Toxic Elements in Soils

Firman E. Bear.

We got the word "toxin" from the Greek word for huntsman's bow. "Toxic" came to refer to the effects of the poisons that man applied to the tips of arrows he used for hunting.

A toxic element is one that brings injury and sometimes death to the living organism that absorbs it.

Plants may suffer injury and die from high concentrations of salts, even salts that carry essential elements. Water of a high content of salt may be toxic to animals. Plants may be harmed by relatively small amounts of a particular element in the soil; animals may be injured by eating the plants in which that element has accumulated.

SELENIUM probably is the most troublesome. It is a serious hazard to livestock and probably to people in a wide semiarid belt that extends from inside Canada southward across the United States into Mexico.

Selenium (Se), a nonmetal, resembles sulfur in its chemical properties. The so-called metallic selenium is a silvery-gray, crystalline solid. Selenium tends to be present in largest amounts in areas where the soils have been derived from Cretaceous rocks. Such soils average about 5 parts per million of total selenium, but some contain as much as 80 p.p.m. Lack of rainfall has prevented the solution of the selenium minerals and the removal of their salts in drainage waters.

Cultivated crops and native grasses seldom contain enough selenium to be dangerous to animal life. The selenium content of wheat from such areas is usually only a few parts per million. When such wheat is mixed with wheat from other regions and with other foods, no trouble is likely, at least among people outside these areas.

The primary problem is selenium-accumulating forage plants, in which the concentration of selenium may reach 15,000 p.p.m. Even at much smaller concentrations, selenium may harm animals that eat considerable amounts of the forage.

The major selenium-accumulating forage plants are certain species of Astragalus (vetch), Xylorrhiza (woody aster), Oonopsis (related to goldenrod), and Stanleya (of the mustard family).

Astragalus is the most widely distributed. About 24 of its more than 200 species are selenium accumulators.

The accumulator plants seem to need selenium to grow well. The element tends to be toxic to nonaccumulating plants. It apparently replaces sulfur in the sulfur-containing amino acids.

Accumulator plants not only are troublesome in themselves as forage for livestock they increase the concentration of selenium in the soil in which they grow. They leave the accumulated selenium behind in soluble form in their dead tissues for more ready absorption by the next accumulator.

Two types of livestock injury are recognized. Alkali disease causes malformation and sloughing off of hoofs. Blind staggers, a more advanced form, leads to death within a short time. Eggs of hens fed seleniferous grain have low hatchability. Concentrations of 5 P.P.m. of selenium in food or 0.5 p.p.m. in milk or water are considered dangerous for people.

The degree of injury to livestock is related to the condition of the range, notably in very dry years. Selenium-accumulating plants tend to be deeper rooted than the grasses. They survive more severe aridity. They may remain as the principal forage for grazing in time of drought.

Selenium toxicity to livestock can be overcome partly by feeding linseed oil meal as a supplement. Sodium arsenite has been found useful as a remedy, but it is too poisonous to have around the ranch. Considerable experimental work has been done with the less toxic arsanilic acid and related compounds, but they are less effective than the arsenite. Sulfates are a partial antidote for selenate toxicity but they are not effective in overcoming the toxicity of selenites or of the organic compounds of selenium.

To solve the selenium problem, a multiple approach has been suggested a careful survey and mapping of the soil of the seleniferous areas, development of a more complete list of accumulator plants, and more exact descriptions of the symptoms of injury to livestock that have eaten selenium-containing forage and grains. The worst areas should be fenced off and posted with warnings to stockmen. Everyone should be aware of using selenium-containing water.

ARSENICAL SPRAYS, when used repeatedly, have brought about accumulations of arsenic to the point of toxicity in orchard soils, notably in Washington. Many orchards have had 15 lead-arsenate cover sprays of loo gallons a tree yearly for a long time.

Arsenic (As) is a gray, brittle, crystalline substance. It occurs combined with sulfur, oxygen, and other elements, and as the element. The arsenates are fixed by soils in a relatively insoluble state and are not lost by leaching. They tend therefore to accumulate. Nevertheless, as much as 12 p.p.m. of water-soluble arsenic has been found in the top 6 inches of soil in orchards.

Barley and alfalfa seedlings are injured at concentrations of 2 P.P.M. of water-soluble arsenic. The tips of their leaves turn yellow a few days after they emerge.

Arsenic toxicity is particularly troublesome where old orchards have been removed and attempts are being made to replace them or to grow grass, grain, or vegetables on the sites.

