HOW WEEDS SPREAD 499 



the roots of crops to penetrate the subsoil. Weeds add to 

 the vegetable matter in the soil when they are plowed under 

 and increase the plant food which is available for useful 

 crops which follow. All these purposes are served to better 

 advantage, however, by growing cultivated plants adapted 

 to the particular use. 



689. Weeds Make Cultivation and Rotation Necessary. 

 Weeds are sometimes commended because they make neces- 

 sary the cultivation of the soil, which might otherwise be 

 neglected to the injury of crops. This cultivation both 

 keeps down weeds and prevents the loss of soil moisture. 

 Another benefit from weeds is that they often force the use 

 of a system of crop rotation which might not otherwise 

 be adopted. Some weeds which are practically impossible 

 to control in grain fields soon disappear when a cultivated 

 crop is grown or the land is seeded to grass. When mustard, 

 wild oats, or other weeds become very plentiful in fields 

 where small grain is grown continuously, they can best be 

 checked by growing a crop of corn or potatoes and culti- 

 vating it thoroughly. This practice is good, even when no 

 weeds are present, but it might not be adopted if the weeds 

 had not compelled its use. Many weeds of meadows and 

 pastures are easily killed by cultivation. Thus the rotation 

 of crops is an efficient means of subduing weeds. 



HOW WEEDS SPREAD 



690. Dissemination. Some weeds have few or limited 

 means of distribution, while others are provided with many 

 agencies of dissemination. Natural agencies, such as the 

 movement of wind and water, play a large part in the spread 

 of weeds. Animals, both wild and domestic, carry the seeds 

 from place to place. The activities of man, however, are 

 perhaps the greatest factor in spreading weeds. 



691. iN'atural Agencies. An important natural agency 

 by which weeds spread is the movement of air currents. The 



