Arsenic is an element which is likely to be toxic in soils. Where an apple orchard has been sprayed for years with lead arsenate, arsenic can build up substantially. This seems to do very little harm to apple trees, but if the area is later subdivided, home owners may have a hard time growing grass or other plants on the contaminated soil. Arsenic is taken up by plants in place of phosphorus but does not substitute for it nutritionally.
Another place where arsenic can be harmful is on old golf courses. Lead arsenate is used for grub-proofing turf and also to control knot-weed, crabgrass and annual bluegrass (Poa annua). If used year
after year, arsenic may build up and override the intake of phosphorus by plants, thus starving them for that element. Growth will be retarded, plants will be stunted and they will mature late. Very sensitive plants, such as the stone fruits, may show injury in the form of shot-holes in the leaves or as marginal scorch.
The remedy for arsenic-sick soils is not easy or cheap: it involves application of as much as 500 to 1,000 pounds per acre of superphosphate. Or the same amount of iron sulfate can be applied. However, superphosphate is not only cheap but safer; iron sulfate used at this high rate will badly injure or kill grasses in lawns; in fact it was once recommended as a weed killer.
Selenium, molybdenum, fluorine and lead also are taken up by plants. These elements are poisonous and thus can harm humans (or animals) who eat contaminated plants. About the only time these elements need concern the home gardener is when they get into soil in the vegetable plot. The most dangerous of these elements is selenium, used by many African violet growers as a soil insecticide. It works beautifully for this purpose. However, selenium is a dangerous and lethal chemical. Never discard a plant grown in selenized soil (or the old soil itself) onto the compost pile. There is too much chance that some of the treated soil will eventually wind up in the vegetable garden, where it could be responsible for sickness and death.
Nutrients are commonly called "plant foods" but, like human food, they must be broken down or "digested" into simpler forms before plants can use them. Soil bacteria and fungi perform this essential job. Also, plant roots can only take up nutrients in solution, so everything must be soluble in water. Nitrogen, phosphorus and potash-the "big three"-plus a number of "minor" elements play definite though often inter-related roles in plant growth. A properly fertilized soil is one in which these elements are always available in amounts adequate to assure maximum flower and "fruit" as well as vegetative (stem and leaf) development.