Plants vary in their tolerance for arsenic. Apples, pears, grapes, and raspberries are among the most tolerant fruits. Rye, asparagus, cabbage, potatoes, and tomatoes are among the most tolerant field crops. Bluegrass, orchardgrass, and ryegrass are among the most tolerant forage plants.

Least tolerant plants include peaches and apricots among the fruits; barley, wheat, peas, and beans among the field crops; and alfalfa, clovers, vetch, and Sudangrass among the forages.

Other fruit, field, and forage crops are intermediate in tolerance.

Most plants, wherever grown, contain traces (normally less than 0.5 p.p.m. dry weight) of arsenic. Plants growing on soils in which arsenates have accumulated have been found to contain as much as 14 p.p.m.

Arsenic toxicity is evidenced by slow, stunted growth and late maturity of plants. Shot-hole and marginal scorch occur on the leaves of the more sensitive fruits, notably peach and apricot and sometimes cherries. The leaves drop prematurely. Legumes tend to die in the seedling stage, following the appearance of small spots of dead tissue scattered over the leaves. Grain crops turn yellow and die back from the tips of the leaves. Arsenic-poisoned corn seems to suffer from too little moisture. Injury is most pronounced during warm, dry months.

Treatment for arsenic toxicity consists in the use of 1 to 2 tons of iron sulfate to the acre, or triple-superphosphate at about the same rate, or, for peach trees, zinc sulfate at the rate of 5 to 10 pounds a tree. At pH values above 6, a chelated form of zinc, about 1 pound to a tree, is preferred.

Other methods of dealing with this problem include growing only the most tolerant types of trees and crop plants and deep plowing to bury the surface soil to a depth of 1 to 2 feet. The plowing does not suffice for peaches. The most important period is when the trees or crops are being established. The best method in replanting orchards is to set the young trees in 2 to 4 cubic feet of uncontaminated soil.

Arsenites seem to be more troublesome than arsenates.

The ferric forms (or oxidized compounds) of iron therefore are more useful than the ferrous forms (or reduced compounds) as correctives.

Plants on soils treated with ferric compounds contain much less soluble arsenic than those growing on soils not so treated. Aluminum sulfate is not a satisfactory substitute for the corresponding iron salt.

MOLYBDENUM serves in plants as a catalyst in enzyme systems that function in reducing nitrate to ammonium in preparation for the synthesis of amino acids and proteins. It has a similar part in nitrogen fixation by the nodule bacteria of legumes and by the nonsymbiotic bacteria.

As far as is now known, toxicity of molybdenum enters the agricultural picture only in relation to livestock production. The element appears to be essential as a respiratory catalyst, but trouble is experienced at concentrations above 10 p.p.m., dry weight, in green forage.

Areas of serious molybdenum toxicity to livestock have been found in California and Florida. The indications are that the element may also be present in toxic amounts in the forage of widely scattered places in the semiarid regions of the West.

The total molybdenum content of soils usually is about 1 to 3 p.p.m. Soils containing 75 P.P.m. of molybdenum have been found in Hawaii. The molybdenum content of soils in the localities of California where cattle are injuriously affected usually do not exceed to p.p.m. Molybdenum differs from most of the essential trace elements in that its availability is highest at soil pH values that approach neutrality. Most of the other trace elements are most available in acid soils.

One remedy for molybdenum deficiency for plants growing on acid soils is to lime the land liberally. Applications of superphosphate tend to have similar but less pronounced effects. Conversely, the available molybdenum in the soil can be reduced by acidulation with sulfuric acid, partly because the pH value of the soil is lowered and partly because it provides a competitive ion for absorption by the plant.

One of the best methods of overcoming molybdenum toxicity is to grow forage crops on the land for harvesting as hay until the more readily available supplies of molybdenum have been removed from the soil and hauled away with the hay.

FLUORINE usually is present in plants within a range of 2 to 20 p.p.m. dry weight. We have no evidence that it is useful as a plant nutrient. Tea is the best known accumulator of fluorine; its dried leaves may contain as much as 400 p.p.m.

In acid soil and following applications of soluble fluorides, the fluorine content of some plants can be raised to levels of 500 P.P.m. or higher. Injury usually results before the fluorine content has been raised to 50 p.p.m. At higher levels the plants may die.

Soils normally contain 100 to 300 p.p.m. of total fluorine, with increasing concentrations in the subsoil. Soils derived from rocks high in phosphate may contain as much as 8,000 p.p.m.

Fluorine is often added to soils in fertilizers and insecticides. It is brought down in the rain, notably in areas around factories where fluorine-containing ores are processed. Yet, as far as we know, fluorine toxicity to plants seldom or never occurs in open fields away from such factories.