Flame Photometric Determination of Exchangeable Calcium and Magnesium in Soils

Abstract

The Beckman Model DU spectrophotometer, with No. 9200 flame attachment, and No. 4300 photomultiplier assembly was used to determine exchangeable Ca and Mg in soils. The study involved an investigation of the instrumental adjustments that would give the highest metal-to-background emission ratio, including effect of flame composition. The effects of Al, Fe, Mn, and PO4 upon Ca and Mg flame emissions were investigated. Thirty-two soils were analyzed chemically, and by 2 flame photometric procedures for Ca and Mg, with special emphasis on the effects of method of extraction and preparation of test solution. Two procedures for photometric determinations of exchangeable Ca and Mg are proposed: A “standard procedure” which calls for the elimination of the acetate ion and the removal of R2O3 and PO4, and a resultant test solution of 0.2 N NH4Cl; A simpler “optional procedure” which consists of the direct flame photometric reading of the soil NH4OAc extract. In the photometric determination of Ca and Mg in NH4OAc solution, the greatest source of error stemmed from the mutual interferences of the two cations, the Ca imparting a slight plus error to the Mg, and the Mg imparting a minus error to the Ca. No such interferences were observed when the determinations were made in the solution of 0.2 N NH4Cl. For soils low in exchangeable Ca and Mg, it is suggested that the exchangeable Ca and Mg may be equally well determined by the simpler “optional procedure”. However, for soils of high Ca and/or Mg contents or where phosphate is suspected, the use of the “standard procedure” is recommended.