Volcanic ash soils have excellent soil hydrological properties, i.e., high water retentivity and high water permeability compared with other clayey soils. They contain abundant allophane and organic matter. Having well developed aggregated structures, volcanic ash soils have large porosities as much as 80 %, retain water inside soil aggregate, allow the water move easily through large pores between aggregate and macropores, and consequently have high permeabilities. In this study, various soil hydrological properties of a volcanic ash soil in Japan are shown and discussed; volumetric ratio of solid, liquid, and gas phases, particle density, hardness, pH, EC, soil organic matter (ignition loss), soil texture, specific surface area, soil water retention curve and saturated and unsaturated hydraulic conductivity. Finally, the changes of soil water retention curve and unsaturated hydraulic conductivity were substantially related to the dispersion and coagulation behavior of the soil, in response to the electrolyte (NaCl) concentration.
Materials & Methods
The soils were sampled from an upland field of Tanashi experimental farm, owned by the University of Tokyo, at the western part of Tokyo, Japan. This experimental field is used for multiple purposes and different crops are planted every year. Soil pit of 1 m depth was excavated and both disturbed and undisturbed samples were obtained. Disturbed samples were obtained from the depth of every 10 cm, and were used for the measurement of soil water content, particle density, pH, EC, ignition loss, specific surface area and unsaturated hydraulic conductivity. Soil water retention curves and unsaturated hydraulic conductivity of Andisols, Alluvial soil and Toyoura sand were measured. To see the effects of electrolyte concentration on the soil water retention curve, three types of water were used; distilled water, 0.1N NaCl solution, and 0.5N NaCl solution. Soil water retention curves were fitted with Mualem-van Genuchten equations, and the unsaturated hydraulic conductivities of the measured and predicted values were compared.
Result I - Soil hydrological properties
Solid phase was about 30 % at the surface 20 cm layer, A horizon, and less than 20 % below the 50 cm layer, B horizon. The high porosity of 70% at the surface soil and 80% at the subsurface soil results from the highly developed aggregated structure of the soil, and it is one of the extraordinary characteristics of volcanic ash soils. High water retentivity of Andisols ensures that enough water for plant growth is retained in the soil pore, while, at the same time, high water permeability of Andisols ensures that water can drain reasonably fast after rainfall, so that enough air is retained in the soil pore for plant root. The soil texture was classified as light clay, pH was around 6, and ignition loss was 17 to 27 %.
Result II - Dispersion and coagulation behavior
Although soil hydrological properties of Andisols change with changing pH because of the pH-dependent charges of allophane, water retention curves and unsaturated hydraulic conductivities were not affected by the electrolyte (NaCl) concentrations used in the measurement. These soil hydrological peculiarities of Andisols were attributed to the stability of soil structures at neutral pH range, where critical coagulation concentration (CCC) was very low. As for Alluvial soil, when the NaCl concentration became larger than the CCC, coagulation occurs and the unsaturated hydraulic conductivity increases. On the other hand, as for the Andisols, the CCC value is very small for the neutral pH range, and allophonic colloidal particles coagulate, forming a very stable aggregate. A typical CCC curve for Andisils is shown in Figure 1, which elucidate the relation of dispersion and coagulation of the soil to pH. In this figure, the EC of the ambient soil solution is assumed to be 0.35 molc m-3 and the CCC is smaller than it in the neutral pH range of 4.5 to 7.5 and otherwise larger than it. When pH is lower than 4.5 or larger than 7.5, the CCC is larger than the electrolyte concentration of the ambient soil solution, and therefore dispersion of allophanes begins to take place. The stability of volcanic ash soils against ambient environmental changes, in the neutral pH condition, may be contributing more or less to all of the surrounding natural environments.
Figure 1: A typical illustration of the behavior of coagulation and dispersion of Andosols in response to the CCC change with pH.