Pore water conductivity    (0 - 500 mS.m-1)   In situ measurements  Rapid, direct readout (5s)      A unique new sensor for spot readings of pore water conductivity. For the first time it is possible to sample the conductivity of the pore water, ECp, directly within the soil.     Unique Ease of Measurement

The SigmaProbe is a completely new sensor, which has been developed at the IMAG-DLO Institute in The Netherlands [1]. Until now, pore water conductivity has been measured either by the time-consuming and error-prone method of extracting pore water from the soil by suction, or by saturated paste conductivity measurement. In contrast, the SigmaProbe is simply pushed into the soil (or other growing medium) and then read directly using either a PC or Psion Workabout.

Sensor Design

The measurement is made possible by use of an integrated circuit specially developed at IMAG in order to measure accurately the permittivity and conductivity of the bulk soil. From this the conductivity of the pore water is calculated using a unique mathematical model. The model eliminates the need for soil specific calibration and makes the readings insensitive to the degree of contact between probe and soil.

The probe tip is specially designed both for ease of insertion into the soil and to enable local measurements of permittivity and conductivity.

It is the combination of these features which enables the pore water conductivity to be calculated reliably from the dielectric readings.

The software provided with the SigmaProbe processes the dielectric data from the probe, calculates and displays the pore water conductivity, and saves the readings to a data file. For a more complete explanation of the theory behind the SigmaProbe, please see "SigmaProbe - theory and design" Theoretical basis of the Sigmaprobe pore water conductivity meter

SigmaProbe reading range The following graph gives an indication of the range of pore water conductivity (s p) that can be measured by the SigmaProbe at different soil moisture levels: (Click the graph for a larger version).

 Soil Moisture Mapping

The ideal technique for mapping soil moisture content in precision agriculture would be very rapid, non-invasive, and would also deliver information about the moisture profile. Not even the SigmaProbe can do this! However water content can easily be calculated from bulk conductivity once the pore water conductivity is known. As a result, it is possible to use the SigmaProbe to calibrate the Geonics-EM38 [3] or Veris Soil EC Mapping System [4], so that their conductivity maps can be converted into soil moisture maps.

Details of other applications - in plant nutrient status, fertigation and land reclamation can be found in the Water Relations catalogue.

 References

This sensor system has been developed in co-operation with IMAG-DLO, Wageningen, The Netherlands. http:// www.ussl.ars.usda.gov/saltoler.htm  http:// www.geonics.com  http:// www.veristech.com