This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
This is comparatively a new way of fighting weeds and further experiment is needed for discovery of all its merits. Professor Henry L. Bolley, Botanist at the State Experiment Station of North Dakota, states that "the preliminary field trials at this Station in 1896, were, perhaps, the first experiments of the kind conducted in any country." Soon afterward the discovery was made in France by M. Aime Girard that Copper sulfate would kill Wild Mustard if applied when the foliage was tender. Since then, in many parts of this country and Canada and in European countries, experimenters have been at work, trying the effects on various plants of different chemicals, seeking to find the reason why the treatment succeeds in some cases and not in others, and to learn how it can be most economically and effectively used. Such experiment has proved the worth of the following chemicals as weed-killers, or herbicides:
Common salt (Sodium chloride). This is the cheapest, handiest, and safest of herbicides, but not the most useful; for, when applied in sufficiently large amounts to kill a pernicious plant such as the Orange Hawkweed, it may also kill other plant-life and so permeate the soil as to check all agricultural growth for a season. Its de-structiveness lies in its power to absorb the moisture in the soil and from the plant tissues, so that they die of thirst; therefore, if it is to be effectual, it should be applied in hot, dry weather. Small areas of Quack Grass and Canada Thistle may sometimes be entirely destroyed by salting freely and then allowing cattle and sheep to bite down the salted herbage, repeating the operation as often as new shoots appear. In places where it is needful to expel all plant growth, salt may be used in the form of hot brine, the solution being so strong as to show forming crystals on its surface.
Copperas, or Green vitriol (Iron sulfate). This chemical, being a by-product of the iron and steel industry, is comparatively cheap, costing only about a cent a pound. As an herbicide it should be used as a spray, in a solution of about a hundred pounds to a barrel of water (52 gallons), which should be a sufficient amount to spread over about an acre of herbage. A dust spray of this chemical has also been used, but is effective only when the plants are wet with dew. Iron sulfate is particularly useful as a grain-field herbicide, applied in dry, clear weather, when there is no likelihood that rain will wash off the plants before the chemical has done its work. Grains and grasses are very resistant to injury from the spray, partly, no doubt, because their growth is from the center and they quickly recover from such slight harm as may have been done to the outer leaves; also, they are smoother in texture than many of the grain-field pests, such as Corn Cockle, Charlock, and King-head, so that the spray does not cling so readily to their slender, blade-like leaves. The spray must be applied before the grain begins to "head" or the weeds to bloom, at a time when both are making the most rapid growth, for then the grain recovers so swiftly as scarcely to receive any check in its growth, and the weeds succumb most readily when they are most green and succulent. In the pea-field also this spray may be used to kill weeds without serious injury to the crop, but not with beans. Clover and alfalfa leaves are blackened, but recover rapidly if the solution has not been too strong.
When successfully carried out, this method of cleaning a field of its undesirable plants pays the farmer very well; for returns from crops that have been relieved from competition with weeds for food and moisture and space to grow, are often half as large again as those from similar fields untreated, and are greatly improved in quality as well as in quantity.
Bluestone, or blue vitriol (Copper sulfate). This well-known fungicide is also a most effective herbicide, if used when the weed foliage is young and tender. The formula for the solution is eight to twelve pounds of Copper sulfate to a barrel of water (52 gallons), using fifty to seventy-five gallons per acre. Professor Bolley found twelve pounds of Copper sulfate to be as effective as one hundred pounds of Iron sulfate. Like that chemical, it should be used in clear weather, when the plants are not likely to be rain-washed for at least twenty-four hours, as such a bath would render the work of no effect and require that it be done over again. It is necessary that sprays shall be fine, like a fog or a mist, in order to be effectual; for drops only roll off the leaves as rain would do, and small drops merely make large ones.
Carbolic acid (Phenol). This can be used only on small areas because of its cost. The crude acid may be used in full strength to saturate the soil about the perennial roots of such plants as Milkweed and Canada Thistle. It does not corrode metals and can be used with any sort of can or pump. When diluted with water it needs to be constantly shaken in order to make a good mixture. It is quick in action, but not lasting. The treated plants, if deeply rooted, often recover and send up new shoots.
Caustic soda (Sodium hydrate, or Sodium hydroxide). To be used where one does not mind killing out all plant growth for a season. Better than Carbolic acid for killing Poison Ivy, Spreading Dogbane, or any other woody and deep-rooted plants. Apply in strong solution, preferably in hot, dry weather, and, when the noxious growth is killed, water the bare spots frequently so as to assist the chemical to leach away.
Oil of vitriol (Sulfuric acid). This can be handled only in glass vessels. It is not used as a spray, but is applied directly to individual plants that are particularly hardy and pernicious. Very great care is necessary in its use, as it destroys everything that it touches; if accidentally spilled it may make painful "burns" on the flesh or eat holes in clothing. It is not recommended, as other chemicals are very nearly as effective and are very much less dangerous to handle.
Corrosive sublimate (Bichloride of mercury). Make a solution in proportions of one ounce of the drug to six gallons of water. Though fatal to the weeds, this, too, is not to be recommended for any general use because of its extremely poisonous nature. Its cost is also much greater than less dangerous chemicals.
Kerosene. Crude petroleum. Either of these will kill plants, and the former is usually always at hand in the farm household. They have the merit of being safe to handle, but are relatively more costly than other herbicides. The saturated soil remains sterile longer than with applications of caustic soda or carbolic acid.
Arsenite of soda. This is a very active poison, and extreme care must be exercised in its use. Do not inhale the powdered drug when making the solution, or the spray as it is delivered; keep to the windward side while working with it. The formula for the solution is one pound of the drug in three to nine gallons of water. White arsenic is cheaper than Arsenite of soda, but needs to be combined with twice its weight of Sal soda in order to be readily soluble in water; the formula being, one pound of White arsenic, two pounds of Sal soda, three to nine gallons of water. These arsenical compounds are the chief ingredients of all commercial weed-killers, and are used on walks, roadways, tennis courts, and all places where the complete and lasting extirpation of all plant growth is required.