Comparison of analytical methods for organic matter in composts and organic mulches

The conventional dichromate (Walkley and Black), carbon analyzer, and weight loss-on-ignition (WLOI) methods are compared for determination of organic matter contents in composts and organic mulches. The objective of this study was to evaluate these three methods for their reliability in determining the organic matter contents of composts and organic mulches that also contain inorganic carbon. The carbon analyzer method overestimated organic matter contents for samples containing inorganic carbon (C) as carbonate or charcoal C. The removal of inorganic C improved the correlation coefficients (r) of results obtained by the carbon analyzer method and the Walkley and Black method (0.95 vs. 0.89). The WLOI method produced results more similar to those obtained with the Walkley and Black method than with a carbon analyzer. Oven drying samples for 16-24 h at 105 degrees C as a basal temperature for WLOI improved results compared with a basal temperature at 70 degrees C, which is commonly used. A heating temperature of 500 degrees C for 12h resulted in organic matter determinations by the WLOI method in the closest agreement with those obtained by the Walkley and Black method.