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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 Leonard Ornstein writes: ===8<============== Cliff: A classical porous-cup tensiometer will measure the matric potential accurately because the water in the polyacrylamideis is always (except for a small delay--of the order of minutes) in equilibrium with the water in the soil, both capillary water and any "bound" water, either by way of the capillary film, or by diffusion across the air spaces in the soil. What makes polyacrylamide an attractive soil amendment is that it can hold more than 20 times its dry weight at 0 bars, and releases water continuously from 0 bars matric potential right up to the wilting point, but it releases most of the water between 0 and -1.0 bars. Len Ornstein At 10:13 AM +0000 2/20/02, owner-sowacs@aqua.ccwr.ac.za wrote: > >In relation to measuring water content when much of the water is tied up >in granules. I know conventional TDR will not work - it does not >recognise the water in the granules when the water in the medium as a >whole has drained to the point where there is no continuous film. I >suspect heat dissapation sensors will not work for the same reason. >One brand of capacitive sensor DID work. This is not a trivial problem >- I would suggest using gravimetric techniques. > >A related comment. It is a good idea to check the moisture release >curve of the poly acrilimide. Many of these materials do not release >water until they are at close to wilting point - far to dry to do >vegetables any good. > >Cliff.hignett@soilwater.com.au > >owner-sowacs@aqua.ccwr.ac.za wrote: > >> I would like some information on the ideal moisture for seedling >>germination. >> >> Tomato and peppers. >> >> Also which probe would you recommend. >> >> I am a chemist and am interested in experimenting on optimising >>growing conditions for seedling germination. >> >> I am using poly acrylamide water retention granules at about 1% %v/v >> >> Can you help >> >> Bernard Irving <bernardirving@hotmail.com> >> >> [Bruce comments >> Assuming these are potted plants, the sensor probe would need to be small >> enough to insert into the pot without disturbing the roots too much, >> and provide a volumetric readout. >> Please provide more information on the application. >> I wonder if anyone can comment on the effect of water retention >> granules on measured water content?! >> >> I am sure the various manufacturers will also contact you directly. >> Bruce] >> > >-- >Cliff Hignett >Soil Water Solutions >45a Ormond Avenue >Daw Park >South Australia 5041 >61 (08) 8276 7706 >cliff.hignett@soilwater.com.au >www.soilwater.com.au ===8<===========