HOW ROTATIONS HELP 505 



labor of crop production, and helps to systematize farm oper- 

 ations. 



659. Rotation Improves the Physical Condition of the 

 Soil. The roots of all plants do not penetrate the soil to the 

 same depth. Deep-rooting plants like clover and alfalfa 

 enter the lower layers of the soil ; when their roots decay they 

 open channels for the passage of air and moisture and make it 

 easier for the crops which follow to draw on the stores of 

 plant food in the subsoil. Constant cultivation and the 

 growing of cultivated crops tend to decrease the supply of 

 vegetable matter in the soil, because favorable conditions 

 for its decomposition are provided. Grain crops add little 

 in the way of vegetable matter unless the straw is returned 

 in manure, as the roots and stubble are not bulky. The 

 grasses, however, grow for two or more years and accumulate 

 a large quantity of fibrous material, which tends to restore 

 the supply of vegetable matter. If a portion of this matter 

 is in the lower soil layers, as in the case of deep-rooting plants, 

 it further improves the physical condition. The varying 

 cultivation which is given to different crops is also of benefi^ 

 for the soil is stirred to different depths and aerated. 



660. Rotations Conserve Moisture. Practically all sys- 

 tems of rotation include, at some time during their course, 

 one or more cultivated crops. Cultivation, by maintaining 

 a surface mulch and lessening evaporation, helps to hold the 

 moisture in the soil. Moisture passes very readily from 

 stubble land, or from any bare, untilled field, but the tillage 

 given a cultivated crop conserves moisture for the crop which 

 follows. 



661. Rotations Conserve Vegetable Matter. Constant 

 cultivation and the removal of crops rapidly reduce the 

 vegetable matter in the soil. A rational system of rotation 

 includes the keeping of more or less live stock to turn the 



