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Sowacs : Peter Broomhall's "graph-shape" discussion:
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Peter, you plotted soil water content (measured by neutron probe
and FDR) against tension as measured with a tensiometer.
>>your graph again:
S *
o *
i *
l *
*
W *
a *
t * * *
e
r
0*****10*****20*****30*****40*****50*****60*****70
Tension kpa
The curve you obtained was surely the uppermost region of the
classic soil water retention curve?
With decreasing soil water content, the soil water potential (as
measured with the tensiometer) decreases rapidly.
The region at the beginning where the tension stays the same (at
about -5kPa) despite reducing soil water content must be the area
above field capacity - a region that irrigators surely want to
avoid. (Anaerobic conditions caused by too much water reduce root
aeration?)
The shape of ther retention curve is different for different
textured soils. Clays tend to decrease at a slower rate, but for
longer and over a larger range. Sands become dry very rapidly,
and the tension drops with decreasing soil water content, much
sooner than in clays.
To redraw your graph over a larger range of tension to show the
water retentionm curve for sands and clays:
S sc c
o s c
i s c
l s c
s c
W s c
a s c
t s c c c
e s
r s s s
0*****1*****10****100****1500*****
Tension kpa (logarythmic scale)
I am not sure if this is what you getting at with the shape of
the graph, if not please explain further..
>Would not a gypsum block react the same way ?
>When the soil water content declines, and resistance increases,
would not there be some variation depending upon different soil
conductivity levels.
Gypsum blocks also measures [mainly] the matric potential, and
therefore would have a similar pattern to the tensiometer,
showing increasingly less change as the soil dries. The range of
most tensiometers is not much drier than -100kPa, since the
continuous "columns" of water break at this tension, and air
enters the tensiometer causing it to fail. Gypsum blocks, too,
depending on their pore size distribution, cannot measure
accurately much below a certain dryness. (But measure adequately
for the range that irrigators are interested in).
The soil solution's conductivity is supposedly buffered by the
gypsum that the block is made up of. It slowly dissolves into the
soil solution and is a good general salinity buffer.
I have never quite understood how this occurs, and have wondered
whether the presence of gypsum would affect soil pH and therefore
have other implications when it comes to measuring soil water
tension - and associated root activity.
(There is a paper on the effect of gypsum-rich soils affecting
neutron probe measurements). See the list of NP references:
http://www.icfrnet.unp.ac.za/~metele/sowacs/np2.txt
Arslan A, Razzouk AK. 1994. Effects of gypsum on the neutron
probe calibration curve. Soil Science 158:3 174-180
Bruce Metelerkamp
>>>>>>>>.
From: Hortech Services Pty Ltd (hortech)
To: bruce@icfr.unp.ac.za
Date: Tuesday, October 15, 1996 1:12 pm
Subject: Re: SOWACS : Matric Potential & Neutron Probe
Inflections
When I have plotted tensiometer readings versus changes in soil
water content at specific depths I have sometimes found the
following type of graph.
S *
o *
i *
l *
*
W *
a *
t * * *
e
r
0*****10*****20*****30*****40*****50*****60*****70
Tension kpa
In some cases the line has an R2 of 0.8 to 0.95.
Has anybody else seen similar results ?
Would not a gypsum block react the same way ?
When the soil water content declines, and resistance increases,
would not there be some variation depending upon different soil
conductivity levels.
regards
Peter
Peter Broomhall
Horticulural Consultant
Hortech Services Pty Ltd
a.c.n. 060 406 957
P.O. Box 370
Kallangur QLD 4503
Australia
P: +61 418 708 573
F: +61 7 3886 0389
hortech@ats.com.au
From: Hortech Services Pty Ltd (hortech)
To: BRUCE@mailgate.icfrnet.unp.ac.za
Date: Tuesday, October 15, 1996 10:19 pm
Subject: Re: SOWACS : Matric Potential & Neutron P
At 02:01 PM 15/10/96 +200, you wrote:
>Peter
>
>You plotted soil water content against tension as measured with
a
>tensiometer.
>How did you measure the soil water content?
Soil Water content as determined by Neutron Probe and EnviroSCAN.
The comparison was done to see if the tension readings
consistently occcurred at same soil moisture deficit. An
important issue if you are to use the tensiometer as an
irrigation scheduling tool.
Looked at soil water content at the 60 cm depth and plotted
against the 24 inch tensiometer reading. Similar for 30 cm vs 12
inch.
>
Peter
Peter Broomhall
Horticulural Consultant
======
Regards
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905
brooz@pobox.com
Bruce.Metelerkamp@pobox.com
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