Lan YeQing;Hu QiongYing;Xue JiaHua;Lan YQ;Hu QY;Xue JH

;Applied Chemistry Department of Nanjing Agricultural University, Nanjing 210095, China.

calcium sodium flocculation cations hydrogen peroxide;red soils;minerals clay minerals;soil colloid chemistry;dispersion ph soil humus humic acids;soil types;soil ph;china

Two typical soils from the south of China were used to investigate the roles of pH and humic acid (HA) on the dispersion of soil clays. Critical flocculation concentration (CFC) of the soil clay suspension was determined using light transmission ata wavelength of 600 nm. The results showed that the effects of pH and HA on dispersion of soil clays were closely related to the major types of minerals in the soil and to the valence of the exchangeable cations. At pH 4, 6, 8 and 10 the CFC for the Na-yellow-brown soil treated with H2O2 increased from 0.32 to 0.56, 6.0 to 14.0, 10.0 to 24.6, and 26.0 to 52.0 mmol/litre NaCl, respectively, when Na-HA was added at rates from 0 to 40 mg/litre. With the same Na-HA addition and three pH (6, 8 and 10) treatments, the CFC for the Na-red soil increased from 0.5 to 20.0, 1.0 to 40.0 and 6.0 to 141.0 mmol/litre NaCl, respectively. Obviously, pH and HA had greater effects on clay dispersion of the red soil (dominated by 1:1 minerals and oxides) than on that of the yellow-brown soil (dominated by 2:1 minerals). However, at pH 6, 8 and 10, and with the addition of Ca-HA from 0 to 40 mg/litre, the CFC of the Ca-yellow-brown soil and Ca-red soil treated with H2O2 increased from 0.55 to 0.81, 0.75 to 1.28, 0.55 to 1.45, and 0.038 to 0.266, 0.25 to 0.62, and 0.7 to 1.6 mmol CaCl2/litre, respectively. Na-soil clays were, therefore, more sensitive to pH and HA than Ca-soil clays.