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					Tas-Regions, September 2001   Andrew Bishop

                     Solarisation: Applications under Tasmanian conditions.


Solarisation is a technique of controlling some weeds and diseases in the soil and is often promoted
as an alternative to chemical use. A plastic material (solarising film) is laid on moist soil in an effort
to raise soil temperatures to a level that is lethal to the seeds of some weed species and soil borne
pathogens. The process effectively uses the power of the sun to destroy pathogens instead of
applying chemicals. This sounds an attractive way to improve on pest management in paddocks
before planting crops without the use of chemicals. There is no doubt that, if successfully
implemented, solarisation procedures reduce the need for chemical inputs and control of pests. So is
there a role for solarisation in Tasmania? This article looks at where solarisation is used in the
world, solarisation-operating principles, past studies and findings in Tasmania, and new
developments in cool climate solarisation films. It also looks at other important issues including the
economics of solarising treatments and associated environmental implications using solarising films.

Solarising uses overseas: where and what.

Solarisation is referred to widely in the literature as being capable of controlling plant diseases and
weed pests. Much work has been done in countries such as the USA and Israel investigating the
potential of polyethylene mulches to control plant pests including weeds. However in these places,
soil temperatures under the plastic mulch have been high enough to denature proteins and physically
destroy weed seeds and fruiting bodies of pathogens. Countries such as Greece, Egypt, and Jordan
also successfully employ solarisation strategies. Researchers from Agriculture Western Australia
report that in Greece, solarising films are laid in hothouses to control soil borne pathogens before
tomato crops are planted. In New Zealand, solarising has been used successfully in trials to reduce
the viability of fungal pathogens by up to 89%! Much of the overseas research into solarisation and
its effect has been driven by the search for alternatives for the replacement of the chemical soil
sterilant, methyl bromide.

Solarising principles: factors that contribute to solarising success.

The core requirement of a successful solarising mechanism is the rapid heating of soil to
temperatures that are lethal to the pests you wish to kill. So what factors contribute to the success or
otherwise of solarisation?

Solarising film properties: Generally clear polyethylene is accepted as the standard film for
solarising with black polyethylene reported as less successful although having good effect over
longer periods of time. Past studies in Tasmania have used black polyethylene sheeting (wild poppy
control) and some limited applications have been undertaken in nursery situations (nutgrass control)
using clear polyethylene sheeting. Recent studies in the USA have tested purpose-manufactured
solarising films that have increased infrared absorbing characteristics of the plastic and have been
demonstrated to be successfully applied in areas of lower solar irradiation intensity. Thus
appropriate selection of film is a factor that contributes greatly to the success or otherwise of the
technique

Air temperature: High air temperatures enhance soil-heating effects reducing the loss of heat from
the treated area. Within Tasmania this may be a limiting factor compared to say California in the
USA, or the dry hot regions of Israel for example. In a protected situation, such as in a polytunnel,


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Tas-Regions, September 2001   Andrew Bishop

this may not be a limitation. Time of year of treatment in Tasmania is also a very important factor
in increasing the chances of higher air temperatures.

Wind speed: Wind speed is considered a factor that can be varied. Generally high wind speeds and
low air temperatures combine to decrease temperatures further via a chilling effect. Although little
can be done to change wind speed on a broad scale, application of the treatment in areas that may be
protected from the wind can reduce chilling effects, without loss of heat from the film’s surface.

Solar irradiation intensity: Although not being able to directly alter the intensity of the solar
irradiation reaching the film surface, selection of an appropriate type of film is one way of
maximising and concentrating the level of radiation received. Lower solar irradiation intensities in
Tasmania compared to areas such as Israel and Jordan are probably one of the largest drawbacks for
the successful application of this treatment in Tasmania.

Soil moisture: Applying solarisation films to dry soil will greatly reduce the effectiveness of the
treatment. In many situations in Tasmania there may be sufficient moisture in soil to be effective.
If not irrigation can be considered to raise soil moisture prior to application of the film.

