Determination of total organic‐C in soils by an improved chromic acid digestion and spectrophotometric procedure

Abstract

An improvement to the Walkley‐Black wet digestion method for the rapid determination of organic carbon over the range 0.2–5.5% in air‐dry soil is described. It permits total recovery of the organic‐C in finely ground soil samples digested with the heat of dilution from mixing N K2 Cr2 O7 with concentrated H2SO4. in test tubes followed by external heating from a hot‐plate digestor. The organic‐C concentrations are determined directly, as the Cr product in diluted soil digests, by absorptiometry at 600 nm with calibration against similarly treated sucrose standards in solution. For the soils tested, there were negligible interferences from carbonates, wood charcoal, coke, Fe+2 and readily reducible Mn; Cl does not interfere with the organic‐C assay in non‐saline soils but for saline soils a correction based on 1/12 Cl‐ assay of the soil is necessary. The present method is compared with Tabatabai and Bremner's dry combustion procedure and Allison's manometric adaptation for calcareous soils. The procedure described here does not require carbonate to be determined and is therefore simpler. In addition it is cheaper, faster and more effective in controlling interferences than dry combustion procedures.