Determination of phosphorus in calcareous soils by Mehlich 3, Mehlich 2, CAL, and Egner extractants

Acid extractants were found to be a problem when determining phosphorus in some soils especially in soils with a higher content of carbonates. Carbona te content, soil pH or extractable calcium cannot exactly predict the behav ior of a particular soil. On the other hand, measurement of pH after extrac tion reflects the ability of soil to change the properties of an extractant . The objective of this study was to find the relationships between pH chan ges during the extraction and the extraction of phosphorus for Mehlich 3, M ehlich 2, GAL, and Egner extractants. Twelve soil samples from various regi ons of the Czech Republic were taken for investigation. The pH/KCl of the s amples was in the range 4.9-7.3. Calcium carbonate was added into the extra cting bottles to the soil samples before extraction. The CaCO3/soil ratio w as from 0 to 0.5. The content of phosphorus and pH were measured in filtrat es after extraction. The ratio P-r=P-meas/P-0 was calculated for all measur ements, all additions of calcium carbonate and all soils. P-meas is the mea sured value of phosphorus after extraction with the addition of calcium car bonate and P-0 is the value of phosphorus determined after the extraction w ithout addition of calcium carbonate. The ratio P-r reflects the changes in the extractability of phosphorus and it is independent of the absolute con tent of phosphorus in soil. The relationships between P-r and pH after extr action for GAL, Mehlich 2, and Mehlich 3 extractants were found. No such re lationship was found for Egner extractant. Correction equations (x=pH after extraction, y=P-r): Mehlich2: y=0.1121x(2)-1.3924x+4.3408, R-2=0.9576 Mehlich3: y=0.0661x(2)-0.9768x+3.4747, R-2=0.9372 CAL: y=0.2456x(2)-2.X287x+8.4766, R-2=0.8315 To reduce the number of pH measurements after extraction a critical value f or pH/KCl and a critical value for extractable calcium was determined for M ehlich 3 extractant. There were 1173 soil samples taken for this experiment . The samples represented all major soil types, climatic regions, and propo rtions of agronomic cultures in the Czech Republic. The critical values for pH/KCl=7.1 and Ca=4,000 mg kg(-1) were found for Mehlich 3. For higher val ues of extractable calcium or pH/KCl than are the critical values, the dete rmination of pH after extraction is necessary.