Optimum seedling moisture content..

[Gerben Bakker]

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Gerben.Bakker@Users.WHH.WAU.NL writes:

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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

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From: Leonard Ornstein <lenornst@pipeline.com>
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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]
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