SPRAYING TO KILL WEEDS 



^ I 'HE use of smothering or poisonous sprays of various kinds 

 to preA^ent the depredations of insects has become a mat- 

 ter of necessity in agricuhural practice, but sprays for eradi- 

 cating weeds are of more recent development. In the battle 

 with weeds a difficulty is met that is not present when we at- 

 tempt to exterminate the insects, for a weed spray must dis- 

 criminate, not between insect and plant, but between good and 

 bad specimens of the same general kind ; between good plants 

 and harmful ones. 



Unfortunately it is not always easy for man himself to 

 make distinctions of this kind. Any plant, good or bad, that 

 gets out of bounds becomes a weed. Xotwithstanding this, 

 however, some progress has been made in compounding weed 

 sprays that can distinguish the weeds from the crops they in- 

 fest. For instance, the wild mustard, which often takes almost 

 complete possession of grain fields, can be controlled by a spray 

 of sulphate of iron. Our illustration shows a grain field in 

 which a strip in the foreground has been sprayed in this way 

 while another strip beyond has been allowed to produce its 

 customary crop of weeds. The complete removal of the weeds 

 in the sprayed strip will be noted. 



The apparent power that sulphate of iron has of dis- 

 tinguishing between mustard and grain is found to be due to 

 the covering of the two kinds of plants involved. The leaves 

 and stems of grain are smooth and covered with a waxy powder 

 called bloom that sheds water as aii}^ other waxed surface 

 would do. The mustard, on the contrary, is rough, hain-, and 