Tasmanian solarising experiences and future applications.

During the late 1990’s, DPIWE conducted a series of studies with respect management of wild
poppy species in commercial poppy crops. One of the treatments tested was solaristion. The film
used in these experiments was black polyethylene sheeting. Temperature logging in treated and
untreated plots indicated solarisation raised soil temperatures by up to six degrees but surface
temperature only reached a disappointing 25 degrees celcius. In these experiments, lethal
temperatures were not obtained. An interesting result was evident though; the increased
temperatures under moist conditions, although not directly destructive of seed structures, appeared
to break the dormancy of many of the weed seeds present causing the seeds to germinate under the
black plastic film. Germinated seedlings either died while under the solarising film or once the film
was removed. The DPIWE weeds section have also used solarisation as a novel approach to
controlling isolated patches of invasive nutgrass in a nursery situation. Application of clear film
over a summer period appeared to reduce the extent of nutgrass occurrence. Based on known
requirements for successful solarising treatments, past experiences in Tasmania, and Tasmanian
climatic and environmental conditions, solarisation as a treatment has less potential for success in
Tasmania than in Israel for example. However in specific situations and by adjusting those factors
that can be varied i.e. soil moisture, film properties, and wind speed it is likely lethal temperatures
could be obtained. The DPIWE Clean Products team is currently investigating the availability of
specialist infrared absorbing film from a Canadian firm to test as part of its ongoing demonstration
of IPM techniques at the Forthside Vegetable Research Station. Significant issues in terms of
application in Tasmania in the future that need to be considered, aside from efficacy, are the
economic and environmental implications of the technology.

Solaristaion- economic and environmental impacts.

Generally when you talk about solarisation being used as an alternative to chemical pest control,
people envisage hectares of land covered in solarising film. Economic barriers are likely to prevent
this in Tasmania in normal cropping regimes. The cost of specialist solarising films is significant
and the more that was used the less economic the application likely outweighing any chemical input
savings. I would therefore suggest that the treatment if applied commercially would be restricted to
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Tas-Regions, September 2001   Andrew Bishop

strategic use on isolated areas of land affected by a particular pest problem, or smaller polyhouse
soil treatments. Following on from the economic questions are the environmental ones. The most
obvious one is what happens to the solarising film once it has been finished with? Sadly, at this
point, there does not appear to be any clear answer. In some situations overseas, the film is actually
ploughed into the ground to supposedly breakdown over time! The films currently developed are
not truly biodegradable and such an approach is far from satisfactory. Alternative approaches are a
re-usable film (implications for labour input removing and reapplying used film) or physical
recycling of the material. The latter would seem the most obvious, but an appropriate infrastructure
would be needed to accept and process the materials with minimal cost (and environmental)
impacts. The raw film material would have a value and this could be passed on to the grower thus
offsetting some of the original investment in the film. Another related environmental question is the
inputs and resources required to manufacture the film in the first instance and offsetting this against
relative environmental implications of chemical inputs. It may be that offsetting the chemical inputs
with the solarising treatment may in fact be more negative environmentally in the bigger picture
than using the chemical treatments in the first instance! Another environmental query is what
happens to the ‘good’ soil flora and fauna if temperatures are sufficiently lethal to destroy weed
seeds and pathogens? This is a valid question and one that must be considered if the biodiversity of
life in soil is to be protected. Solarisation is not necessarily a selective treatment.

Conclusion

Solarisation does work and could be applied in Tasmania in some situations. Most likely
applications would be strategic use on small isolated problem patches affected by susceptible weeds
or pathogens. Polyhouse situations may also be a possibility. The DPIWE Clean Products Team
intends to explore future potential applications of this technology. The author identifies the two
major challenges requiring solutions as the economic and environmental viabilities of the treatment.



For more information contact:

Andrew Bishop
Leader (Clean Products)
6421 7634




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