Measuring Soil Water Potential (soil suction) In the 0 to -20 KPa Range with a Solid Matrix Equilibration Sensor.

Water movement in soil occurs predominately in the water potential range between 0 and -20 kPa.  Although this region is accurately measured by tensiometer in the field, there are often times where their drawbacks limit their use.  Ideally, an instrument could be constructed that measured effectively across the wide range of possible in situ water potentials.  In reality, this has proven to be a difficult task to date.  The goal of this research was to investigate methods to construct a sensor that measured water potential accurately from 0 to 20 kPa that could be monitored continuously in the field.  Using various porous matrices, we modified a currently available water potential sensor to optimize its performance between 0 and -20 kPa.  Each variation was tested in specially designed vacuum chamber that could quickly and accurately come to equilibrium with the test material.  An onboard microprocessor allowed the individual sensor calibration to be stored internally to avoid sensor to sensor variability.  Data indicate that selecting a relatively narrow pore size distribution in the porous medium would allow good resolution in the 0 to -20 kPa range, but a very limited resolution outside it.  Still, combined with other sensors, this new approach gave important information about the water status in the soil.