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.