ISO 22190:2020 pdf download – Soil quality – Use of extracts for the assessment of bioavailability of trace elements in soils
4.1.2 Neutral extracts for measurement of actual availability (ISO 21268-1)
For regulatory purposes simple and cheap methods are required and a simple extraction that simulates the pore water quality is desirable. A neutral aqueous solution (i.e., limiting changes of the soil pH during extraction as much as possible) can be used for this purpose. The concentration of trace element measured in a neutral extract is assumed to reflect the concentration in the pore water [as well as ionic strength, temperature, pH, DOC (Dissolved Organic Carbon)]. These properties may show a variation during the year and can be influenced by external factors (e.g., rain, drought, addition of manure). Extraction of a soil sample with demineralised water may have impact on the soil. For the purpose of estimating the actual availability of trace elements, it is desirable to reduce the influence of external factors and to obtain data that are more independent of the time of sampling. Extraction procedures have been developed using aqueous solutions containing a fixed concentration of a specific salt (neutral extract) in order to simulate the soil pore water. The stronger the extract (high ionic strength), the higher the amount of trace element released from the soil solid phase. On the other hand, the concentration of extracted DOC is also dependent on the choice of the neutral extract especially the concentration of divalent cations (Ca 2+ ) affects DOC. The ratio soil/ extract also affects the DOC concentration [4] . With a higher amount of DOC extracted, a higher amount of extracted trace elements can be expected, especially trace elements with a high affinity for binding to DOC (e.g. Cu, Pb, Cr).
4.1.3 Acid extracts for measurement of potential availability (ISO 17586)
Strong acids like HNO 3 can be used as an estimate for the potential available fraction. It will be clear that the acid extraction has a very large impact on the composition of the solution. The obtained solution has no relevance anymore to the pore water. A strong acid is a stronger solvent and will give the amount sorbed on the CEC, but also the trace elements in acid soluble salts, which is the amount that comes potentially available. At pH 0,5 the potential available fraction is estimated and this pH is approached with 0,43 mol/l HNO 3 . Non potential available trace elements included in the soil matrix are not extracted at pH 0,5. These are only extracted with a method for the total concentration like aqua regia. The difference between aqua regia and the acid extract are the non-available trace elements and therefore aqua regia is not suitable to estimate the available trace elements.
The soil parameters pH, clay and organic matter are already measured in the first tier. Specification of the organic matter and Fe-/Al-oxides are part of the second tier. The parameters in the CaCl 2 extract can only be measured if the extract is available and is therefore part of the second tier. Having these parameters, it is possible to predict the actual availability from the potential availability using transfer functions (see Annex B) [12] or geochemical modelling. The application and use of different parameters in geochemical modelling is described in Reference [13] and ISO 17402. Results of the methods are also useful for deriving soil quality standards [15] and soil protection guidelines [16] .