CHEMICAL WEED-KILLERS OR HERBICIDES The use of chemicals as herbicides offers no spe cific cure- all against weeds. Cultivation, short rotations, watchfulness against the introduction and scattering of weed seeds, are all of more fun damental importance than chemicals in combating weeds. There are, however, various cases in which chemicals intelligently used are more expeditious and economical than any other means for weed killing. A practical difficulty is so to use the her bicide as to kill the obnoxious plants without working permanent injury to the soil or to neighboring cultivated plants. This difficulty limits the chief usefulness of chemicals as weed-killers to the following cases: (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 hawk weed and mustard, is much more sensitive than the associated useful plants to the action of some herbicide.
Chemicals useful as herbicides.
Any soluble chemical, even including the various commercial fertilizers, if used in sufficient amount, will kill plants. Some act directly and quickly as poisons, e. g., arsenic and carbolic acid; others, such as salt, have little or no direct poisonous effect but kill the plants primarily by drawing the water from the tender foliage, or by holding the moisture of the soil so that it cannot be absorbed by the roots. It is important in this connection to note that in either case the herbicide is most effective on young plants that are in active growth. Effectiveness in one or the other of these ways, together with cheapness and convenience of application, are the things to determine choice among the various compounds available. Without attempting to list all of these, we include those whose worth has been best estab lished by trial.
Salt (sodium chlorid) is probably more commonly used than any other compound, chiefly because of cheapness and handiness. Its action depends almost
wholly on the withdrawal and retention of moisture from the plant, therefore 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 road ways and like surfaces and for certain lawn weeds.
Blue vitriol (copper sulfate).—This is more pow erful in herbicidal action than salt, but its cost prohibits its general use. For most purposes it is best used in solution, 2 to 10 per cent being effec tive. 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, as noted on page 117.
Kerosene.—This and other coal-oil products will kill plants. Because of handiness it is frequently used, but it is weak in efficiency, and relatively more costly than any other chemical here listed. A pint of crude carbolic acid will do better service than two gallons of kerosene, and costs much less. When crude petroleum is available at very low price it is commended.
Carbolic acid.—This is one of the quickest and most valuable herbicides. The crude acid is rela tively cheap. It is not quite equal to the arsenical poisons for penetrating the soil or in lasting effects, but it is often preferable because of cost or convenience. It does not corrode metals, hence, may be applied with any can or pump. An effective method is to squirt the strong acid from an ordinary oil can on the roots or crown of in dividual weeds. If it is to be sprayed or sprinkled broadcast on the foli age or ground, it should be diluted with 15 to 30 parts of water, and this mixture agitated fre quently during use.
Sulfuric acid (oil of vitriol).—This, of course, is destructive to everything it touches. It can be applied in the crown or about the roots of coarse or especially hardy plants, provided the user is willing to kill the adjacent vegetation, also. In general, carbolic acid will be preferred, partly be cause sulfuric acid can be handled only in glass vessels.