pH-value as measure for the alkalinity or acidity of a medium

The pH-value describes the activity of hydrogen ions in an aqueous medium. It is defined as the negative logarithm (base 10) of the molar concentration of hydronium ions (H3O+) in a solution. pH-values lower than 7 are associated with high hydronium ion concentrations and acid reactions. In alkaline solutions pH-values are greater than 7 and the hydronium ion concentrations accordingly low.

The pH of a medium is the result of complex processes. In soil the ph-value is mainly influenced by vegetation, soil organisms, organic substance, clay content and atmospheric deposition. In suspended particulate matter the respective influential factors are mainly organic substance, clay content, microorganisms and the ambient water.

In biology, medicine and environmental sciences the pH-value is an important parameter because many chemical processes are pH-dependent, for instance metabolic reactions, nutrient cycles, and binding and release of metal ions and carbon.
By influencing the mobility of nutrients and heavy metals the pH-value is of special importance for plant growth. A slightly acid soil is optimal for many plants because the solubility of most nutrients is best within the pH-range of 6.3 - 6.8. Very low pH-levels are critical because aluminium can be released in toxic concentrations. At high pH-levels, however, there is a risk of manganese deficiency in soils.

The pH in soil is determined electrometrically in a soil/water suspension and in a soil/electrolyte suspension (calcium chloride and potassium chloride). The pH in suspended particulate matter is measured in a calcium chloride suspension.


  • pH-value (water)
    The pH in a water suspension is a measure for the actual acidity of the sample. It is related to the concentration of free hydrogen ions (H+) in the suspension.
  • pH-value (calcium chloride)
    The pH-value in a suspension of sample material and a diluted calcium chloride solution corresponds to the potential acidity.
  • pH-value (potassium chloride)
    The pH-value in a suspension of sample material and a diluted potassium solution corresponds to the potential acidity.


  • Fine insoluble mineral or organic particles in the water phase
  • Soil is livelihood and biosphere for humans, animals, plants and soil organisms. All the substances brought in are transported, transformed and/or accumulated in the soil.

Sampling area

Sampling period

2002 - 2017