Evaluating Changes in the State of Organic Matter Present in Agricultural Soils based on Loss on Ignition Method

Abstract

Loss on ignition (LOI) method is widely regarded as a quick, inexpensive method for evaluating organic matter characteristics in terrestrial soils and littoral sediments. Previous studies examined the LOI method alone to evaluate the state of organic matter, finding that changes in the state of organic matter in littoral sediments could be represented by LOI method-based indices related to combustion temperature: (LOI₃₀₀−LOI₂₀₀)/LOI₆₀₀ and LOI₃₀₀/LOI₆₀₀. However, these new indices have not been applied to agricultural soils. In this study, laboratory experiments were conducted to examine the validity of our proposed indices for evaluating the state of organic matter in agricultural soils. Paddy soil, Andosol, and cow manure compost were allowed to decompose over three months. Measurements of pH, redox potential (ORP), and weight LOI at 200, 300, and 600°C were measured at one and three months after the experiments started. Temporal decreases in ORP were observed, highlighting the decomposition of organic matter present in each material. The (LOI₃₀₀−LOI₂₀₀)/LOI₆₀₀ metric changed over time for all materials, whereas the LOI300/LOI600 metric was almost constant. We concluded that (LOI₃₀₀−LOI₂₀₀)/LOI₆₀₀ can represent changes in the state of organic matter in agricultural soils. Furthermore, we discovered that LOI₂₀₀/LOI₆₀₀ can represent changes in the state of organic matter with reasonable accuracy, rather than the more complicated (LOI₃₀₀−LOI₂₀₀)/LOI₆₀₀. This finding should make LOI measurements easier and more affordable.