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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Arie Nadler seemed to take exception to my comments suggesting that a TDR may not be suitable for use in a peat soil. While his emotional support for the TDR method is admirable, My intention was to suggest that while the TDR rarely needs to be calibrated, it is not a good idea to use it without calibration in soils which have a wide range of pore sizes - particularly where there is a discontinuity in the pore size range which may cause most of the soil to drain while leaving small zones still holding substantial quantities of water. Such 'discontinous pore size ranges' usually only happen in man made soils, eg potting mixes and 'reconstructed' soils, but a 'peat' soil is also a likely place for it to happen. I strongly recommend he calibrates against volumetric water contents. To take his points one at a time. 1. When I demonstrated the experiment to collegues with an interest in TDR's they said that the phenomena was 'known' and published - I have never followed it up. 2. I am not sure what you are driving at here, but I guess you are saying that the water is indeed 'registered'. Fair enough, it has detected the water - perhaps I should have said 'the TDR does not correctly register' such isolated water. Try an experiment with sand containing small (1mm) lumps of clay - then you can test it against a reliable measure of volume water content. 3. I have not actually tried this, but I believe the tensiometer would indeed work, so long as the ceramic was in contact with enough ' high moisture content' zones so that it could equilibrate. Generally the tensiometer cup would be large in relation to the size of the micro variability to which I am referring. If the 'clay lumps' in the experiment suggested above were of the same size range as the tensiometer cup then you are correct, the tensiometer would not work properly. 4. ?? Something on which 'sowac' people might like to comment is the use and interpretation of volumetric moisture sensors in swelling soils. My experience with peat soils is limited, but in my experience, most of them shrink as they dry. If so, then ANY volumetric measure of water content would be extremely difficult to interpret (including the type which uses an oven). Does anyone use TDRs or capacitive sensors in swelling soils? Cliff Hignett > Arlie nadler wrote - > " I would NOT recommend TDR in that environment. The TDR only works when > there > is a continuous water film. Isolated water does not register". > > and > "I see no reason why tensiometers would not work ". > > 1. I am interested in being reminded of a recent, reviewed journal > article stating the requirement of continuity to enable water > sensing by a TDR probe. > 2. If you soak a coarse filter paper (a few layers perhaps to > increase the effect) and place it between the probes rods without > actually touching the rods, you'll notice the change in apparent > length. > 3. Aren't tensiometers more prone to 'water film continuity' ? > > 4. I would hate to use the term 'filtering' but what about the > credibility of our little precious website? -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU