THE ROTATION OF CROPS 221 



acids which are able to dissolve mineral plant food 

 not soluble in pure water and thus render it avail- 

 able to plants. 



Plants, although they require the same elements 

 of plant food, take them in different amounts and 

 different proportions. 



Plants differ in the extent and depth of root 

 growth and therefore take food from different parts 

 of the soil. Some are surface feeders while others 

 feed on the deeper soil. 



Plants differ in their power to take plant food 

 from the soil ; some are weak feeders, and can use 

 only the most available food ; others are strong feed- 

 ers, and can use tougher plant food. 



Plants vary in the amount of heat they require to 

 carry on their growth and development. 



THE ONE CROP SYSTEM 



We are now ready for the question. What effect 

 has the continuous cultivation, year after year, of 

 the same kind of crop on the soil conditions neces- 

 sary to the best growth and development of that 

 crop or any other crop? Suppose we take cotton 

 for example. 



How does cotton growing affect soil humus? 



During the cultivation of cotton, the organic mat- 

 ter or humus of the soil decays in greater quantities 

 than are added by the stalks and leaves of the crop. 

 Therefore, cotton is a humus wasting crop, and the 



