490 THE PRINCIPLES OF SOIL MANAGEMENT 



this definition any plant which grows where it is not 

 desired is a weed. 



347. Objectionable qualities of weeds. Weeds are 

 objectionable for several reasons. Some of the objec- 

 tionable effects of weeds are: (1) They may remove 

 moisture needed by the crop. (2) They may use food 

 needed by the crop. (3) They usurp the light and heat 

 supply. (4) They interfere with tillage and harvesting, 

 operations, and perhaps also with the planting of the 

 following crop. (5) They leave the soil in a condition 

 unfavorable to the growth of the following crop. (6) 

 They decrease the value of the crop by introducing im- 

 purities which are both injurious and expensive to 

 eliminate. 



348. The control of weeds. The control of weeds 

 depends on their character and habits of growth. Each 

 situation develops its own peculiar crop of weeds. They 

 arise as a result of the character and condition of the 

 soil, and the character and habits of the regular crop. 

 It is a type of the natural association of plants. In the 

 wheat fields of the Northwest, mustard is troublesome; 

 in maize, it may be quack-grass, sonchus, daisy or morn- 

 ing-glory. In meadows, it may be the thistle, yarrow or 

 daisy. These weeds gain a foothold because their cycle 

 of growth so closely corresponds with that of the 

 crop. According to the character and occurrence of the 

 weed, one of two methods of control or eradication must 

 be employed: (a) If its propagation is dependent on 

 seed-production, then seed-production should be pre- 

 vented. (6) If propagated vegetatively, then the develop- 

 ment of the aerial portion must be prevented for a 



