Improving clay content measurement in oxidic and volcanic ash soils of Hawaii by increasing dispersant concentration and ultrasonic energy levels

• Ultrasonication tested in clay measurement in hard to disperse tropical soils
• Increasing ultrasonication energy increases clay content in oxidic and andic soils
• Resistance to dispersion controlled by oxide content and surface charge properties
• Pumice and oxide nodules may complicate interpretation of particle size analysis.
• Ultrasonication promising alternative to improve particle size analysis

Quantifying clay content is a fundamental step in predicting and managing soil behaviors such as nutrient and water retention. However, clay measurements are underestimated when using standard methods of dispersion in soils rich in oxides and volcanic ash-derived non-crystalline minerals. Increasing levels of the chemical dispersant and ultrasonic energy are two simple techniques found to increase dispersion and clay measurements in temperate soils, but their effects are less known for oxidic and volcanic ash soils. In this study we investigated the effects of increasing dispersion concentration and ultrasonic energy on clay measurements for a range of oxidic and volcanic ash soils from Hawaii. While doubling and tripling the standard sodium hexametaphosphate concentration of 0.441 g L− 1 did not increase estimates of clay content, increasing levels of ultrasonic energy up to 1600 J mL− 1 significantly increased measured clay content for all oxide and volcanic ash soils. The response to ultrasonication was dependent on soil carbon, oxide content, and surface charge, with more energy needed to disperse soils higher in carbon, oxides, and positive charge. Scanning electron microscopy revealed damage to the sand fraction in some soils when ultrasonicated, but the extent of this damage was viewed as negligible. Porous sand-sized particles resembling pumice grains were also observed in some soils, suggesting that conventional particle size analysis and clay interpretations may not adequately describe surface related behaviors.