Simple modification of the Walkley-Black method for simultaneous determination of organic carbon and potentially mineralizable nitrogen in tropical rice soils

Summary

A modified version of the Walkley — Black method of organic C determination is described, which allows simultaneous determination of organic C and potentially mineralizable N in soils. As usual the organic matter of the soil is oxidized by acid dichromate reagent by heat of dilution. The excess of chromate left after C oxidation is titrated with standard ferrous sulfate solution instead of ferrous ammonium sulfate to avoid contamination due to NH4 +. The NH4 + released due to oxidative action of acid dichromate is determined by distilling a suitable aliquot from the known volume with 50% of NaOH solution. The ammonium released by this method with 15 surface soils showed highly positive correlations with organic C (r=0.969**), total N (r=0.976**) and the NH4 + produced under waterlogged conditions (r=0.985**). The modification is very simple and is easily adaptable for estimation of potentially mineralizable N in soils alongwith organic C determination.

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Sahrawat, K.L. Simple modification of the Walkley-Black method for simultaneous determination of organic carbon and potentially mineralizable nitrogen in tropical rice soils. Plant Soil 69, 73–77 (1982). https://doi.org/10.1007/BF02185705

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Key words

  • Acid dichromate
  • Anaerobic incubation
  • Release of ammonium
  • Total N