508 THE PRINCIPLES OF SOIL MANAGEMENT 



Tap-rooted plants, and others with few surface roots, 

 do not exhibit this action. Alfalfa and root crops are 

 likely to leave the soil quite compact as compared 

 with the crops mentioned above. 



The effect of sod is generally beneficial, and this is 

 one of the reasons for using a grass crop in a rotation. 



361. Certain crops check certain weeds. By rotating 

 crops, the weeds that flourish during the presence of one 

 crop upon the land may be greatly checked by succeed- 

 ing crops. Some weeds are best destroyed by smothering, 

 for which purpose small grain and notably corn or sor- 

 ghum sown for fodder are effective. Others are most 

 injured by cultivation, 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 

 carried on wherever crops are raised, the use of different 

 crops involving different methods of soil treatment is 

 of great service. 



362. Plant diseases and insects checked by removal 

 of hosts. Many plant diseases and many insects spend 

 their resting stages and larval existence in the soil. 

 A continuous growth of any one crop upon the soil 

 favors the increase of these species by providing each 

 year the particular plant upon which they thrive. A 

 change of crops, by removing the host plants, causes 

 the destruction of many diseases and insects through 

 their inability to reach their host plants. A long rota- 

 tion, 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 



