Dry oxidation methods 6B2 and 6B3 in Rayment and Lyons (2011) are based on the Dumas method and are preferred as they consistently recover more soil organic carbon than the Walkley and Black method (Chatteijee et al.
Therefore, data from analysis of archived samples from the NSFP, by the Dumas method were compared with data reported by Colwell (1977) based on the Walkley and Black method using precision data from currently operating laboratories; a scenario that will be faced by many users of legacy data and therefore worthy of examination.
In summary, subsamples were sent to each of the participating laboratories in any round, from a finely ground homogenised bulk sample of soil, for soil organic carbon analysis using the Dumas method, without and with pretreatment to remove carbonates as needed, i.e.
Sub-samples from NSFP samples were analysed at DEDJTR Macleod by the Dumas method using a LECO TruMac instrument (LECO Co, St Joseph, USA) without and with pretreatment to remove carbonates, i.e.
Repeated analysis of homogenised soil samples from the laboratory quality control program at DEDJTR Macleod were collated to provide data on the variability of the Walkley and Black method and the Dumas method (Table 2).
For consistency, when comparing methods, the pro-numerals Y or y always represent variables containing means or individual results, respectively, of data from the Walkley and Black method, while X or x are corresponding variables containing data from the Dumas method. When comparing laboratories, data from the first laboratory is always represented by Y or y while that from the second is represented by X or x.
Determinations of soil organic carbon using the Dumas method were in the range of 0.09-9.49% C with a mean of 2.37% C in the ILPP data.
Both statistical approaches highlighted that application of the Walkley and Black method by Laboratory 2 was not recovering as much soil organic carbon as the Dumas method as applied by Laboratory 1.
Re-analysing these samples with the Dumas method showed that the total oxidisable carbon ranged within 0.17-1.83% C for cereals (mean of 0.49% C) and 1.06-9.83% C (mean of 3.63%) for pastures.