Re: Volumetric vs Gravimetric

[Len Ornstein]

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Bill:

Jason made the CORRECT point that there could be a large change in 
water content under conditions where a tensiometer shows a small 
change in water potential. For example, a tensiometer will read very 
near 0 in uncompacted peat at saturation; drain 1/2 the water from it 
and the reading may still be only -0.03 bars. That's a small 
tensiometer sensitivity to a large change in water content.

The point that I was making is that it's only when the change in the 
tensiometer reading gets large, that you have to worry that 
conditions are approaching a point which might stress the crop. That 
is, universally, the tensiometer, INDEPENDENT OF SOIL TEXTURE, tells 
you when you ought to irrigate. Any device which only measures soil 
water content (neutron probe, TDR, etc.), requires additional 
information that "corrects for" the soil texture to translate the 
reading into a useful guide for when to irrigate.

This was an unsolicited endorsement for tensiometers, not a 
tensiometer putdown!

Len Ornstein

>
>   On the other hand, what the crop cares about is water potential, not
>   soil water content, so the low sensitivity of a tensiometer to large
>   water content changes near field capacity poses no threat to the
>   crop. But when the water potential starts to dive, even if the change
>   in water content is small, you better start to irrigate!
>
>   Len Ornstein Ph. D. >>
>
>I am not sure that I understand this issue of the "low sensitivity of a
>tensiometer to large water content changes near field capacity".  I would say
>that it is opposite, with the tensiometer very sensitive.  Am I missing
>something in prior messages?
>Regards, Bill Pogue, Irrometer Company, Inc.