Comparison between Walkley-Black and Loss on Ignition Methods for Organic Matter Estimation in Different Moroccan Soils

Abstract

The present study aimed to compare two different methods for the determination of soil organic matter (SOM); the wet acidified dichromate oxidation methods “Walkley-Black (WB)” and the combustion’s one (Mass Loss on Ignition (LOI)); in order to find out the best one in terms of suitability, environmentally -friendly, time-efficiency and economical benefit. Forty-five Moroccan soil samples (El Guich, Koudia and Merchouche area) from 0-20 cm depth with different classes, textures and organic matter contents were evaluated. According to the results, a strong linear relationship was observed between LOI-OM and WB-OM methods for all the sites. However, El Guich and Koudia recorded much larger regression coefficients (0.745*** and 0.611*** respectively) compared to Merchouche site (0.452**). Therefore, the SOM by walkley-Black method can be easily calculated from the LOI method at 500 ± 25 ºC for 2 hours using linear equation in order to minimize the analysis time and consumption of chemicals for routine laboratory. In addition, it was noticed that the SOM contents calculated by the LOI method are higher in Merchouche soils compared to that of WB due to the presence of a large amount of clay minerals in the Merchouche soils (43.9 <% clay <56.1) in comparison with those of El Guich (5 <% clay <7.5) and Koudia (16.1 <% clay <20.4). This study revealed that both methods were reliable but the W.B method was more accurate and suitable for calcareous soils and those with high clay contents. However, the LOI method is an accurate and beneficial for non-calcareous soils with low clay content.