A comparison of three methods of organic carbon determination in some New Zealand soils

SUMMARY A dry combustion (Dumas) reference method for measuring total organic C ( T ), is compared with two other methods: the wet acidified dichromate oxidation, Walkley‐Black procedure (an estimator of organic C) and loss on ignition ( L ) (an estimator of organic matter) for 40 samples from various depths from five New Zealand mineral soils, spanning a range of textures and cultivation treatments. Regression analyses reveal the following: (1) an excellent linear relation exists for all samples between Walkley‐Black procedure and reference method results, with a mean conversion factor 1.25, which compares with the original value 1.32 recommended by Walkley & Black (1934). (2) L and T are less well‐related by an equation of the form L = a + bT. In contrast to the traditional assumption that b ≃ 1.72 and a ≃ 0, a strong ‘intercept effect’ ( a > 0) is revealed, reflecting removal on ignition of non‐organic soil constituents. An improved regression of the form L = a + bT + cC , where C = percentage of clay, obtained by inclusion of the clay as an independent variable, points to the role of clay in releasing these constituents, chiefly adsorbed water.