Residues, Soils, and Plants

Victor R. Boswell.

Since the mid- 1940's there has been a renewed and heightened interest in questions involving residues of agricultural chemicals in soil. This has occurred because agriculture has been recently supplied with a number of entirely new synthetic compounds for pest control, of largely unknown stability and toxicity to plants when present in the soil.

During the Second World War, when DDT first came into use in this country, one of its properties that especially intrigued us was its unusual persistence. It seemed almost too good to be true that a single application of a small amount of DDT solution upon window or door screens, walls, or trim would remain effective as a fly killer for many weeks. There were hopes that it would prove highly persistent in the soil for the killing of harmful insects there. It appeared highly desirable that we should have an insecticide of such persistence that a single treatment of the soil for insect control would remain effective for years. DDT has proved to be just that kind of remarkable substance. Entomologists in the eastern United States have found that DDT applied to the soil at 25 pounds per acre in 1945 was still effective against Japanese beetle grubs in 1950. In certain tropical and other soils, however, DDT loses its effectiveness relatively rapidly.

This stability or persistence of an insecticide in the soil that may be highly desirable for insect control can, however, turn out to be a serious disadvantage under some conditions. Is it harmful to plants, to soil bacteria, or fungi? How long will this or that insecticide remain in the soil without leaching out,without gradually vaporizing, or without being rendered inactive to plants or insects by chemical change or by the action of soil micro-organisms? If any amount is harmful to plants or to the soil bacteria and fungi, is it also so persistent that residues reaching the soil after dusting or spraying crops will accumulate to an extent that will harm plant growth? If residues can accumulate, how fast? How often and how heavily can a particular substance be used without danger of lowering the productivity of the soil after some years? Can a harmful amount of such residue be removed or corrected by special treatment of the soil? These are questions that must be answered not only for insecticides but for any agricultural chemical that is purposely applied to or that incidentally reaches the soil.

ONE OF THE FIRST REPORTS of crop injury believed to be due to an accumulation of an insecticide in the soil appeared in 1908. Symptoms of injury were evident in apple trees in an orchard in Colorado that had been sprayed repeatedly with lead arsenate. Soil analysis showed 61 p. p. m. (parts per million) of arsenic in the soil of the orchard, much more than the amount naturally present. Later work suggests that the injury in question was not due directly to the arsenic in the surface soil of the orchard. The old work, however, did demonstrate that ordinary use of an arsenical insecticide in an orchard results in an arsenic accumulation in the surface soil.

During the years 1930 to 1933, in South Carolina, several investigators reported a series of observations and studies of toxicity of arsenic in the soil to crop plants. They found that on heavy soils Cecil clay and Davidson clay in those instances cereals and cotton were uninjured by thousands of pounds of calcium arsenate per acre. Vetch and cowpeas were injured by 1,000 to 1,500 pounds per acre or more. On the clay soil soybeans were uninjured by large amounts, but on Norfolk sandy loam 200 to 300 pounds per acre caused serious injury. They found that the crops on soils low in iron content were injured by much smaller amounts of arsenic in the soil than when they were grown on soils high in iron. Soybeans were seriously injured on Norfolk sandy loam following only 3 years of cotton dusted with calcium arsenate to control boll weevil. In Louisiana, about the same time others also found that a given amount of calcium arsenate caused much more injury on a light soil than on a heavy one. On Crowley silty clay neither 50 nor 150 pounds per acre had any effect on rice. On Crowley very fine sandy loam, however, 50 pounds reduced the rice yield 45 percent and 150 pounds reduced it 65 percent.

In the 1940's, work in central New Jersey showed that most vegetables are sensitive to arsenic. Lima bean, snap bean, and turnip were especially sensitive; they were killed by 1,000 or 2,000 pounds per acre in the surface 3 inches of soil. Early growth of all crops was retarded, and some crops very seriously, but if the plants survived long enough for the roots to penetrate below the treated zone, they made considerable recovery.

