How2Day: Characteristics of Acidic Soils

Acidity is one of the more important characteristics in determining what will and will not grow in your soil. Acidity is measured by pH on a scale from 1 to 14. Soils with a pH lower than 7 are considered acidic, while those with a pH higher than 7 are alkaline or basic. While most plants prefer a neutral or slightly acidic soil with a pH of 6.5-7.0, strongly acidic soil will not support most plants. Fortunately, acidic soil can be neutralized by mixing in lime or other alkalizing materials.

Acidic soil develops naturally over hundreds of years, often as a result of natural events. High rainfall washes bases out of the soil over time. When these are removed, they are unable to neutralize acidity, and the soil begins to become more acidic. The effect is especially severe in sandy soils that have less basic material to begin with and drain water through much more quickly. The "parent material" -- the type of rock the soil was derived from -- also makes a difference. Soils made from certain materials such as limestone, naturally react with and neutralize acids, while soils made from granite don't have this property. Finally, decaying plant matter acidifies soil gradually over time by producing hydrogen protons, which react with carbon dioxide to form carbonic acid.

As plants grow, they absorb alkaline nutrients from the soil. If those plants are harvested, the mineral can not return back to the soil. Over time, this can cause the soil to become more acidic, particularly if high yield crops like alfalfa are used. If only part of the crop is used (such as when only the grain is harvested and the rest of the plant is returned to the soil) acidity will develop more slowly.

According to the Oklahoma State Extension, manganese and aluminum in soils are usually in a solid form where they are not absorbed by the plants. When the soil pH drops below 5.5, however, the manganese and aluminum become dissolved and can be absorbed by the plants, acting as poisons. Aluminum, damages the root system, interfering with the ability of plants to absorb water and nutrients. Manganese damages the part of the plant which is above ground, causing it to be stunted and unhealthy.

Although most plants fairly neutral soils, some plants (known as ericaceous plants) do well in acidic soils. Plants such as heather, Camellia sinensis (the tea plant), and plants from acidic environments such as wetlands will thrive in acidic soil. Other plants (known as calcicole) prefer alkaline soils and will be unhealthy in even moderately acidic soils. Cabbage and many other vegetables as well as alpine plants will do poorly in acidic soil.

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