State of the art and future developments in soil analysis for bioavailability assessment

For an evaluation of soil fertility status, a multiplicity of soil-analytical methods is in use in many countries. Very often a separate extractant is used for almost every single nutrient. For evaluation of the heavy metal burden of soils, however, in most countries only the 'total' concentration is measured after aqua regia treatment. To date, there is no generally accepted method of estimating the bioavailability of heavy metals. A single extraction of soils with 0.01 M CaCl2 is proposed. This extraction is simple, cheap and environmentally friendly. Moreover, the extracting solution matches the soil solution with respect to pH, concentration and composition. Results using this extractant are compared with results obtained by some conventional extraction procedures in use in The Netherlands. For some nutrients and heavy metals analytical results are judged against crop response. Heavy metal concentrations in soils determined by this extraction procedure give a better indication of bioavailability than do total soil contents. Understanding of plant requirement for nitrogen and of heavy metal behaviour in the soil-plant system with the CaCl2 extractant compares favourably with that obtained using other extractants. Using modern methods analyte concentrations in extracts are easily measured.