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Subject: Re: microprobes -Reply -THETAPROBES AND ROOT ZONE
MEASUREMENTS WITH CS615 SENSORS
(Reply to mail received from: rdbriggs@mailbox.syr.edu)
Russell Briggs said:
>>.. we are measuring soil temperature continuously, and soil
moisture periodically, with the Theta Probe.
So you are using one probe and moving around to the
different treatments in different pots? To my mind, the
thetaprobe is better suited to the situation where a
once-off measurement is required. The disturbance created
by inserting the probe, particularly if one has to auger
first, and you are working in smallish pots
will mean that one can soon not have enough
undamaged soil sample to work with?!?
Would not a permanent sensor buried in the pot at palnting
be a better choice?
>>Our approach is to measure moisture in the rooting zone, then
at the base of the rooting zone with the Theta Probe. In order to
provide access, we plan to use a soil auger to remove a plug of
soil, then insert the Theta probe into the soil.
Do you replace the augered soil afterwards? And do you use
a different position each time?
What about root damage at each auger, and will augering not
encourage greater root growth in that disturbed soil?
>>We have made a device that serves as a template for the three
prongs, just smaller than the prong diameter. Before we did
this, the simple act of pushing the probe into the soil in the
field cause one of the probes to break.
Agreed - the very first time I pushed the thetaprobe into
[hard dry] soil, I bent a pin slightly. (I wonder if this
affects the calibration?)
Not only can the probes bend/break - but the act of pushing
the probe in compresses the soil in the immediate vicinity
of the pins, thereby changing the volume relations of the
soil and water. (Try this in a very loose soil and watch
the value change as the soil is compressed by pushing the
soil from the sides onto the sensor.)
>>I would also appreciate a copy of your internal report
mentioned in your posting (Little and Metelerkamp. ICFR Bulletin
10/96)
Due out soon, will send ASAP.
>>Regarding your second posting, this is an extremely important
issue. The greatest difficulty, in my estimation, is that the
root systems for pines become so complex relative to spatial
extension, that the impact of surface weeds may be impossible to
quantify and eventually may become irrelevent. In the first
year(s) of seedling establishment, I would hypothesize that the
surface weeds are important, but become much less important as
the pine roots explore greater volumes of soil, extending both
laterally and deeper.
Yes, but by looking at whether the surface layers dry out
faster with weeds will give an indication of this. (If
there is no tree growth suppression despite a more rapidly
drying topsoil, then the lower soil water supplies will
have been preoved to be important).
>>We in the Forest Soils division of S-7 are constantly
reminded that these are trees and much of the action occurs below
the top 50 cm. When (at what point in development) this occurs as
seedlings grow is quite interesting and your proposed work
appears to be a good idea.
We, in contrast are continually reminded that "pines have
surface roots" (as compared to the 20m deep euaclypt roots,
anything else is shallow!)
How surface is "surface" remains to be seen!
My proposed modus operandus is as follows:
The water use is to be inferred from the rate of drying of
the topsoil of the various tree seedling:weed types plots.
The onset of competition will be inferrred from the
difference in water use [drying rate] between a seedling
with no weeds (in a weed-free plot), and those in the
various weedy plots.
To determine the latter, the difference between the water
use of the seedling:weed combination and the weed only will
be used as the additional water use of the seedling in the
weedy situation.
Any difference therefore must be due to restictions due to
competition by the weeds (of whatever nature).
To try and explain this in a "word-equation":
competition influence =
seedling-only use MINUS (seedling-in-weed water use)
where the latter is calculated from
(seedling+weed use) MINUS (weed only use).
Soil evaporation in the weed-free (and so bare soil) plot,
would be a necessary (and real cost in water terms) loss.
It also becomes minimal very rapidly with our high
radiation and heat, and low rain situations. Any objections?
>>Question - would the sensors be placed in the approximate
mid-point depth of root extension for the first year? If so,
what about the second year?
We also want to see when the competition starts - so the
weeds are planted over the entire area, and the above
method of monitoring will be used in conjunction with root collar
diameter monitoring.
>>Would it be feasible to install two sensors at each depth-
sensor 1 at the midpoint of predicted first year root depth,
sensor two at the deeper depth representing the midpoint of
cumulative root extension at year two. That would provide a
vertical gradient in year one (roots to no roots) as well as
a lateral gradient.
Would be nice as follow up study, but we want to have a
first stab at assessing which are the real "baddy" weeds as
far as water competition goes. We will then look at them in
more detail.
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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