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The measurement of low organic matter contents in soils

Four methods were compared to measure the small amounts of organic matter present in many Namibian soils and to detect organic amendments added to improve fertility. Two Walkley-Black wet oxidation methods involving titration (WBT-C) and colourimetry (WBC-C) and two loss on ignition methods involving combustion at 360°C after drying at 105°C (LOI-105) or 150°C (LOI-150) were studied. The wet oxidation methods measure readily oxidisable organic carbon while loss on ignition gives an estimate of total organic matter content. Total organic carbon measurements using high temperature combustion and infrared detection of carbon dioxide (LECO-C) were the reference against which other methods were compared. The WBC-C method was considered the best alternative since it was less variable than other methods, correlated closely to LECO-C results and could detect as little as 0.1 mg-C in a sample. It was the most convenient method to analyse large numbers of samples, was economical with technical time and reduced chemical use and production of hazardous wastes to 10% of the WBT-C method. The LOI-150 method gave slightly lower values than the LOI-105 method and was marginally better correlated to LECO-C measurements but both appeared to overestimate organic matter content due to weight loss from the inorganic fraction.