Ishiguro Munehide
Laboratory of Soil Conservation, Research Faculty of Agriculture, Hokkaido University

Allophanic Andisol, which is a typical volcanic ash soil in Japan, has a significant pH-dependent charge. The predominant clay minerals are allophane and imogolite. The positive charge increases and the negative charge decreases as pH decreases; therefore, anion movement becomes slower and cation movement becomes faster as pH decreases in the soil. Therefore, both cations and anions movements are strongly affected by the soil pH. In natural condition, the soil is well flocculated because the soil particles have both negative and positive charges. However, at higher or lower pH, the soil disperses well because electric repulsive force generates among the soil particles; the negative charge predominates at higher pH and the positive charge predominates at lower pH. The saturated hydraulic conductivity becomes lower at higher pH and lower pH due to the generated electric repulsive force; the soil swells and dispersed soil particles clog the soil pores. However, when dilute H2SO4 is percolated in the soil, the hydraulic conductivity does not decrease, while it decreases when dilute HCl or HNO3 is percolated in the soil. This is because SO42- strongly adsorbs on the soil surface at low pH and the soil remains flocculated, while monovalent anions such as Cl- and NO3- locate in the diffuse layer and the overlapping of diffuse layers causes swelling and dispersion of the soil.
Because a soil has pH dependent charge, the soil structure is affected by pH and the soil structure change causes the hydraulic conductivity change. The electric charge characteristics of soils are very important when we consider solute transport and soil permeability.