WEED POISONS 223 



Chemical Weed-Killers or Herbicides 



The usefulness of chemicals as weed-killers is largely limited to the 

 following cases (Jones) : — 



1. When an especially obnoxious weed, as poison ivy, occurs in a 

 limited locality and is to be destroyed regardless of consequences to 

 soil or neighboring plants. 



2. When the aim is to render the soil permanently sterile, as in 

 roadways, tennis courts, and the like. 



3. When the weed plant, as orange hawkweed and mustard, is 

 much more sensitive than the associated useful plants to the action of 

 some herbicide. 



Kinds of herbicides (L. R. Jones). 



The chemicals used as herbicides, the worth of which has been 

 established, are the following : — 



Salt (sodium chlorid), is more commonly used than any other com- 

 pound, chiefly because of cheapness and handiness. It should be 

 applied dry or in strong solution ; and it is most effective in hot, dry 

 weather. Salt can be used in any weed-killing operation, but it is most 

 valuable on roadways and like surfaces and for certain lawn weeds. 

 Hot brine (one pound salt to one gallon water) is useful on walks and 

 roadways. 



Blue vitriol (copper sulfate). — This is more powerful in herbicidal 

 action than salt, but its cost prohibits its general use. For most pur- 

 poses it is best used in solution, 2 to 10 per cent being effective. It is 

 often used on gravel walks and similar surfaces, but salt will generally 

 be found cheaper and arsenical poisons more effective. Its chief value 

 is against charlock or mustard. 



Copper sulfate solution, containing 8 to 10 pounds of blue vitriol to 

 50 gallons of water, and applied at the rate of 40 to 50 gallons per acre, 

 is a good formula. 



Iron sulfate (copperas) solution, containing If to 2 pounds of iron 

 sulfate to the gallon of water (100 pounds iron sulfate to 52 gallons of 

 water), is a good herbicide. Use at the rate of 50 to 75 gallons per 

 acre. 



Kerosene. — This and other coal-oil products will kill plants. It is 

 weak in efficiency, and relatively more costly than any other chemical 



