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Journal American Rhododendron Society

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Volume 16, Number 4
October 1962

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Native Rhododendrons and other Acid Soil Plants
Edgar T. Wherry, Philadelphia, Pa.
Résumé of lecture presented to Tappan Zee Chapter, April 25, 1962

        Some fifty years ago the extreme importance of acidity vs. alkalinity in biochemical processes came to be recognized, and it occurred independently to workers in different countries that the soil reaction might be significant in plant growth. The speaker's contribution was the application of dyes, which show color changes with various degrees of acidity, to testing soils in the field, a procedure now so commonplace that one can buy "test kits" at any store handling horticultural supplies. At the time Dr. F. V. Coville, chief botanist of the U. S. Department of Agriculture, was engaged in the highly successful project of bringing the Blueberry into commercial cultivation, and on learning of my work in another branch of the Department as a laboratory chemist, he arranged for me to travel over the eastern states to obtain soil acidity data on members of the Ericaceae; thus I was destined to become a plant-explorer
        In the course of field trips, photographs were taken of any rare or notable plants found. As Kodachromes had then not been invented, my procedure was to take the pictures with a plate camera, provided with a long bellows, stopping the aperture well clown as to gain perspective and giving a time exposure. The negatives were then printed on standard lantern slide plates, and the plant parts colored in with transparent water colors. These slides will be shown to demonstrate their various theories.
        To take up some points about soil acidity which will be of interest to rhododendron growers. There are some popular misconceptions, which should be corrected. When rain falls on exposed rocks for long periods, these are broken down into a mixture of sand and clay, which as soon as the all important plant nutrient nitrogen accumulates, through the life-activities of micro-organisms, becomes soil. The basic elements, the more important of which to plants are potassium, calcium, and magnesium, tend to leach out and be carried down through crevice, ultimately reaching springs, streams, and the sea. The general rule is that the top of a hill tends to become ever more acid, the land affected by water emerging from its base, more alkaline (unless of course occupied by acid-producing mosses as in a bog). This has been checked by indicator tests thousands of times: yet it directly contradicts statements to be found in older books and popular articles based upon them.
        The misunderstanding arises from confusion of words. When a horticulturist or agriculturist finds his ornamentals or his crop plants sulking, turning yellow or seeming otherwise unhappy, he says, the soul is sour. Acids taste sour, so the conclusion is jumped at that the offending soil is acid. Sour soil is correspondingly a distorted soil, usually so water-logged that air cannot get to the plant roots and toxic waste products cannot get out. It is often too alkaline, and may indeed smell of ammonia or ammonium sulfide. To cure this, the soil must be aerated, or lightened by stirring in sand, humus, etc. A "'sour" soil is not an acid soil.
        Some thousands o£ tests with the indicator dyes from Maine to Florida demonstrated that by and large, Rhododendrons and other Ericaceae thrive best in soils of a considerable degree of acidity. If their seeds chance to fall into non-acid places, they will grow for a time, but they are physiologically weak, and when subject to any serious stress, whether connected with climate, competition with other plants, or attack by parasites, are likely to succumb without offspring. In other plant families some members may also do best in acid soil, but others are better adapted to "circumneutral" conditions. The latter term is used in preference to alkaline because in the eastern United States with relatively high rainfall, soils never have a chance to attain any significant degree of alkalinity. The pertinent "pH" numbers are circumneutral, 6 to 8; acid, 4 to 5.5.
        In making observations as to the acidity in which a plant is thriving, care as to certain details is important. The least amount of indicator dye in which the color-change can be seen should be applied, for the tiny bit of acid in a soil cannot swing over an excess of dye. The soil sample must be taken from as close to the roots as possible, not merely from anywhere in the general vicinity. Because of the leaching effect already mentioned, a soil may be intensely acid at the surface, moderately acid a few inches down, and circumneutral only a foot down.
        Once an error gets published in a supposedly authoritative journal, it is likely to get repeated by those numerous copiers forever after. While I was collaborating with Dr. Coville, the statement appeared that unlike most rhododendron, R. roseum would grow in limy, alkaline soil, so should be suited to ordinary non-acid gardens. Dr. Coville had made plans to send me where the observations had been made. We visited one of the largest limestone quarries in the state, at Leroy, New York, and this rock neutralizes acids effectively. No question about it, there was the Rose Azalea in fine bloom high above. But when we got up there, it proved to be rooted not in limestone at all, but in a silica rock, which yielded definitely acid soil. The second point to remember is to make tests at the plant root level.
        (It was pointed out that if a top dressing of lime is applied to acid-soil plants, they may be stimulated; this is because nitrogen-fixing micro-organisms grow in the limy layer, and the nitrates produced dissolve in rain water and act as a fertilizer. However, should the acid humus fail to hold the lime in the upper soil layers, so that it reaches the roots, the plants are likely to be unable to withstand the stresses of the following winter.)
        Finally, considering the geological setting of lower Rockland County, the two prominent formations here are Triassic diabase ("trap-rock") and red-shale. Both of these rocks contain moderate amounts of lime, so they are capable of yielding circumneutral soils. However, they are over large areas so exposed to leaching by the rain, that their soils become sufficiently acid for ericaceous plants. Should any of these when set out in a new site prove to show leaf-yellowing, or to be susceptible to fungus attack, one of the low-acid pockets may have been occupied. Iron chelates may correct the yellowing, but it is possible to increase soil acidity: one can add peat moss (provided no testing it is found to be acid itself), conifer debris, fresh oak leaves, decayed sawdust, etc.
        Chemical; such as ammonium sulfate can be used, but they are dangerous unless carefully scattered. A mixture of powdered sulfur with humus is both safe and effective. Concluding, acid-soil plants thrive best in acid soils.


Volume 16, Number 4
October 1962

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