478 



FIELD CROPS 



live. Many of them are destructive to only one of the crops 

 commonly grown. They are not generally capable of move- 

 ment for any considerable distance during a season, but 

 increase very rapidly from year to year if a single crop is 

 grown repeatedly on the same land. The change of crops 

 from one field to another helps to keep these pests under 



Figure 153. — Samples of soil from (1) a grass plot, and (2) from one which has 

 been in corn continuously for a number of years. Note the absence of vege- 

 table matter in the sample from the corn field. 



control. As most plant diseases are unable to maintain 

 themselves for more than three or four years in the soil 

 without their particular host crop on which to grow, the crop 

 may be returned to the land at the end of such a period with 

 little fear of injury. The same statement is true to a lesser 

 extent of insects. Some of them will go from field to field, 

 but the greater part of them will die for lack of suitable food 

 if crops on which they do not feed are introduced. 



655. Rotations Aid in Keeping Down Weeds. Some 

 weeds grow best in certain crops or under certain conditions; 



