pH

Soil chemical and biological processes are often influenced by how acidic the soil is. The soil pH value is a measure of its acidity. Pure water has, by definition, a pH value of 7, which is considered neutral. In undisturbed forest soils, the pH value is lower. This is due to natural acidification processes—primarily the formation of carbonic acid when carbon dioxide dissolves in soil water. A typical pH value in water from mineral soils with low organic matter content is in the range of 5.2–5.4.

Chemical weathering contributes to neutralising acidic soil

Chemical weathering, the breakdown of soil minerals, contributes to neutralising acidity in the soil. This is a slow process, but it causes the pH of the soil water to increase as it moves downward through the soil profile. As a result, the pH value is generally higher deeper in the soil than in the upper layers.

Does soil acidification lead to increased forest damage?

During the last two decades of the 20th century, extensive research was conducted on how acidification caused by industrial emissions affected forest soils. It has been established that deposition of sulphuric and nitric acids has lowered soil pH in areas with high deposition, in some cases by more than one pH unit. This has led to increased mobility of metals, such as aluminium and certain heavy metals, and contributed to deterioration in groundwater quality and acidification of streams and water bodies. Many researchers believed that soil acidification contributed to observed increases in forest damage, but direct causal relationships have been difficult to prove.

More information on forest soil pH and how it is measured can be found in the sections “Forest soil pH” and “pH measurement in soil samples” below.

Forest soil pH

Soil pH provides a good indication of the chemical state of the soil. In general terms, pH is an intensity measure of soil acidity. It is not possible to determine total soil acidity from pH alone, since pH only reflects the concentration of hydrogen ions (H⁺) in solution. Hydrogen ions bound to soil particles are not included in the pH value.

Many processes influence soil pH, such as decomposition of organic material, weathering of primary minerals, cation exchange, and the activity of microorganisms. If pH is known, certain general conclusions about soil conditions can be drawn. The humus layer typically has a very low pH, often around or slightly below 4 in a mor layer.

In mineral soils, a pH below 5.5 indicates that the proportion of exchangeable aluminium (Al) in the exchange complex is high. A pH in the range 7.8–8.8 indicates the presence of calcium carbonate (CaCO₃) in the soil. Most Swedish soils are slightly acidic (pH < 7) due to the influence of carbonic acid (H₂CO₃), organic acids, and leaching of weathering products.

pH measurement in soil samples

When measuring pH in a soil sample, the sample is shaken in water or a salt solution. This produces a suspension consisting of the solid phase of the soil and a liquid phase containing dissolved substances from the sample. When the solid phase has separated from the solution—either by sedimentation or centrifugation—the pH is measured in the liquid.

The pH value measured by the pH meter is controlled by the soil’s acid–base status, as the buffering systems in the soil will either release or consume H⁺ ions in the solution. Some buffering systems are not sufficiently fast to immediately affect the pH of the solution. Therefore, the sample is usually allowed to reach equilibrium with the solution by waiting, for example overnight, before measuring pH.

Weathering of primary minerals contributes to the neutralisation of H⁺ in the soil, but this process is so slow that it does not noticeably affect a pH measurement. Soil buffering systems help stabilise the pH measurement. Measuring pH in pure water, on the other hand, is difficult because the total concentration of H⁺ is very low. This problem may also occur when measuring pH-H₂O (with deionised water as the suspension liquid) in weakly buffered soil samples, such as those from the C horizon in sandy soils or coarse moraines. In such cases, the pH value may drift and take a long time to stabilise.

To obtain more stable pH values, a neutral salt solution (e.g. 0.01 M CaCl₂) can be used instead of deionised water as the suspension liquid. The pH measured using 0.01 M CaCl₂ (pH-CaCl₂) is generally about 0.5 pH units lower than when using deionised water (pH-H₂O). The maps of forest soil pH presented above are all based on pH-H₂O.

For determination of pH-H₂O for soil samples in the National Forest Soil Inventory, the procedure is as follows:

• The sample is air-dried.
• The sample is sieved to remove particles larger than 2 mm.
• A representative subsample (1.0–5.0 g, depending on composition) is weighed into a 50 ml plastic container.
• 25 ml of deionised water is added, and the container is sealed and shaken to suspend the soil in the water.
• The suspension is allowed to stand overnight.
• The pH is then measured with a glass electrode in the solution, which has reached equilibrium with the soil sample.