ROTATIONS KEEP DOWN WEEDS 507 



grown repeatedly on the same land. The change of crops 

 from one field to another helps to keep these pests under 

 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. 



663. Rotations Aid in Keeping Down Weeds. Some 

 weeds grow best in certain crops or under certain conditions; 

 others thrive under totally different conditions. The small 

 grains offer particularly favorable conditions for the growth 

 of many weeds. The spring grains are sown before many of 

 the weed seeds germinate, and ordinarily no effort is made to 

 control weeds which come up in them, so that they are allowed 

 to grow unmolested till harvest. Even less opportunity is 

 afforded to combat weeds in fall grain, except that the grain 

 begins growth earlier in the spring than many of the weeds 

 and is harvested earlier than some of them mature their 

 seed. By harvest, most of the annual weeds have ripened 

 their seeds and have thus had every chance to increase. 

 Meadows and pastures offer less favorable conditions for 

 annual weeds, as the crops and weeds are cut or eaten off by 

 stock, and when a good sod is established it affords little 

 opportunity for weeds to get a start. Biennial and peren- 

 nial weeds, however, often thrive in meadows and pastures, 

 if the field is left undisturbed for several years and there is 

 no chance to destroy them by stirring the soil. Cultivated 

 crops offer opportunities for the destruction of weeds of all 

 classes. In other words, weeds increase rapidly in grain 

 crops, some classes decrease while others may increase in 



