Factors Contributing to the Clay Dispersion and Aggregate Stability of Thai Oxisols

Aggregate stability tends to give a better availability of soil water. Clay dispersion makes the soil dense, difficult for root penetration. The better understanding of these two soil properties is necessary to evaluate the suitability of soils for plant production. We collected soil samples from 10 sites from the southeast coast and northeast plateau of Thailand, at upper (0 - 5 cm depth) and lower (5 - 20 cm depth) levels for under udic and ustic Oxisols soil samples. The objective of this study was to investigate the contribution of major aggregate binding agents such as soil organic carbon (SOC), Fe and Al oxides, clay minerals and kaolin crystal size to the stability of soil aggregates. Soils were acidic (pH < 6.5), low to medium cation exchange capacity (3.34 - 14.40 cmol kg -1 ), SOC ranged from 2.08 - 4.56 % for udic and 1.11 - 1.56 % for ustic soils. The udic soils showed higher contents of crystalline, non-crystalline, and organic forms of Fe and Al than those for ustic soils. The mean coherently scatterings domain (CSD) values for kaolin crystal size were 115 and 152 nm for udic and ustic soils, respectively. The mean water dispersible clay (WDC), water dispersible silt (WDSi), and dispersion ratio (DR) were 92, 43 g kg -1 , and 0.17 for udic soils, and 218, 45 g kg -1 , and 0.30 for ustic soils, respectively. Mean weight diameters (MWD) were 1.01 and 0.62 mm for udic and ustic soils, respectively. The clay flocculation index (CFI) and aggregated silt and clay (ASC) were 0.83 and 650 g kg -1 for udic soils and 0.66 and 619 g kg -1 for ustic soils. The SOC, non-crystalline, organic form of Fe and Al oxides, and crystalline Al oxide showed positive correlation to MWD and CFI. A negative correlation between crystal size of kaolin and aggregate stability was found. Graphical abstract