Exchangeable base cations
Since soil particles are electrically charged they can bind ions dissolved in the soil water at their surface. The bond is unspecified and any ion with a charge opposite the one at the surface can take part in the bound. Moreover, an ion is not bound to a certain charge at the surface of the particle. The electrostaticly bound ions are exchangeable and can change places with other positively charged ions.
Cations (+) bind to negatively and anions (-) to positively charged soil particles. The capacity of a soil to bind cations at particle surfaces is denoted cation exchange capacity (CEC). For anions the corresponding expression is anion exchange capacity (AEC). In Sweden most soils have an essentially larger CEC than AEC. As a consequence cations to a larger extent than anions are bound in Swedish soils. Examples of exchangeable cations are calcium(2+), magnesium(2+), potassium(1+), ammonium(1+), hydrogen(1+), and aluminium(3+). Anions are for example sulphate(2-), nitrate(1-), chloride(1-) and phosphate(3-). Figures within parentheses show the charge of each ion, respectively.
The unit for exchange capacity is cmolc kg-1 (centimole charge per kg soil). Mole is a SI-unit denoting number of particles. One mole equals Avogadro's constant, which is 6.023*1023. The number of ions bound per surface charge depends on the ionic charge. A specific surface charge can thus bind twice the number of potassium ions as the number of calcium ions, because potassium is a univalent ion whereas calcium is bivalent. Of this reason the unit for exchange capacity is centimole charge instead of the corresponding number of ions.
The capacity of soils to bind ions is above all linked to small particles, mainly clay minerals and humus. Humus particles may be big, but they have a large inner surface where ion exchange can take place. Generally, CEC and AEC of soils rich in clay and humus are high. Especially high AEC have soils with precipitated iron and aluminium oxides. Organic material in the humus layer may have a CEC of about 100 cmolc kg-1. The CEC of clay minerals vary between 10 and 100 cmolc kg-1.
Among the cations some are denoted base cations and some acid cations. Common base cations are calcium(2+), magnesium(2+), potassium(1+) and sodium(1+). Hydrogen ions(1+) and aluminium ions(3+) are denoted acid cations. Most base cations, except for sodium(1+), are nutrients important for plant growth. The proportion of base cations in percent of the CEC is denoted degree of base saturation. The dominating base cation is generally calcium(2+).
Through ion exchange processes an equilibrium is maintained in the soil water. If the composition of soil water is changed this affects the composition of exchangeable ions. An example is acid atmospheric deposition resulting in decreasing pH in the soil water, and increasing release of Al(3+) to the soil water, in turn competing out bound base cations. Thereby the exchangeable base cations buffer soil water and groundwater to lowering of pH.
Through exchange processes plants can take up exchangeable cations, which are important as nutrients. Through roots base cations are exchanged to hydrogen(1+), and anions to bicarbonate(1-) or hydroxide(1-). Because plants take up more positively than negatively charged ions, the uptake causes biological acidification of soils. When ions taken up by plants are returned to the soil through decomposition of dead plant parts, the acidification is compensated. In these cases the biological acidification is temporary. In general plants are harvested and the ions that were taken up are, thus, removed with the biomass when it is taken away from the site. In these cases the biological acidification is made permanent.
Except for the capacity of a soil to bind cations (negative charges of particles), the amount of base cations in soil depends on processes bringing or removing them. In the long-term, base cations are supplied through weathering and atmospheric deposition, or through human activities like, for example, fertilisation. Base cations are lost through leaching, plant uptake and harvest of plants. A number of studies have shown that the removing processes in many ecosystems, among others Swedish forest soils, today are larger than the supplying processes. In this way the store of base cations as well as the degree of base saturation decrease.