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Further to measuring SWC (Soil Water Content) close to the soil
surface - I would like to share my experiences on this, and
bounce my methodology for a trial I will be measuring ...
I tested the ThetaProbe and the "Speedy Soil Moisture Meter"
against gravimetrically detemined soil water content. (The bulk
density was measured and used to convert to volumetric SWC).
The three methods agreed fairly well on a clay soil with only the
manufacturer's generalised calibration equation.
But of more interest was that even in a very small area (100 mm
diameter), there are considerable differences in SWC as measured
with the ThetaProbe. This I put down to the nature of the sensor
measuring at a much smaller point than is the case where a sample
is taken for gravimetric or Speedy measurement.
The variability in the upper few centimetres of top soils seems
real. In an effort to explain and prove this, when a centimetre
of topsoil was removed, the variability was greatly reduced.
Thus, I hypothesise that it is the influence of the thickness of
the uppermost layer of soil that is almost bone dry that causes
the variability; - Because the sensor is only 60 mm in length,
a dry topsoil of 2 mm compared to 20 mm could reduce the measured
soil water content by a third.
Removing this unevenly deep dry upper topsoil appeared to reduce
the "problem" of variability.
This is currently being published as an internal report but
available on request: Little and Metelerkamp. ICFR Bulletin 10/96
The Speedy meter operates on the pressure produced when the water
in a known mass of soil & water reacts with a known mass of
carbide to produce gas.
********
While on the "air" I am also about to measure the topsoil SWC.
The trial is an attempt to assess whether pine tree seedlings are
suppressed by weeds due to competition for water, nutrients or
light in our South African forestry situations.
The weeds are to be planted by hand! in a regular grid, and
various species and categories assessed. The weeds will be
planted right up to the trees, or kept at a spacing of 1m.
Where should one measure the top soil water content in order to
assess the competition for water?
My idea was to place a sensor almost beneath each tree, and then
another in the middle of the row (as far from the tree as
possible), in the weeds. The extent of the hydraulic gradient
away from the tree position would indicate which was using water
more rapidly. The absolute dryness across different weed types
would indicate which type was a greater water hog.
I am planning to use the CS615 sensors which integrate over
300mm. So I could integrate the top 150mm if I place them at 45
degrees angle.
With a limited number of sensors, we have decided to concentrate
on the topsoil, but the woody weeds could well have their
influence below this level. The pines are slow growers only
closing canopy after several years, and not much is known about
their weed interactions in our conditions.
Placement of these long sensors presents a problem in that they
must be buried under disturbed soil prior to planting (and cannot
simply be pushed in after planting).
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
URL http://www.icfrnet.unp.ac.za/~metele
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From: Michael Savage (savage)
To: SoWaCS@aqua.ccwr.ac.za
Date: Tuesday, December 10, 1996 8:12 am
Subject: microprobes
Sue Philips asked about measuring soil water content within a few
cm of the soil surface. I have used the Theta probe (from Delta-T
Devices) for this. The probe can be inserted near the vertical (-
rod length is 60 mm, diameter is 3 mm and rod spacing is less
than 25 mm). They could be inserted at various angles (-
including the horizontal position) to give other depths. If there
is a temperature response to the analog voltage output, then it
would be most important at the soil surface where temperature
would vary the most. It is therefore also advisable to measure
soil temperature. Usually, we used a 12 V battery to supply
continuous power to the sensor. The sensor can be connected
directly to a cheap voltmeter - for logging, a CR10X or other
datalogger can be used.
The sensor has the same problems as other TDR systems - 1. air
gaps surrounding the probe created on insertion; 2. compression
of the soil on insertion.
These two problems can be minimized by very careful insertion -
for example, coring out soil using a tool with a diameter
slightly less than the rod diameter. This may also minimize the
second problem.
Other sensors (such as the Campbell Scientific 615 (rod length is
300 mm, diameter is 3.2 mm and rod spacing is 32 mm) which has a
much longer rod length) would need to be buried near horizontal
to sense such small depths.
Michael J Savage, Ph.D.
Department of Agronomy University of Natal
Pietermaritzburg 3201 South Africa
Tele: 27 331 2605514 FAX: 27 331 2605426
E-mail: savage@agron.unp.ac.za
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