Comparison between Walkley- Black and Loss- on- Ignition methods for organic carbon estimation in soil from different locations

The present study aims to find out the best method for soil carbon determination using two methods, the wet acidified dichromate oxidation, Walkley-Black (W.B) and loss on ignition (LOI) methods. This study was employed on 30 soil samples from 0-20cm depth from three Duhok, Irbil and Sulaimaniya of Kurdistan region-Iraq. The following parameters are analyzed: organic C and the particle size distribution of the soil samples. The results of LOI and W.B methods were statistically analyzed by simple linear regression equations to interpret their relationship. The highest values of soil organic carbon (SOC) were found at Loss on ignition method ranged between 4.19 - 29.70 g kg-1 soil, whereas W-B method had the lowest values ranged between 3.0 – 19.0 g kg-1 soil for overall sites. There was a strong relationship between these two methods, so the amount of SOC using both methods had significantly a very strong correlation (R2=0. 93, p 0.001), (R2=0. 87, p 0.0001), (R2=0. 75, p 0.001) in Sulaimaniya, Duhok and Irbil respectively. This study also revealed that silty clay and clay soils gave the highest values of the SOC by the two methods compared to the loamy soil. However, the soils that have more clay and carbonate contents gave higher values of SOC at high temperature using L.O.I technique. The results of this study revealed that both methods were reliable but W.B method was more accurate and suitable for soils with high clay contents and for the calcareous soils. Also, LOI is an accurate, if the fine fraction is present in low percentage.