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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 Gerben.Bakker@Users.WHH.WAU.NL writes: ===8<============== Len, So here we see the disadvantage of a non-eye communication. I did not intend to offend you Len or mislead anyone else. I didn't expect such hostile reactions from other anonymous persons. My goal was to DISCUSS my opinion that it is not as self-evident as it may seem that the measured value is the real value. Critical sounds are always welcome in my point of view. I was certainly too short when I said that polyacrylamide could move 'freely' (I did put it between quotes though) through the soilwater. You are right when you say that the polymer is crosslinked and that this will keep the polymer together. However we did experimental research on various aqueous polymers (not only cross-linked ones) in relation to their retention and temperature dependency in tensiometercups. Simply said the polymers were put in a tensiometercup and the cup in its turn in demineralized water (maybe to short again). All the polymers including the crosslinked ones gave a decrease in their osmotic potential in time. The cause was not exactly determined but we assumed that one of the reasons could be that it is due to loss of smaller chainparts through the pores of the cup. This is where I also refer to articles of for example Peck & Rabbidge and Bocking and Fredlund. Imperfect linking in crosslinked polymers (whatever the cause) could in our opinion also cause separation of chains and thus pressuredrops. Please tell me if we are wrong. The noticed pressure loss was about 0,1 m H2O per month. We also measured temperature dependencies in all polymers varying around 1 m H2O per degree Celcius at a high polymer concentration. If you know an aqueous polymer which does not have these properties as you say, I would be very interested. Gerben Bakker - - - - - - - - - - - - - - Original Message - - - - - - - - - - - - - - Leonard Ornstein has offered some explaination on the properties of polyacrylamide: ===8<============== To Gerben: Dry polyacrylamide will draw water from the soil, but water-saturated poylacrylamide acts as a water reservoir for the plants, since it gives up most of its water in the interval of 0 to -0.2 bars matric potential. Unlike salt, commercial polyacrylamide is a cross-linked polymer which CAN NOT enter the pores of the tensiometer cup, so it can not contribute to the osmotic potential within the tensiometer. Also, the temperature dependence of the osmotic effect of polyacrylamide is EXTREMELY small. To Bruce: Between 0 and about -1.0 bars, the dielectric constant of a water-swollen polyacrylamide gel is very close to that of pure water. To ALL: I am NOT a seller of polyacrylamide soil ammendments. I do use it as the moisture-sensitive element in my self-regulating irrigation valve, the Irristat. I am also the co-inventor (1959) of its use as the separating medium for PAGE, polyacrylamide gel electrophoresis. So I know something about it. Len Ornstein > >Gerben.Bakker@Users.WHH.WAU.NL writes: > >===8<============== >Polyacrylamides provide retention because of their osmotic potential. When the >granules are mixed with the soil the suction of the soil will increase in a >same manner as it will when salt is added, dependent on its concentration. >This means that plants have more difficulties in getting the water out of the >soilmixture. As long as the total potential of the soilwater is less than a >plant can overcome (wiltingpoint of that plant) the plant will survive. So >when measuring the waterpotential of the soil with a normal tensiometer the >expectation is that you measure a higher suction than without the granules. >Polyacrilamides however are actually long chain molecules that can move >"freely" through the soilmoisture. A normal tensiometer has pores in its >ceramic that allow polyacrylamide chains to enter the tensiometer filling. The >result is a decrease of the measured potential. Thus the suction the plant >experiences is higher than the measured one. An other problem of polymers used >as retentionmedia is that their osmotic potential is not negligible dependent >on the temprature. > >Gerben Bakker > >- - - - - - - - - - - - - - Original Message - - - - - - - - - - - - - - > >From: Leonard Ornstein <lenornst@pipeline.com> >===8<==============Original message text=============== >The only effect of the polyacrylamide granules is to provide more >moisture retention per unit volume of soil than in their absence. Any >tensiometer will return the true matric potential in the soil, >independent of the presence or absence of the polyacrylamide. > >For well-aerated, open-textured potting medium, a matric potential of >about -0.1 atmospheres (bars) will encourage rapid germination at >temperatures above 25 degrees Celsius. Such conditions also encourage >mold growth, so an application of an appropriate fungicide is >recommended. > >I don't agree with Bruce; A matric potential readout, which measures >water availability to the plant, rather than a volumetric readout, is >to be preferred. > >Len Ornstein > >[Thanks for correcting me Len. >I agree that matric potential will be preferable, especially where >there are water retention granules that will increase the volumetric >water content but not the matric potetential. >This raises the interesting question of the real purpose of retention media. >My understanding is that the main benefit seems to be that more water can be >held in a given volume of soil, (than would otherwise be possible), which >will become available as the soil dries. >This means that the intervals between irrigations is increased. > >My point about the presence of polyacrylamide is whether this could >bias any of the sensors that operate by use the measuring the >soil's dielectric characteristics. >Bruce] > >At 8:11 AM +0000 2/18/02, 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] >===8<===========End of original message text=========== >- - - - - - - - - - - - End of Original Message - - - - - - - - - - - - >===8<=========== ===8<=========== - - - - - - - - - - - - End of Original Message - - - - - - - - - - - - ===8<===========