WEEDS AND POISONOUS PLANTS 



329 



thistle interferes with the harvestifig and handHng of a crop, 

 and may greatly reduce its value. Thus a forage crop, such 

 as hay, is much less valuable if it contains a large proportion 

 of weeds that domestic animals will not eat or that, if eaten, 

 may injure them. The presence of weed seeds in a crop of 

 wheat or oats may greatly reduce the market value of the 

 grain. Sometimes parasitic fungi or insects live on weeds and 

 pass from the weeds to related useful 

 plants. Some weeds are poisonous 

 to man or to domestic animals ; 

 these will be discussed on a later 

 page. 



348. Some Ways in which Weeds 

 May be Useful. — When land is 

 allowed to lie fallow — that is, 

 when it is not cultivated for one 

 or more years — it becomes covered 

 by a growth of grasses and weeds. 

 When these die, their substance 

 helps to enrich the soil by adding 

 to its supply of organic materials. 

 In this way a soil that has become 

 poor may be considerably improved 

 by the growth of weeds. However, 

 as a general thing the same or 

 greater good can be accomplished 



more quickly by a proper rotation which includes at reason- 

 able intervals a crop of alfalfa or some other member of the 

 pulse family. Weeds also help, during the time of the year 

 when crop plants are not being grown, in holding the soil to- 

 gether by means of their root systems, thus checking erosion, 

 and also preventing the washing away of the nitrogen-con- 

 taining substances that are so important to soil fertility. 



349. Annual Weeds. — A large proportion of common 

 weeds are annuals. Being annuals, they must depend upon 



Fig. 181. — Pigeon grass. 

 .\fter Weed. 



