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Re: SOWACS : Matric Potential & Neutron Probe Inflections

At 09:32 AM 10/10/96 +0200, you wrote:
>
>Irrespective of what the soil water content is, it is traditional
>to use certain matric potential values as threshholds for Field
>Capacity (full point) about -10kPa, wilting point (WP -1500kPa) 
>and then irrigators use 50% between these as a "time-to-refill" point.
>
In the early seventies on my reccommendation we in the Western Cape
discarded the -15000kPa point. The reasons being that I found the past
guesstimated 50%   ( which had worked quite well actually ) to roughly equal
100kPa reading of tensiometers----under our soil conditions.  To the relief
of the lab staff who never had much liking for the high pressure equipment
we changed the plate pressures to 10 and 100kPa to obtain what we called
easily available water.   The chaps in the field then decided to use 50% of
that! basically as the result of thumb suck as they did not consult the
relevant graphs from 10-20-30----90-100kPa analyses which show some rare
results with our soils.
>
I often have wondered when I see this 50% figure quoted, whether
>this refers to 50% by volume, as it is usual - and much easier 
>(and more logical) to work in mm of water applied.
>
> I concur.We give the lab anal results as mm/m or mm/root depth.


>>> As far as economics, tensiometers are cheap compared to the
>>> pressure transducers and a data logger. 
>>The farmers I have contact with will not agree. Lots of
>tensiometers are abandoned in orchards.
>
Tensiometers are just like bicycles; you first fall off when you learn to
ride, but you keep on trying. To be a good farmer you have to be a
professional i.e. trained for the job. How can you shoot down tensios
because some farmers fall off tensios? Shoot the farmers instead please.
Tensiometers became so popular in the Cape that a SA type is being
manufactured in Cape Town.

>I, too would stick to a set irrigation cycle...

I have censored my gripe reply to that.  This is 1996. In what wonderful
soil-plant-weather-environment can one get top economic returns with a set
cycle?  As Trevor Finch said yesterday : they are working blind if they do
not measure their soil moisture.
Jean/
>
>