain influence on the mechanical 

 condition of the soil caused by the development of its 

 peculiar root system which varies with different crops. 

 Alternation of deep and shallow-rooted crops in the 

 rotation require the use of other layers of soil. Each 

 plant has certain insect enemies and is subject to certain 

 fungous diseases which naturally become worse if the 

 plant crop is grown in the same place year after year. 

 Some crops are more exhaustive on the humus supply 

 of soils than others, and rotation enables the main- 

 tenance of this humus. 



Proper rotation facilitates diversification of crops 

 which is essential to the proper distribution of farm 

 labor. 



Plant-food. 



The demand on soil for plant-foods from year to 

 year is varied by rotation. An " exhausted " soil is 

 frequently depleted in some one food element due to 

 growing continually some crop that uses more of that 

 element than it does of other plant-foods. Such a soil 

 is often exhausted only for the particular crop which 



