Determination of phosphorus in calcareous soils by mehlich 3, mehlich 2, cal, and egner extractants

Abstract

Acid extractants were found to be a problem when determining phosphorus in some soils especially in soils with a higher content of carbonates. Carbonate content, soil pH or extractable calcium cannot exactly predict the behavior of a particular soil. On the other hand, measurement of pH after extraction reflects the ability of soil to change the properties of an extractant. The objective of this study was to find the relationships between pH changes during the extraction and the extraction of phosphorus for Mehlich 3, Mehlich 2, CAL, and Egner extractants. Twelve soil samples from various regions of the Czech Republic were taken for investigation. The pH/KCl of the samples was in the range 4.9–7.3. Calcium carbonate was added into the extracting bottles to the soil samples before extraction. The CaCO₃/soil ratio was from 0 to 0.5. The content of phosphorus and pH were measured in filtrates after extraction. The ratio P₁ =P_meas /P_O was calculated for all measurements, all additions of calcium carbonate and all soils. P_meas is the measured value of phosphorus after extraction with the addition of calcium carbonate and P_o is the value of phosphorus determined after the extraction without addition of calcium carbonate. The ratio P_r reflects the changes in the extractability of phosphorus and it is independent of the absolute content of phosphorus in soil. The relationships between P_r and pH after extraction for CAL, Mehlich 2, and Mehlich 3 extractants were found. No such relationship was found for Egner extractant. Correction equations (x=pH after extraction, y=P_r):