ROTATION OF CROPS 477 



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 Uttle 

 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 benefit, 

 for the soil is stirred to different depths and aerated. 



652. 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 next crop. 



653. 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 

 bulkier and less valuable products of the farm into more con- 

 centrated and more readily salable products. With proper 

 care given to the manure, a large part of this vegetable matter 

 may be returned to the soil. While grain crops and cultivated 

 crops are exhaustive of vegetable matter, grass crops, because 

 they have extensive root systems, materially increase the 

 vegetable matter in the soil. 



654. Rotations Lessen Damage from Insects and Dis- 

 eases. Most of the plant diseases and injurious insects are 

 decidedly limited in the number of plants on which they can 



