Re: Tensiometers: data variability vs TDR

[Len Ornstein]

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With regard to tensiometers vs TDR/capacitance measurements :

In a soil environment, if SUFFICIENT time is provided to reach 
equilibrium with the water vapor in the gas phase, any reading (below 
the tensiometer's limit of around 0.9 bars) will be accurate, even if 
there is absolutely NO contact between the ceramic cup and the soil. 
The problem is that while with capillary continuity, equilibrium can 
be reached fairly quickly because capillary flow is reasonably rapid, 
diffusion of water vapor in the gas phase is slower and the low 
concentration of water in the gas phase, even at saturation (0 bars) 
is very low, and the rate of equilibration is controlled by both the 
diffusion constant and the water vapor concentration. Equilibration 
in gas phase, only, can take a few hours.

TDR detects the presence of water between the probes because of its 
large dielectric constant (compared to both dry soil and air). But 
the response (the apparent length) depends critically on the 
geometry; GROSSLY different distributions between the probes of the 
same amounts of 3 materials (air, soil and water) of different 
dielectric constants will rarely give the same reading.

Also, remember that the tensiometer  "sees" exactly what the roots 
see, and what's important to the plant; if the matric potential is 
high (near 0 bars) the water is readily available to the plant. 
However, relatively high water content (as might be seen, for 
example, by TDR in a dense clay) may
be at a matric potential of - 3 bars, which makes the crop work quite 
hard to extract it!

Len Ornstein



>
>Arie Nadler seemed to take exception to my comments suggesting that a TDR may
>not be suitable for use in a peat soil.  While his emotional support for the
>TDR method is admirable,

snip

>
>3. I have not actually tried this, but I believe the tensiometer would indeed
>work, so long as the ceramic was in contact with enough ' high moisture
>content' zones so that it could equilibrate.   Generally the tensiometer cup
>would be large in relation to the size of the micro variability to which I am
>referring.   If the 'clay lumps' in the experiment suggested above were of the
>same size range as the tensiometer cup then you are correct, the tensiometer
>would not work properly.

snip

>  > 3. Aren't tensiometers more prone to 'water film continuity' ?
>>