Exchangeable base cations
The amount of base cations depends on the soil’s capacity to bind cations as well as on inputs and losses. Inputs occur through weathering, atmospheric deposition, and human activities (e.g. fertilisation). Losses occur through leaching as well as plant uptake and harvesting.
Soil particles are electrically charged and can therefore bind different ions present in the soil solution at their surfaces. The binding is non-specific, meaning that any ion with a charge opposite to that of the particle surface can participate in the binding. Nor is an ion bound to a specific charge on the particle surface. The electrostatically bound ions are exchangeable and can be replaced by other charged ions.
The importance of exchangeable ions in soil
Through exchange processes, the exchangeable ions are in equilibrium with the soil solution. Changes in the composition of the soil solution therefore directly affect the composition of the exchangeable ions. An example is acid deposition via precipitation, which lowers the pH of the soil solution and increases the release of Al³⁺ into the soil water; this, in turn, competes with and displaces bound base cations. In this way, exchangeable base cations buffer soil water and groundwater against decreases in pH.
Exchangeable ions can be taken up by plants through exchange processes and are therefore important from a plant nutrition perspective. Uptake occurs as roots exchange base cations for hydrogen ions (H⁺), and anions for bicarbonate (HCO₃⁻) or hydroxide (OH⁻). Since plants take up more positively charged ions than negatively charged ones, plant uptake leads to biological soil acidification. When plant uptake is returned to the soil through the decomposition of dead plant material, this acidification is compensated, and the biological acidification is only temporary. However, plants are often harvested, whereby biomass and its content of absorbed ions are removed from the site; in these cases, the biological acidification becomes permanent.
Several studies have shown that, in many ecosystems—including Swedish forest soils—removal processes currently exceed inputs. As a result, both the pool of base cations and the base saturation in the soil decrease.
Determination of exchangeable base cations
Exchangeable base cations are determined by first weighing out 2.00 g of humus or 15.00 g of mineral soil into a 250 ml polyethylene shaking bottle. Then, 100 ml of 1 N NH₄OAc solution (buffered to pH 7.0) is added. The sample is shaken for 90 minutes in a mechanical shaker. The suspension is filtered through filter paper (ash-free filter paper, Munktell 00K, diameter: 18 cm). The concentration of base cations in 10 ml of the extract is analysed using ICP-AES (inductively coupled plasma atomic emission spectrometry).
Contact
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PersonJohan Stendahl, head of department and researcherBiogeochemistry of Forest Soils