Why pH is Important In Soil

The term pH often confuses gardeners. Although the technical physics and chemistry behind the pH theory call for a college course or two, actually in use, this scale is no more difficult to understand than a thermometer. It measures the hydrogen ion concentration in a given substance on a scale of from 0 to 14.

Since the hydrogen ion concentration has a direct relation to the acidity or alkalinity of a substance, with a device for measuring that concentration, we can tell how "sour" or "sweet" a given soil may be. This measuring can be done in two ways: First, an electric bridge can measure this directly. Such a device is too expensive for the home gardener but most county agents have one, as do state experiment stations that make soil tests. Secondly, although not quite as hairline accurate, the small pH test kits sold in garden centers are adequate and usually give a reading within a tenth of a point or two. They are chemical and indicate pH by a change in color of test papers. For a small price, the gardener can determine one of the most important factors in gardening.

But why is pH important? First, it tells us what chemical nutrients can be available to a plant if they are present. A pH test does not tell you, for example, that phosphorus is present, but only if the soil is not too acid or too alkaline to allow phosphate to be released.

The point at which all elements are available to plants that do not require highly acid soil has its center at pH 6.5. On the pH scale, 7.0 is considered as neutral, where acidity and alkalinity balance out each other. Thus somewhere between 6.0 and 7.0 is perhaps the best point to strive for in altering soil acidity.

Acid-soil plants such as rhododendron, blueberry, mountain laurel, and others are able to thrive in acid soil because they do not use nitrogen from the soil directly, but depend on a special type of fungus on their roots, called mycorrhiza, which is able to change ammonium nitrate into nitrate nitrogen. These plants also tolerate some free aluminum at their roots, an element which is released at a pH of 5.5 or lower.

Not only does acidity decrease the availability of elements directly, but also it reduces the activity of soil bacteria until at 5.0 such activity might cease entirely. Since organic matter is available to plants only after bacteria have digested it first, the effect on growth is obvious. Another effect of pH is the release of toxic elements such as aluminum, which is released at a low pH (below 5.5) and also at a high pH (8.5). The prevalence of disease is also affected by pH, both by acid and alkaline soils.

The physical condition of the soil can also be involved with pH, wince liming to increase alkalinity improves the "crumb" structure of soils containing large amounts of clay.