Redox potential (Eh) of soil has been measured to know the redox state in paddy soil. However, it was difficult to get reliable data because the Eh values in the field fluctuated largely. Although the measurement method had been examined to improve the problem since the 1950’s, it still remained to be analyzed. To improve the problem, we used the cyclic voltammetry (CV) to examine the reaction of the platinum electrode. The results obtained were as follows. The causes that fluctuated the data consisted of two factors. One was the chemical reactions of platinum
electrode with the surrounding materials. The other was associated with the property of the soil. The platinum electrode induced reaction with oxygen in the oxygenated medium and made oxide layer in the surface of the electrode. This layer disturbed to make the Eh stable. Polishing the sensor could remove the layer. In the reductive state, the sensor adsorbed some materials in the reductive state around its surface. This also decreased the stabilizing time. In addition, the diffusivity of ions in the soil was too small because of the large surface area and electro double-layer of soil solid. These factors decreased the moving speed of electron or ion
transport and non-equilibrium condition was formed in the soil.
In infiltratioin process of water to dry soils, rise of the temperature just before the wetting front arrival is observed. This phenomenon is caused by wetting heat released by adsorption of water vapor onto the soil and or wetting of the soil. The temperature change affects not only soil water potential, infiltration itself due to change in viscosity of the water but also bio-chemical phenomena by changing temperature environment. However, few quantitative experiments and analytic studies have been made on this phenomena of natural soils, many problems remain to be solved on this matter. In this research, to evaluate the soil temperature change quantitatively, infiltration experiment was conducted on three types of natural soils under thermally insulated condition and the temperature change was monitored. In addition, wetting heat of the soils was measured by calorimetric method. The experimental results showed that the temperature increased at just before the wetting front by 2.3°C, 21.6°C and 5.8°C for the Masa soil, Ando soil and the red yellow soil, respectively. We observed strong relationship between the increases in the temperature and the wetting heats of each soil. The wetting heats of each soil showed strong relationship with the cation exchange capacity.
Optimum conditions for predicting unsaturated hydraulic properties using the evaporation method were investigated. Numerical simulations for evaporation from a saturated silt soil column were carried out. Sensitivities and correlations for two selected parameters of the van Genuchten-Mualem model were evaluated with the response surface in terms of the pore-water pressure heads inside the soil column. The pore-connectivity coefficient, I, which describes the tortuosity factor for the unsaturated hydraulic conductivity, had a strong negative correlation with the shape factor, a, and a positive correlation with the saturated hydraulic conductivity, Ks. It is therefore necessary to carefully select initial values for these correlated parameters. Higher potential evaporation rate, longer column, and pressure heads measured closer to the soil surface were more valuable for the parameter estimation based on the response surface analysis. The hydraulic parameters were optimized for a Fujinomori silty soil. The optimized hydraulic parameters were more reliable for longer column and pressure heads measured closer to the soil surface. Furthermore, using pressure heads at two different depths in the objective function resulted in smaller standard deviations for Ks and I than those optimized with single depth data.