The program, then, is to apply calcium arsenate on vermiculite as early as possible in spring, even if crabgrass has germinated, and to follow this three weeks later with a spray of Liquid or Super Sodar (on some containers this may be listed as AM A).
TOXIC RESIDUES IN SOILS
I have gone to some lengths to describe the action of these 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.