The Walkley-Black method has been the preferred method for GLP customers and researchers, but that trend is changing. Many GLP customers are now requesting the Weight Loss on Ignition method or even both methods. Each method for determining soil organic matter has its advantages. On soils very high in organic matter, the Weight Loss on Ignition method is better because incomplete oxidation of organic carbon may occur in the Walkley-Black method. In this method there is no chemical reaction, which could be limiting since we are only measuring weight loss of the sample at high temperatures.
Soil texture influences the rate of organic matter decomposition.
Surface soils with a high clay content generally have a higher organic matter content due to the extra water holding capacity of clay and the resulting increases in vegetation produced. Soils with a high level of organic matter will hold more plant available water than soils with little organic matter. The organic matter acts much like a sponge. Soils with low levels of organic matter may have reduced biological activity.