Agroscope Reckenholz-Tänikon Research Station ART, Reckenholzstrasse 191, CH-8046 Zürich

The methods of soil extraction with water and CO2 satutrated water

The chemical processes of two Swiss soil tests for phosphorus availability were examined to allow a comparison: Extraction in the ratio of 1 g of soil to 2.5 ml of CO2-saturated water (CO2-P) and in the ratio 1:10 with pure water (H2O10-P), both followed by colorimetric P determination. Apart from orthophosphate up to 80 % of the result are due to other P species. Significant amounts of extracted organic P are not detected by this method. The extracts are saturated with respect to P: Using less soil with a constant volume of extractant, the P concentrations in the extract remain similar and may even increase. Likewise, only a small fraction of P added to the soil prior to the extraction are recovered in the extract. CO2-saturated water extracts between half and the fivefold of P compared with pure water. Calcium plays an important role: The addition of 1 % lime or Ca2+ to lime-free soils reduces the extracted P significantly. For lime-free samples CO2-P can be estimated from H2O10-P if H2O10-Ca is included in the calculation. With lime containing soils there are no useful correlations between CO2-P and H2O10-P.These «weak» extraction methods measure the «intensity», i.e. the P concentration in mg/L, and not soil P contents in mg/kg or kg/ha and can therefore not be used for an estimation of P fluxes or P balances.

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