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Acidity and Alkalinity
Acidity and alkalinity of soils are the result of (1) the chemical composition of the rock from which the soil is derived, and (2) the partial or complete decomposition of vegetation. The degree of acidity or alkalinity of the soil, measured in terms of pH, is known as the soil's reaction. A basic indicator of soil health and fertility, soil reaction is easy to determine and, in most cases, easy to control.
Farmers in semi arid regions often use gypsum (calcium sulfate) to reclaim alkali beds and add calcium to soil that is excessively alkaline. By replacing the sodium of alkali soils with calcium, gypsum can often improve drainage and aeration. As a hydrate, however, in which the sulfate is chemically combined with water, gypsum is not strictly a natural product, and is thus avoided by many organic growers.
A few plants, like blueberries, flourish in fairly acid soil, but most garden crops, lawn grasses, trees, and shrubs prefer soils that are just slightly acid (pH 6.5 to 6.0).
This method is far from precise, but it can give you a rough idea of where your soil stands. If your soil is too acid, it may be brought back to a favorable pH by applying limestone. Agricultural-ground limestone is the commonest and safest liming material. Of the two types of ground limestone, calcitic and dolomitic, the latter is preferred because it contains magnesium in addition to calcium, thus fertilizing the soil as well as neutralizing it.
General guidelines for applying limestone are as follows: to increase pH by one unit spread on every 1,000 square feet of sandy soil 30 pounds of limestone; on a sandy loam. spread 50 pounds; on a loam, 70 pounds; and on a heavy clay, 80 pounds.
Another good material with which to lime soil is unleash hard wood ashes. If you have a fireplace or wood stove, save the ashes. Ii you can't put them directly on the soil, store in a dry place, since rain quickly leaches out the lime and potash in them. Coal ashes are of little or no value
Deficiency of calcium and magnesium is a third possibility. The best explanation may be that in acid soils, chemical reaction can lock up major nutrients, especially phosphorus, making them unavailable to plants. Heavy use of inorganic, high-analysis fertilizers causes soil to become more acid, as does heavy use of sulfur-containing fungicides. Organic gardeners don't have to worry about that, but the same result can stem from using organic fertilizers that have an acidifying effect.
When using ground limestone, don't expect a tremendous response the first year you apply it. The year after will be better. Liming should be repeated every fourth or fifth year, depending on the indications of soil tests. Where it is really needed, liming produces dramatic results. Because of this, there is a temptation to over lime, especially on a small garden plot. Over liming is as bad as not liming at all. A pH of 7.0 is a signal you've overdone it.
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