Developing a Simple Method to Estimate Clay Content for Tropical Soils That Resist Dispersion.

Clay content is an important soil property with significant effects on key soil behaviors including nutrient and water retention, nutrient cycling, and carbon sequestration.  Thus quantifying clay content is a critical step in predicting and managing soil behavior. Measuring clay involves dispersing larger soil aggregates into their individual sand, silt, and clay particles, but many tropical soils characterized by iron/aluminum oxides or amorphous minerals resist dispersion, resulting in the underestimation of clay. The objective of this study was to determine if clay content measurements could be improved for dispersion-resistant soils by increasing either chemical dispersant concentration or ultrasonication energy level. Soils analyzed included the Hali‘i (Anionic Acroperox), Hilo (Acrudoxic Hydrudands), Leilehua (Ustic Kanhaplohumults), and Lualualei (Typic Gypsitorrerts) series. The Hali‘i and Hilo soils possess dispersion-resistant characteristics, while the Leilehua and Lualualei soils possess less dispersion-resistant minerals. The standard pipet method was followed for this experiment using three sodium hexametaphosphate (Na-HMP) concentrations: 0.4408g/L (standard), 0.8816g/L (double), and 1.3224g/L (triple). Four ultrasonication energy levels included 0 (standard shaking), 100, 200, and 400J/mL, and a 1600J/mL level was studied on the Hali‘i and Hilo soils.  Increased dispersant concentrations were found to only increase clay content for the Hilo soil for doubled (21.4%) and tripled (18.5%) concentrations as compared to the standard (13.7%), but these low clay contents still indicated incomplete dispersion. Increased ultrasonication energy levels resulted in significant increased clay contents for the Leilehua, Hali‘i, and Hilo soils, with clay contents peaking for the Leilehua (88%) at 100J/mL, the Hali‘i (54%) at 1600J/mL, and the Hilo (51%) at 1600J/mL.   Only ultrasonication appeared to be a viable alternative to improve clay content quantification in these dispersion-resistant soils.  Future research will investigate the use of VNIR spectroscopy to predict clay content.