The greatest accumulations of arsenic residues from the spraying of crops have been noted in the orchard soils of the Pacific Northwest, especially Washington, where heavy lead arsenate sprays have been applied annually for many years. Most of the arsenic residue is confined to the surface 6 to 8 inches of soil, the amounts found below 8 inches rarely exceeding those occurring naturally in the soil. The old orchard trees have been apparently uninjured by the great accumulations sometimes found because the arsenic accumulated only in the surface 6 to 8 inches after most of the tree roots were well established in the deeper levels of the soil. In some orchards amounts up to approximately 1,400 pounds per acre of accumulated arsenic trioxide have been determined in the surface 8 inches. Arsenic up to 30 times and lead 40 times the amount occurring naturally have been found. Although a large amount of arsenic in the surface soil may not harm the trees, it is definitely harmful to many kinds of cover crops, and cover crops are essential to profitable tree yields over a long period of time. As lead arsenate has accumulated, legume cover crops have become progressively poorer in many orchards.

During the depression years of the 1930's many of the less productive orchards in Washington were pulled out, and efforts made to grow alfalfa or annual crops on the old orchard sites. Alfalfa and beans often died on those high-arsenic tracts although they thrived on immediately adjacent sites that received no spray residues. Of the vegetable gardens observed on numerous old orchard sites, none was entirely successful and many were failures. Several years after heavily sprayed trees had been removed, rye and potatoes grew fairly well but beans and peas still showed marked sensitivity. After several years tomatoes, asparagus, and grapes showed intermediate sensitivity, but they grew poorly on land from which the trees had been recently removed.

If the roots of orchard trees encounter the high accumulations of arsenic found in the upper layers of some old orchard soils, the trees may be definitely injured. Specific arsenic toxicity symptoms have appeared in peach and apricot trees planted on land from which old sprayed apple orchards had been removed.

As the years pass, the arsenic toxicity of the former orchard soils is gradually decreasing. With the cessation of applications of arsenicals, with the gradual leaching effects of rainfall and irrigation, and with continued culture of the less sensitive crops, the productivity of those soils in the Pacific Northwest should be ultimately restored after many years. In South Carolina also it was noted that after the arsenic applications (to cotton) were stopped, productivity gradually returned to those soils that had been damaged.

SINCE 1945 large quantities of new synthetic organic insecticides have been used. The most important of these can be classified in one or another of two general groups of substances : (1) chlorinated hydrocarbons, such as DDT and BHC; and (2) phosphorus compounds such as parathion and HETP. With no background of earlier experience with these or related substances for use as insecticides, there was no basis for knowing whether any one of them would prove to be more persistent or toxic in the soil to plants, than lead arsenate, for example, or less so. The almost unbelievable insect-killing power of some of them, the apparent stability of DDT, and even the "newness" of these materials all combined to give a genuine urgency to the questions that arose about them. The instances of damage to orchard and cotton soil by arsenic accumulations stood as warnings of what might result from long-continued use. If one of these new substances should, when mixed in the soil, happen to be many times as toxic as lead arsenate is to common crop plants ( some are many times as toxic as lead arsenate to insects in the soil) and as persistent as lead arsenate or more so, there would surely be trouble ahead. From the first, some feared that under certain conditions of use farmers might encounter damaging effects to crops by soil residues in a shorter time and of a more troublesome character than had resulted from using lead arsenate. Soon there were not just one or two compounds to consider, but a dozen of them; and it was important to find out as soon as possible what their potential long-time effects might be.

Perhaps one of the most striking observations on plant response is that various insecticides, when present in the soil in appreciable amounts, may definitely reduce rate of growth, total growth, and yield (as of seed or fruit ) without producing above ground any symptoms of injury whatever. This inability to detect any harmful effect by inspection of the above-ground parts of the plant may very well result in overlooking many instances of unsuspected insecticide residue injury. The crop may appear entirely normal, but if there are no exactly comparable plants nearby on residue-free soil, the retarded growth cannot be detected unless it is rather severe. Definite symptoms of injury show up, usually, only when growth has been retarded so severely that it would be noticed whether other symptoms were present or not. Leaf and stem discolorations and malformations are among the last symptoms to appear. Much harm may be done before they become evident.

Below ground, the situation is not quite so difficult to detect. In general, plants that are retarded in growth by DDT or BHC will show root abnormalities although the tops appear normal. The moderately affected plants may show only somewhat stunted and shortened root systems. In more severe cases the roots are sometimes discolored and abnormally short, numerous, and virtually without root hairs. Extreme injury is characterized by very numerous short, thickened, stubby roots. The roots appear to have been stopped in growth soon after starting, with successive flushes of roots emerging only to suffer the same fate.