HOW WEEDS CAUSE LOSSES 



525 



ing them. It is easy to see that an acre of wheat will yield 

 less when Canada thistle or cockle or kinghead are growing 

 in it than when the wheat occupies all the land. In the 

 same way, Johnson grass reduces the yield of cotton, and 

 weeds of many kinds prevent corn from making a full crop. 

 The greatest damage is often done early in the season, by 

 shading and stunting the crop plants before they get well 

 started. Weeds take 

 plant food which is 

 needed by crops. It is 

 next to useless to apply 

 manure or fertilizer to 

 land and then allow 

 weeds to use it. Weeds 

 also take moisture from 

 the soil at a time when 

 crops need it most. 



Weeds lower crop 

 yields by harboring in- 

 sects and diseases. In 

 some cases, weeds are 

 infested with the same 

 diseases as are crop 

 plants. This is true of 

 the root rot of cotton 

 and other plants in Fig. 157. Canada thistle. 



the South, which may maintain a foothold in fields by 

 living on weeds when crops it does not affect are grown 

 there. Many weeds of the mustard family help to spread 

 club-root of the cabbage and turnip. When the diseases 

 do not actually live on the weeds, the latter may make con- 

 ditions favorable for their development on crop plants. 

 Thus rust and mildew are produced most readily in shady, 



