What Causes Acidic Soils?

A range of natural and man-made actions cause acidic soils. A pH below 7.0 is acidic, and each pH unit lower indicates a tenfold increase in acidity, so soil at pH 6.0 is ten times more acidic than soil at pH 7.0. Soil is a mixture of organic matter from decayed plants and other organisms and minerals such as clay and sand. The chemicals in these materials affect the acidity of soils, and external influences such as rain and fertilizer applications also play a role in changing soil acidity.

As wind and rain weather rocks, the small particles that break away mix with other materials and become soil. Rocks are alkaline or acidic, and the parent rocks that form the base of soils affect their pH levels. For example, soils that have sandstone or shale as their parent rock are acidic.

Some alkaline soil chemicals dissolve in rain, and in high rainfall areas they're washed out of the soil, which increases its acidity. Compounds of calcium, potassium and magnesium are some common alkaline soil chemicals that are leached away in heavy rain. When these chemicals leave the soil, its acidity increases. Other soil chemicals react with rainwater to form acidic chemicals. One example of this is aluminum oxide.

Calcium, potassium and magnesium are also taken up by plant roots in varying levels, depending on the plant type. Legumes remove significant amounts of these chemicals from soil.

Plants exude acidic chemicals from their roots, and decaying plants are also acidic. Plant roots give out carbon dioxide, which is acidic, and organic acids. When the plants die, their decaying materials also increase soil acidity, though the level of increase varies according to the type of plant. The soil in coniferous forests is more acidic than the soil in deciduous forests, and the soil in prairies is less acidic than the soil in forests.

When nitrogen fertilizers such as ammonium nitrate, urea and anhydrous ammonia react with chemicals in the soils, they increase their acidity. Nitrogen fertilizers convert into different nitrogen compounds in soil, and as they do this they use up alkaline chemical compounds. High phosphate fertilizers also have an acidifying effect.

Nitrogen fertilizers that include an alkaline chemical don't cause acidic soils. One example is calcium nitrate fertilizer.

Fertilizers made of animal or plant waste increase soil acidity as they break down. Animal manure and compost are two organic fertilizers. As they decay in the soil, they give off carbon dioxide.

Burning fossil fuels causes nitric and sulfuric acid to dissolve in rain, making the rain more acidic. When this rain falls on soil, it increases the soil acidity. It's possible nowadays to remove acids from the gases given off by power stations that burn fossil fuel.