Toxic Residue In Soil
I have gone to some lengths to describe the action weed killers because this has a bearing on the residues the chemicals leave in soils. We can dismiss chlordane's four to five day residual effect (on plants) as unimportant. This must, however, be distinguished from its four to five year residual effect on soil insects.
Both calcium and lead arsenate toxicity (carelessly caused by excessively heavy doses) can be overcome with a heavy application of superphosphate (about 75 pounds to 1,000 square feet) or with ferric sulfate (iron sulfate, but be sure you use the ferric form, not ferrous). The rate on ferric sulfate is quite high - 10 pounds to 100 square feet - but for spot treatment might be feasible. This heavy a dose of iron sulfate will, of course, kill grasses and other plants so the remedy may be as bad as the disease.
One of the worst weed killers, in so far as residual effect goes, is sodium arsenite. This is an old material: there has been some kind of a sodium arsenite weed killer on the market since 1888. It will sterilize soil so that nothing can grow there for two to three years. This is excellent on drives and walks, where I use it regularly. I spray a thin line along a fence where no mower or tillage tool will go. It keeps weeds from growing there for years. Fortunately, it does not wash once it has become fixed on soil, so I can spray within a few inches of desirable plants.
Ferric sulfate, 10 pounds to 100 square feet, is the remedy for sodium arsenite toxicity. Next, put on gypsum to neutralize any remaining sodium particles.
Two Major Herbicides
I recall how, when I was working with 2,4,5-T in 1943 and 1944, I would say to myself, "But what is this doing to soil bacteria?" I couldn't see how this stuff could kill plants and not be equally toxic to bacteria. Yet nothing seemed to happen to the bacteria or other microorganisms, even after repeated spraying. Later, we found that this was not a toxic chemical that destroyed directly, but affected growth abnormally. Since they attacked the cambium layer of broad-leaved plants, bacteria (having no cambium layer) were safe.
Later an even more astonishing fact came to light. When we tried to trace 2,4-D and 2,4,5-T to see what happened to them in soils, they had disappeared. Bacteria had actually used them as food - had "eaten" them completely. With each successive application, the period these chemicals could remain in soil uneaten became shorter and shorter. This meant that the population of chemical-eating bacteria was increasing and using up this strange food faster and faster. In one series of tests where bacteria of this type were transferred from one flask to another, always with plenty of 2,4-D to eat, they used up over 98 per cent of the chemical after 70 transfers in four days.
Many of the chlorinated phenoxy compounds cause a similar response. Since their breakdown in soils is linked to bacteria, temperature plays a vital part in their disappearance. For example, if either 2,4-D or 2,4,5-T are applied just before soil freezes in winter, bacteria are inactive and do not consume them. As a result, the chemicals persist in the soil and if they come in contact with certain deep-rooted, hard-to-kill weeds, will destroy them through prolonged exposure to 2,4-D effects.
The one drawback to this method is that no plants susceptible to 2,4-D can be seeded early in spring in a soil treated in late fall with 2,4-D because it will still be there. After two weeks during which soil temperatures are in the 60s, it will be safe to sow seeds of most vegetables and flowers.
Substituted Urea Compounds. Under such names as Neburon, Diuron, Monuron and Fenuron, these are being used to control weeds under a wide range of conditions. They differ considerably in such qualities as solubility, persistence in soils, species of crops on which they are safe, weeds they kill and in other respects. They are considerably more residual than 2,4-D, so when a weed killer is wanted for a period of weeks, selection of a proper formulation of a substituted urea compound is perhaps the answer. These chemicals are digested and destroyed by soil bacteria but at a much slower rate than 2,4-D.
Aminotriazole: This specialized weed killer is perhaps the best we have for control of nut-grass, poison ivy and Canada thistle. It deserves special mention because of the furor it raised in the fall of 1959 during the cranberry fiasco. Aminotriazole seems to be considerably more persistent in soils than any 2,4-D type of material or certain forms of the substituted ureas. In laboratory tests, only about one-fifth of it disappeared 35 days after exposure to soil organisms.
I feel that because of this persistence, aminotriazole (in the hands of the amateur) should be confined to use around ornamentals where such hard-to-kill weeds as nut-grass, poison ivy and Canada thistle must be destroyed.
Additional Materials: New weed control chemicals are emerging from the laboratory; it is impossible to keep up with them. In addition, there are hundreds of materials already in commerce. There is, however, one fundamental and universal principle which must be followed in handling such materials, namely, read and adhere to the package directions.
If you only knew how many hours of work and testing went into the preparation of directions to protect you from personal injury and to protect your plants, you might appreciate how important it is to put on your bifocals and read all the print on the package. Above all, if a manufacturer thinks it is important to tell you his product has a residual period of 10 weeks, believe him. He isn't anxious to limit his sales; hence any restriction of this kind is put on the package for your protection.
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