WEEDS: INJURY TO CROPS AND NATURE OF 3 



oats. One might thus make a list covering practically 

 all kinds of weeds. A peculiar feature is observed, how- 

 ever. It is never said of the weedy plant that it is not 

 injurious to crop yield until it becomes so abundant on 

 the man's farm or in a particular neighborhood that it 

 seems practically impossible for the farmers concerned to 

 get rid of it. It is then tolerated and finally the evil ef- 

 fects condoned in the manner indicated. 



" A growing crop or plant gets its food material by ab- 

 sorbing the soil solutions. The excess of water is evapo- 

 rated by the leaves. No more water is taken up by the 

 crop than is evaporated by the leaves. 



" If the leafy growth of a cereal crop is not normal in 

 extent, but weakened and insufficient because of crowd- 

 ing out by other plants which exclude light and proper air 

 supply, then the rate of water evaporation from the leaves, 

 and consequent absorption from the soil, will be greatly 

 diminished. Thus even if there is an abundance of water 

 and food materials, the cereal crop does not get its just 

 supply if it is unduly crowded by other weedy growths. 

 When drouthy times occur the weeds are able to do 

 their greatest destruction." 



" We have made many examinations of this question 

 and have almost invariably come to the conclusion from 

 our studies that where a normal stand of grain or other 

 crop is upon the ground the yield of the crop will be 

 found to be reduced in approximately direct proportion 

 to the weeds which are developed on the ground during 

 the growth season. This assumes that the water and 

 soil conditions are normal. If, however, such conditions 

 are abnormal during the maturing period of the crop, 

 the loss in yield occasioned is often very much greater 

 than that indicated. This is especially true if a drouthy 

 time occurs just while the grain is filling." 



A part of the depletion of our soils must be attributed 

 to the growth of weeds on the farms, some weeds being 



