CONSTRAINTS OF LOSS ON IGNITION AS A PREDICTOR OF SOIL ORGANIC CARBON IN PIEDMONT SOILS

Soil organic matter is a critical component of soil and is an important control over soil structure, water retention, and nutrients. Loss on ignition (LOI) is commonly used as a predictor of the amount of soil organic matter (SOM) followed by assumption regarding the percentage of SOM that is soil organic carbon (SOC). We compared LOI and SOC to assess the accuracy of LOI as an indicator of SOC. This study analyzed approximately 2000 samples from 152 soil cores collected in 2012 from from Greenbrier Farms in South Carolina. Cores were 60 cm deep and subdivided into 2 cm increments to a depth of 10 cm and then in 5 cm increments to the bottom of the core. The soils were all Ultisols with a thin, poorly developed A-horizon and distinctly reddish brown B horizon. The clay fraction was primarily kaolinite, and gibbsite has also been reported. The red color was likely from the presence of iron oxyhydroxide, most likely ferrihydrite, which increases with depth based on color. LOI was determined by weight difference between pre- and post-combustion after combusting samples at 450^oC for 12 hours. The combustion temperature and time was calibrated to minimize structural water loss from a kaolinite standard. SOC concentrations were measured using a LECO TruMac C/N macrodeterminator. Although LOI was bounded by measured SOC, we found that as depth in the soil increased, SOC decreased, and LOI generally increased but with a wide range of values. Thus, no clear relationship between LOI and SOC existed, particularly at depths below 10 cm. The increased LOI with depth suggests that structural water was lost during combustion despite calibrating the method to kaolinite. We hypothesize that ferrihydrite was a source of structural water during combustion as the literature indicates that iron oxyhydroxide dehydration reactions begin at temperatures as low as 145^oC. The results suggest that LOI should not be used as a proxy for SOC for ferrihydrite-rich soils such as Ultisols like those in the Piedmont of the southeastern United States. Further research is required to determine whether ferrihydrite and/or some other mineral is the source of structural water and whether combustion temperatures can be adjusted to eliminate the loss of structural water.