PRINCIPLES OF CROP-ROTATION 505 



tively little phosphoric acid. Potatoes remove very- 

 large amounts of potassium. A rotation of crops is, 

 therefore, less likely to cause a deficiency of some one 

 constituent than is a continuous growth of one crop, 

 and it utilizes more completely the available nutrients. 



358. Root systems of different crops. Some crops 

 have roots that penetrate deeply into the subsoil, while 

 others are only moderately deeply rooted, and others 

 quite shallow-rooted. Among the deeply rooted plants 

 are alfalfa, clover, certain of the root crops, and some 

 of the native prairie grasses. Representing those having 

 moderately long roots, are oats, maize, wheat, meadow 

 fescue, grass, etc., and among those having shallow roots 

 are barley, turnips and many of the cultivated grasses. 

 As plants draw their nourishment from those portions 

 of the soil into which their roots penetrate, the deeper 

 soil is not called upon to provide food for the shallow- 

 rooted crops, and the deep-rooted crops remove rela- 

 tively less of the nutrients from the surface soil. It 

 therefore happens that a rotation involving the growth 

 of deep- and shallow-rooted crops effects, by utilizing 

 a larger area of the soil, a more economical utilization 

 of plant nutrients than would a continuous growth of 

 either kind. 



359. Some crops or crop treatments prepare food 

 for other crops. It is quite evident that the growth of 

 leguminous crops, even when not plowed under, leave 

 in the soil an accumulation of organic nitrogen trans- 

 formed by bacteria from atmospheric nitrogen. This, 

 in the natural course of decomposition and nitrification, 

 becomes available to cereal or other crops that may follow 



