Comparison of soil analyis methods for K, Mg and Ca

For the analysis of potassium (K), magnesium (Mg) and calcium (Ca) in soils different methods are used in Switzerland: Extraction with pure water (H2O10), with CO2 saturated water (CO2), with ammonium acetate and EDTA (AAE10) or with calcium chloride (CC). During the extraction, water-soluble compounds are dissolved, lime is decomposed by acids, cations are liberated by ion exchange with the extracting agent or with dissolved calcium and additionally, there are reactions taking place during the filtration. Which of these processes dominates, depends on element, extracting agent and lime content of the soils. Only H2O10-K and CO2-K as well as CC-Mg and AAE10-Mg of lime-free soils are each based on the same reaction types. Therefore, they are correlated which may allow acceptable conversions of one result into the other. At individual locations AAE10-K and CO2-K were correlated, but the extrapolation to CO2-K = 0 yielded AAE10-K values varying between 6 and 250 mg/kg. This corresponds to potassium, which is not extracted with the CO2 method. Its proportion depends on the different cation exchange characteristics of the soil clay fractions. Since the CO2-K and H2O10-K values expressed as mg/kg depend on the arbitrarily defined ratio of soil to extracting agent, they represent no total content of a soluble soil K fraction, but serve as a measure of the K availability in the soils.