Hovedkationerne

Hovedkationerne is a Danish term used in soil science and hydrochemistry to denote the major exchangeable cations found on soil particle surfaces, as well as the principal cations in natural waters. The typical set includes calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+); ammonium (NH4+) is often included as an exchangeable cation in soils, depending on the nitrogen cycle and microbial activity.

In soils, hovedkationerne occupy the majority of the cation exchange capacity (CEC) on clay minerals and organic matter. They play a central role in soil fertility, nutrient availability, and soil structure. Calcium and magnesium promote flocculation and stable soil aggregates, while potassium and ammonium supply essential plant nutrients. The balance among these cations, expressed as base saturation or Ca/Mg ratios, influences soil physical properties and pH buffering.

Measurement and interpretation typically involve extracting the soil with a neutral salt solution, commonly 1 M ammonium acetate at pH 7, to displace exchangeable cations. The displaced cations are then quantified and expressed as cmol(+)/kg soil. This information informs fertility assessments and liming or fertilization strategies, as management practices shift the relative abundance of Ca, Mg, K, Na, and NH4+.

Influencing factors include soil weathering, parent material, drainage, fertilizer use, and acid deposition. Liming generally increases Ca and Mg relative to hydrogen and aluminum, improving structure and buffering. Excess sodium can lead to dispersion and poor structure, particularly in sandy or poorly drained soils.

In water chemistry, hovedkationerne refer to the major cations in solution, notably Na+, K+, Ca2+, and Mg2+. Their concentrations affect hardness, salinity, and buffering in freshwater and marine systems.