Abstract
Tensiometric method using ethylene glycol (EG)-water solution was discussed to enable longterm matric potential measurement of upland soil in National Institute of Agro-Environmental Sciences, Ibaraki, Japan, where water-using tensiometer has sometimes been frozen due to daily minimum air temperature lower than -10°C in winter.
Apparent matric potential measured by EG-water solution was 7.5% larger than true matric potential. Deviation of the apparent matric potential was corrected by specific gravity of the EG-water solution. Decrease in specific gravity of EG-water solution enclosed in tensiometer, resulted from EG diffusion through porous ceramic cup, was predicted from initial specific gravity, diffusion parameter, and duration day. Thus, the decrease in specific gravity is not necessary to be monitored during field measurement. Relationship between freezing temperature and specific gravity of EG-water solution should be measured on stirred (or disturbed) condition, because decline in the freezing temperature with increasing specific gravity is affected by the presence of disturbance. Frozen cases of the EGwater solution during the matric potential measurement in the field had good correspondence with freezing conditions estimated by laboratory experiment. It was suggested, consequently, that long-term matric potential measurement through winter season was possible, provided that the initial specific gravity of EG-water solution and replenishing period of EG into tensiometer were considered, and that specific gravity more than lower limit (critical specific gravity) corresponding to minimum air temperature was maintained.
