Optimum seedling moisture content..

[Leonard Ornstein]

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Leonard Ornstein writes:

===8<==============
Gerben:

I was not offended; but I was concerned that others might be mislead.

Non-cross-linked water soluble polymers WILL move through the pores 
of a typical tensiometer.

Peck and Babbage did use such polymers, and their tensiometers 
ultimately failed because the linear polymers slowly leaked out.

When a cross-linked polymer is synthesized, unless considerable care 
is exercised, the resulting gel can contain a small amount of 
un-cross-linked linear polymer. In polyacrylamide used as a soil 
ammendment, the presence of such "impurity" has no impact on its 
performance, but if it used as the filling for a tensiometer, the 
shift you experienced is exactly what is to be expected as the 
"impurity" slowly leaks out.

The temperature coefficient you measured when using polymer as a 
filling for a tensiometer is undoubtedly real. Truly neutral, pure, 
un-charged polyacrylamide gels have minute temperature coefficients. 
Gels contaminated with appreciable quantities of acrylic acid, show 
changes with temperature, pH and salt concentration. The 
polyacrylamide gels prepared for use as soil amendments are not
prepared with very high-purity (expensive) monomer, and so may show 
some temperature sensitivity. The gels used in my Irristats, do not.

Hope this helps.

Len Ornstein

At 5:59 AM +0000 3/5/02, owner-sowacs@aqua.ccwr.ac.za wrote:

>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
>
===8<===========