Relationships between Sulfochromic and Permanganate Oxidizable Carbons in Some Soils of North and North-West of Iran

Original Walkley-Black (OWB) method has been extensively used for measuring Soil Organic Carbon (SOC), mainly because of its convenience. However, the reliability of this method is still under speculation. In recent years, Permanganate Oxidizable Carbon (POXC) has been suggested to be more useful than the total SOC. In the present study, SOC contents of the 20 non-calcareous soil samples (0-20 cm) were determined by the OWB method and its modified versions (WB with external heating and WB using colorimetric determination) to understand their relationships with easy-to-find Soil Organic Matter (SOM) determined by the method of Loss-On-Ignition (LOI) at two temperatures (400 and 550°C). The POXC was also determined in the sampled soils and applicable relationships were specified between these methods. Eight of the 20 soils were selected to determine the accuracy of different WB methods using total organic carbon by CHN elemental analysis. Results showed strong power relationships between LOI and OWB methods. The WB method with external heating exhibited the highest recovery (95.3%) among the tested methods. This could be attributed to the finer soil particles used in the proposed method (less than 0.15 mm) than what has been used in the conventional method (less than 0.5 mm). The POXC method showed a high correlation with OWB method and, on average, accounted for only 4.1% of the Total Organic Carbon (TOC). This would likely reduce the value of POXC as an independent parameter to derive the labile fraction of SOC.