Accounting for C in soil will require a degree of precision sufficient to permit an assessment of any trend through time. Soil can contain many chemically diverse forms of organic and inorganic carbon. We measured the C content of 26 substrates by three methods commonly used for soil C (Walkley-Black 1934; Heanes 1984 and Leco Corp.1995). The Heanes and Leco methods were essentially equivalent in their capture of organic C but the Leco method captured almost all the inorganic C (carbonates, graphite). The Heanes and Walkley-Black methods did not measure carbonates but did measure 92% and 9% respectively of the C in graphite. All three of the common soil test procedures measured some proportion of the charcoal and of the other burnt materials. The proportion of common organic substrates (not the carbonates, graphite or soil) that was C by weight ranged from about 10 to 90% based on the Heanes and Leco data. The proportion of the organic fraction of those same substrates, as measured by LOI, that was C by weight ranged from 42 to 100%. The relationship between Walkley-Black C and total C (by Heanes and Leco) for the 26 substrates showed that Walkley-Black C was a variable proportion of total C. Finally, there are apparent artefacts in the Cr 'acid methods used: dichromate digestion should contain at least 7-10 mg C in the sample or over-recovery of C might be reported. Our observation here that common soil C procedures readily measure C in plant roots and shoots, and in burnt stubble, means that there will probably be intra-annual variation in soil %C because avoidance of these fresh residues is difficult. Such apparent intra-annual variation in soil %C will make the detection of long term trends problematic.