CROP ROTATION 245 



305. Certain crops check certain weeds. — By rotating 

 crops the weeds that nourish during the presence of one crop 

 on the land may be greatly checked by succeeding crops. 

 Some weeds are best destroyed by smothering, for which 

 purpose small grain, and notably corn or sorghum grown for 

 fodder are effective. Other weeds are most injured by til- 

 lage, to accomplish which the hoed crops are needed ; while 

 others can best be checked by the presence of a thick sod on 

 the ground for a number of years. In the warfare against 

 weeds that must be waged wherever crops are raised, the use 

 of different crops involving different methods of soil treat- 

 ment is of great service. 



306. Plant diseases and insects. — Many plant diseases 

 and many insects spend their resting stages and larval exist- 

 ence in the soil. A continuous growth of any one crop on the 

 soil favors the increase of these species by providing each 

 year the particular plant on which they thrive. A change of 

 crops, by removing the host plants, causes the disappearance 

 of many diseases and insects through their inability to reach 

 their host plants. A long rotation, such as is frequently 

 used in Great Britain, is particularly effective in eradicating 

 those diseases that persist in the soil for a number of years. 



In the case of diseases that affect more than one species 

 of plant, as does the beet and potato scab, there is need for 

 special care in arranging the rotation. Such considerations 

 may make it desirable to change the plan of a rotation. 

 Another feature of the relation of crop-rotation to plant dis- 

 eases is that the more thrifty growth obtainable under rota- 

 tion assists the crop to withstand many diseases. 



307. Loss of plant-food material between plantings. — Many 

 systems of crop rotation permit a more constant use of the 

 land than is possible with continuous growth of most annual 

 crops. As a soil bearing no crop on it always loses more 

 plant-food material in the drainage water than does one on 



