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pH and soil acidity

Themes: What is soil acidity and pH? | Measuring soil pH | Factors affecting soil pH | Quiz

This red hydrangea thrives on acidic soils. The pH of the soil, which is a measure of the acidity of the soil, governs to a large extent what is suited to and will grow well on a particular soil. Thus, agricultural crops, many of which go on to become our food, require soils in a particular pH range in order to produce their best yields. Both garden and wild flowers have a pH range in which they grow best. Similarly most organisms that live in the soil will operate in soils in a pH range that suits them. For example, one of the most common species of earthworms, Lumbricus terrestris, has a strong preference for soils with a pH above 5.5 to just over 7.0. Other species of earthworm such as Allolobophora longa can tolerate pHs less than 5.5 but only down to about 4.5. Below pH 4.5 in Western Europe at least, there are usually few earthworms in the soil. pH also has an important effect on the availability of plant nutrients. Several nutrients become less available at the lower end of the pH range, i.e. below pH 5.5 and the availability of a few tails off significantly at pHs above 7.5. It is important that those who use the soil for purposes that depend on soil nutrients should be aware of the best pH range for key nutrients.

An acid is a substance that dissolves in water to release hydrogen ions. pH is the measurement of the concentration of these hydrogen ions in a soil solution, i.e. a mixture of soil and pure water. There are two types of acidity. There is the active acidity which relates to the hydrogen ions in solution in the water that occupies the pores in the soil and occurs around particles. Rainwater entering the soil will pick up various ions once in the soil, such as those of calcium, magnesium, hydrogen and aluminium, and the types and proportions of different ions in the solution influence the pH of the soil and its acidity or alkalinity. If there are a lot of nutrient ions such as calcium and magnesium, many more than of hydrogen and aluminium, then the pH of the soil solution will be higher. As the proportion of hydrogen ions increase relative to ions such as calcium and magnesium, so the soil becomes more acid. The composition of the soil solution is very important as an indication of the health of the soil and its ability to support plants and organisms.

As plants regularly extract nutrients from the soil solution via their roots, one might think that the solution would be easily depleted of nutrients and the soil would quickly become more acid and infertile. Fortunately, most soils are buffered against rapid loss. The fine particles in the soil such as those of clay and humus are negatively charged and act as a sort of magnet for positively charged ions such as calcium, magnesium, hydrogen and aluminium that can become available by weathering of minerals (See Chemical Weathering), breakdown of plant material, or addition of fertilisers. These positively charged ions on the surfaces of particles are termed reserve acidity. In due course as the soil becomes more and more leached of nutrients, leaving behind mainly acidifying ions such as hydrogen and aluminium, the pH will then begin to drop and the soil fertility decrease. pH measurements of the soil give us a fairly simple measurement of the state of this balance at any one time.

Themes: What is soil acidity and pH? | Measuring soil pH | Factors affecting soil pH | Quiz