Soil Organic Matter (A choice of methods)

ompic Soil organic matter influences many of the physical, chemical and biological properties of soil. Some of the properties influenced by organic matter include soil structure, water holding capacity, nutrient contributions, biological activity, water and air infiltration rate and pesticide activity. Due to the importance of organic matter in the soil, the number of requests from GLP customers to test the organic matter level has increased markedly in the past 5 years.

Two common methods for analysis of soil organic matter are the Walkley-Black acid digestion method and the weight loss on ignition method. AGVISE Laboratories offers both of these methods of soil organic matter analysis to GLP soil characterization customers. The Walkley-Black method, used since the 1930’s, uses chromic acid to measure the oxidizable organic carbon in a soil. Concern for the disposal of the chromium and the hazard of using this very strong acid by laboratory technicians, prompted the development of the Weight Loss on Ignition method in the early 1980’s. The Weight Loss on Ignition method is based upon measuring the weight loss from a dry soil sample when exposed to high temperatures (360oC). The weight loss that occurs at this temperature is then correlated to oxidizable organic carbon.

Both of these methods are approved and accepted by organizations such as the North Central States Soil Testing Committee. The Weight Loss on Ignition method is the most commonly used method of analysis by independent crop consultants and fertilizer and chemical retailers on routine soil samples. 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. The Walkley-Black method is more accurate and more precise on soils with less than 2.0% organic matter. On soils very high in organic matter, the Walkley-Black method may result in low test results, due to the incomplete oxidation of the organic carbon in the sample. The Loss on Ignition method is better suited to soils with greater than 6.0% organic matter. 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: Soil Texture influences the rate of organic matter decomposition. Soils with a high clay content generally have a higher organic matter content, due to slower decomposition of organic matter.

SOIL BULK DENSITY: Soil bulk density is correlated with soil organic matter levels. Soils with a high bulk density are likely to have low organic matter levels and are more prone to nutrient leaching.

WATER HOLDING CAPACITY: Soil water holding capacity is influenced by the organic matter level in the soil. Soils with a high level of organic matter will hold more plant available water than lower organic matter soils.

SOIL pH: Soil pH influences organic matter decomposition. Microbial activity at very low or very high soil pH will influence the rate of organic matter decomposition.

CATION EXCHANGE CAPACITY (CEC): The cation exchange capacity of a soil is greatly influenced by the organic matter level. A high organic matter soil will have a much higher cation exchange capacity than a low organic matter soil.

MICROBIAL BIOMASS: High levels of biological activity in a soil require a significant amount of organic matter. Soils with low levels of organic matter will have reduces biological activity.