The effect of organic matter is to buffer high alkalinity. It absorbs certain food elements so they are still available to plants in spite of high pH. Most alpines will be found growing in pockets where organic matter has been trapped, while on the Great Plains, shrubs like the buffaloberry (Shepherdia) grow along water courses where flood waters deposit richer soils.
About the only direct effect of too high a pH is aluminum toxicity. At readings above 8.5, aluminum is released and will seriously injure plant roots (just as it does at pH 5.5). Aluminum is also harmful because it makes phosphorus unavailable to plants. Among the vegeta-bles which are seriously injured by even small amounts of free alu- minum are lettuce, onions and beets.
LIME: EFFECTS ON pH AND SOIL CONDITION
Ground limestone is a dual-purpose mineral. We apply it primarily to overcome acidity but, in addition, we receive a bonus in soil conditioning (see Chapter Eleven) that alone is often worth the cost and effort of application. An application of ground limestone can give a heavy clay loam a loose, friable, well-aerated character. Such a soil allows water to penetrate readily without running off, and turns easily under the plow.
This change in condition is due to an electrical-chemical-physical reaction known as flocculation. In this reaction, each lime particle acts as an acid radical to attract several clay particles. These clumps of clay with a lime core form crumbs, which make the soil much more porous than it was before treatment.
The number of particles attracted in a clump varies with the type of clay. In general, clays from northern climates produce larger crumbs. For example, I have seen photographs taken with an electronic microscope which showed eight clay particles clumped around a single lime core. Theoretically, then, a northern clay soil treated with enough lime to attract all the clay it contains should be eight times as permeable as it was before treatment. This degree of improvement is, of course, never reached in practice.
On some southern clays I have found that an application of as much as half a pound of ground limestone to a square foot (500
pounds to 1,000 square feet) has in most cases loosened them to a surprising degree without raising the pH excessively. If you use anything like this much lime, I recommend that you make a soil pH test in a month or so, just for safety. If the test shows the pH is too high, sulfur can be used to reduce the reaction without seriously affecting the crumbs of clay and lime.