Caustic soda.—A strong solution of this makes a cheap and effective herbicide, commended especially for pouring on soil where it is desired to destroy poison ivy or other deep-rooted or woody plants. Of course, soil so treated will be rendered sterile for some time, but the soda will gradually leach away. Like salt, this is most effective if applied in hot, dry weather.
Arsenical compounds.—One or another of the soluble arsenical compounds form the most effec tive herbicides known. These form the basis of all or nearly all of the various proprietary "herbi cides" or "weed-killers." Such compounds are handled by leading horticultural supply houses, and, so far as the writer has tested them, are highly efficient. The only reason for seeking elsewhere is their high price. Soluble arsenical poisons as a rule can be bought considerably cheaper in the drug trade and are similar in action. The simplest to employ is arsenate of soda. This needs only to be dissolved in water for use, at the rate of 1 pound in 3 to 9 gallons of water. White arsenic is still cheaper, but according to Schutt's formula, which the writer has used, it must be combined with sal soda, which is somewhat both ersome. (White arsenic, 1 pound ; washing soda, 2 pounds; water, 3 to 9 gallons.) An important characteristic of these arsenical poisons is that they endure for a long time and do not readily wash or leach away. For this reason they are the most useful herbicides to use on roadways and other plain surfaces, as explained below.
More specific directions for use.
Any of the above chemicals will kill any plant if applied directly to it in sufficient amount. In addition to the more general advice included in the above account, the following specific directions are adapted to special cases.
Gravel roadways, gutters, tennis courts and like surfaces can be kept free from weedy growths by the application of any of the above. If salt is used it should be scattered freely in the dry form. Caution is necessary where it is liable to be washed on to lawns, lest it damage the grass bor ders. Carbolic acid or arsenical poisons are pref erable, being both less liable to wash and more enduring in their action. One quart of crude car
bolic acid in 8 gallons of water, or one pound of either arsenical compound mentioned above in a like amount of water, will suffice to cover a square rod or more of surface; and one, or, at most, two applications per year, will be sufficient.
Charlock, known also as kale or wild mustard (Brassica arvensis, Fig. 143), is easily destroyed in oat-, wheat-, or other grain-fields by spray ing with a solution of 1 pound of copper sul fate in 4 to 6 gallons of water (2 to 3 per cent solution). A force pump should be used, supplied with fine nozzles. The treatment is most effec tively made when the grain is 3 to 6 inches tall, since at this stage the large charlock leaves spreading above the grain are easily covered by the spray. One barrel or less of the solution (30 to 50 gallons) suffices to cover an acre and destroy the charlock, and this amount causes little or no damage to the grain. This same treatment is reported to be more or less effective against a variety of other common grain-field weeds. The wild turnip (Brassica eampestris) and some allied cruciferous weeds are less easily killed because the spray does not adhere to their smooth leaves.
Experiments by the Cornell Station gave the tollowing general conclusions: Wild mustard grow ing with cereals or peas can be destroyed with a solution of copper sulfate, without injury to the crop. A 3 per cent solution (about 10 pounds to the barrel, or 40 gallons of water), at the rate of 40 to 50 gallons per acre, gives very satisfactory results.
The following notes on the effect of the copper sulfate solution on different plants are from obser vations and reports from various sources: "Plants reported killed by copper sulfate solu tions: wild mustard, wild radish, wild barley, penny-grass (if young), shepherd's-purse, wild buckwheat, lamb's-quarters, ragweed, sow-thistle, hemp-nettle, bindweed, dock, dodder.
"Plants reported severely injured: curly dock, black bindweed, dandelion, sow-thistle and senecio.
"Plants reported as not injured; wild rose, pop pies, pigweed, spurge, corn-flower, field-thistles, chamomile, couch-grass, bent-grass and horsetails.