SOIL MOISTURE MEASUREMENT BY SOIL IMPEDANCE MEASURING METER : LABORATORY CALIBRATION AND FIELD EVALUATION

Abstract

Soil impedance measuring meter  is relatively  a  new  device  and  utilises the  principle  of measuring  the dielectric constant of the soil, and hence water content by soil electrical impedance method at 100 MHz. The probe is supplied with a general calibration by the manufacturer and very little has been published on the materials-specific calibration.  Hence, the output  (mV) of the probe was calibrated versus volumetric water content  (8) of silty clay  loam  soils, loamy  very  fine sand  soil  and chalky material. For 8v <0.6 m 3m-3, a third order polynomial relationship between 8v and the probe 's output was found suitable (r2=0.99) for calibration . A good correlation was  found  between  moisture  content  measured  by  this new  probe and  that  with  neutron  probe  under  field  condition, for  silty clay loam  soil (r=0.96) and  for chalky  material  (r=0.97).

The probe is comparatively small and easy to install  but  its effective  sampling volume  is limited  (42.4 cm2).  Therefore, it is most suitable for homogeneous soils and pot experiments. The sensitivity of the probe is greatly influenced by conditions close to the central rod and a 0.5 mm annular gap between this rod and the surrounding material resulted in 42% reduction in output.