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I would agree with Bruce that using pure water and pure air as two end points might not give correct calibration slopes (ie changes in soil moisture) (Although I have now lost track of who is replying to who, and is this Sowacs or Trickle, ...) For example, a neutron probe is routinely 'calibrated/checked' in a drum of water. (Its water drum or standard count should not change.) A typical calibration equation is... vsw(%) = 0.7 * (SoilCount/WaterCount) The 0.7 should be 1.0 if its response was linear between pure water and pure air. The 0.7 is because the response of a neutron probe is approximately linear in the soil moisture range of practical interest, but is then non-linear as the moisture approaches 100% pure water. In summary, you cannot just take pure water and pure air readings and assume a linear soil-moisture equation between these two points. However, the water reading could be used to check the sensor and derive a sensor specific calibration equation (As is done with the Environscan, for example). (Obviously sensors other than the neutron probe could have a different calibration curve.) Specifically on thermal probes, my very limited experience is that they showed some hysteresis - different readings depending on whether they were getting wetter or drier. Has anybody else checked this ? On the question of changing calibrations because of... >* Change in soil compaction. >* The movement of crop roots around the probe. >* Slight ground disturbances (surface tilling). >* Soil organisms - worms/ants etc. making a home near the device. >* Soil cracking. it still seems to me that the best check/calibration is to integrate the point soil moisture readings to find the total water content in the profile, and compare changes in this with the volume of water applied. This is a giant, in-situ, calibration. >We experienced much heartache to go to all the trouble to >calibrate the probe to the soil then install it and to then have >natural soil conditions void the calibration a week later. I agree - the real problems of spatial variation of soil moisture, wetting patterns from drippers, extraction patterns of roots, etc make soil specific laboratory calibrations ... (I can't think of a polite word) ... of interest. ---- Trevor Finch Research Services New England 8/16 Nicholson St, Balmain NSW 2041 Australia email: rsne@mpx.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323 ----