【Objective】In this study on humic acid in soil, the technique of light scattering was adopted to compare effects of three different electrolytes common in soil, i.e. Na⁺, Ca²⁺ and Na⁺-Ca²⁺mixture, on agglomeration kinetics of humic acid colloids relative to concentration of the electrolyte and elucidate effects of interface behaviors of ions different in valence and complementary ions on humic acid dispersion stability and agglomerate structure. 【Method】The technique of dynamic light scattering was used to monitor effective hydrodynamic diameters of humic acid agglomerates, which were found increasing with time in all the three electrolytes, regardless of concentration. Based on the time-dependent changes in agglomerate diameter, total mean agglomeration rate, critical agglomeration concentration and activation energy were calcuated, and agglomeration mechanism analyzed. Fractal dimension of humic acid agglomerates 60 d after agglomaeration was determined. 【Result】Resuls show: (1) agglomeration of humic acid colloids was quite similar in all the electrolytes: effective hydrodynamic diameter of humic acid agglomerates linearly increased with time in electrolytes low in concentrations and followed a power law in those high in concentration. The effects of Na⁺ and Ca²⁺ on agglomeration of humic acid colloids in the experiment were much higher than those of the ions different in valence as is described in the Schulze-Hardy law; (2) in the electrolyte of Na⁺-Ca²⁺ mixture, Ca²⁺ ions dominated agglomeration process, and agglomeration process in the electrolyte of Ca²⁺ differed from that in the electrolyte of Na⁺-Ca²⁺ and the sharper the lower the concentration; (3) in the electrolyte of Na⁺-Ca²⁺ mixture, Na⁺, as complementary ion in the electrolyte of Na⁺-Ca²⁺ mixture, had certain complementary effects on critical coagulation concentration, inter-particle activation energy and agglomerate structure, especially the last; and (4) the humic acid agglomerates in the electrolyte of Na⁺-Ca²⁺ mixture was quite compact in structure, varying between those in the electrolytes of Na⁺ and Ca²⁺. It is, therefore, feasible to control compactness of the agglomerates in structure in the electrolyte by adjusting ion composition of the electrolyte. 【Conclusion】 The agglomeration of humic acid colloids induced by Ca²⁺ depends not only on compression of the surface electric double layer on the counter ions, which reduces electrostatic repulsion and causes agglomeration, but also on polarization of Ca²⁺ in the strong electric field that induces and contributes to formation of covalent and bridge bonds between Ca²⁺ and oxygen-containing functional groups on the surface of humic acid. In addition, the competitive adsorptions of Na⁺ and Ca²⁺ on the surface of humic acid inhibits agglomeration of Ca²⁺, thus forming agglomerate moderate in compactness. All these findings may provide some new ideas and theories for exploring colloidal interface reaction and mechanism for agglomeration of soil colloids.