Dean Lanyon on TRICKLE-L spoke of the wetting and drying cycles
due to irrigation negatively affecting a soil's structure.
We have observed a similar phenomenon in un-irrigated [rain-fed]
plantations of eucalypts here in South Africa. The trees have, at
one site that we are studying, dried the soil to such a degree
that large vertical cracks formed during dry years.
Whether this is a permanent change, and how common or widespread
we do not know. We do not even know if it is damaging to the soil
structure as a whole. It must surely damage roots that had
traversed soil blocks that have subsequently separated.
On a more positive note, these large cracks could have benefitial
results in large rain storms. The relatively bare soil under
plantations is genarally better at infiltrating rain water than
adjacent grasslands. But under large deluges, a lot of water is
cetain to runoff and so not enter the groundwater system. With
the existance of large cracks, the water would flow
preferentially down to below the perhaps saturated top soil, and
so allow a slower and more complete wetting up event, while
reducing runoff considerably. Of course these cracks would close
if the soil was at all a swelling type of clay, and so the drying
of the soil would not be exacerbated by the cracks, at least
initially anyway.
These cracks were only found under trees that had been
established by completely breaking up the grass root matte by
ploughing and discing the soil. Trees established with less
intensive site preparation grew slower, and did not dry the soil
as much (so used less soil water), than the those with only light
preparation. In the poorest case, planting positions prepared
only as a small hand-cultivated pit did not establish or grow
well, but the soil was not dried as completely, and so cracks did
not form.
Dean's post below supports that argument that tilling the soil
provides an environment amenable to root growth. We postulate
that this is the reason that the intensivley prepared tree plots
were able to perform so well, grow so fast both above and below
and thus use the soil water more completely. They were also able
to put more of their growth into above-ground growth rather than
having to allocate carbon and energy below ground to supply the
tree with what it needed.
FIRST COPPICE
Following on from this, the first coppice crop after the trees
were felled, have NOT shown the same dramatic differences in
yield across more and less intensive site preparations
treatments. This has worried us as we would have thought that the
better performing trees would have continued to outperform
neightbours into the coppice....
We have not yet looked at the site to see hwat the status of the
cracks is now, but this may prove illuminating in solving the
root causes of the cracking.
If anyone knows of other experiences along these lines, we would
be pleased to hear about them
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
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
URL http://www.icfrnet.unp.ac.za/~metele
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From: Dean Lanyon (IRRIGATION-L)
To: IRRIGATION-L@LISTSERV.GMD.DE
Date: Monday, July 8, 1996 7:32 pm
Subject: Re: EFFECTS
On the subject of negative effects of irrigation and drainage I
would like to make the point that any irrigation that puts the
soil
through wetting and drying cycles has the ability to alter soil
structure. Whether this is good or bad will depend on the
functional
requirements of the plant roots and the initial status of the
soil
structure. If we take, for example, a soil that has been tilled
to produce a loose
collection of aggregates conducive to rapid root growth, the rate
decline in soil physical fertility from this state will depend,
partly,
on the application rate of water. In general, flood irrigation is
considered
the most destructive while soils that are wetted by capillary
rise
retain the initial soil structure for a greater length of time.
This issue of irrigation methods influencing the rate of decline
of soil
physical fertility has received little attention in the
literature
and if anyone has comments or references in this area I would
appreciate the advice.
Regards,
Dean Lanyon
Dean Lanyon
c/o ISIA, Tatura
ph. +61 58 335 323
fax. +61 58 335 299
mobile 0419 535 852
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