RE: CULT: Solarization

From: "Chris Hollinshead" <cris@netcom.ca>


Here is an item on Soil Solarization that appeared in the Summer 1996 issue
of the Canadian Iris Society Newsletter.

Christopher Hollinshead
Mississauga, Ontario  Canada  zone6b
AIS(Region 16), CIS, SSI
Director-Canadian Iris Society (CIS)
Newsletter Editor-Canadian Iris Society
E-mail:  cris@netcom.ca <c*@netcom.ca>
Canadian Iris Society (CIS) website:  http://www.netcom.ca/~cris/CIS.html
Siberian-Species Convention 2003 website:
http://www.netcom.ca/~cris/2003/index.html
Iris-talk info: http://www.netcom.ca/~cris/CIS/iristalk.html


Soil Solarization is an effective way to control many soilborne problems.
Covering the soil with clear plastic for 1-2 months can generate high enough
temperatures in the top 6”-12” of soil to kill pest insects, nematodes, weed
seeds, and many disease organisms. This process has proven valuable for home
gardeners, because soilborne pests, particularly nematodes, accumulate in
crops grown in the same place year after year. The beneficial effects seem
to last for several seasons.  Solarizing Soil" on the opposite page shows
how to prepare a bed for solarizing.

Solarize the Soil
In the past,  it has been assumed that if certain disease pathogens, such as
verticillium wilt and fusarium wilt were present in the soil, they would
remain there and you would always have problems with those diseases. Over
the past 10 years,  a technique called solarization has been developed that
may solve many garden disease problems. Solarization was developed in Israel
and has been tested at a number of universities across this country. It is a
process that produces very high levels of  heat and humidity in the soil,
which pasteurizes the soil, destroying  harmful bacteria,  fungi, some
nematodes virtually every type of insect larva, and the stock of weed seeds
near the surface. Solarization has been found to be an effective control
against such pesky disease problems as verticillium wilt in tomatoes,
potatoes, and eggplants.  It knocks out fusarium wilt in tomatoes and
onions. It is effective against rhizoctonia in potatoes and onions, and
eliminates a variety of nematodes that attack potatoes and other crops.

An unexpected and unexplained benefit of solarization is that it also
enhances the soil's ability to grow especially robust and healthy plants.
Greater yields have been seen in beds that have been solarized. Solarization
destroys harmful organisms,  but it seems that certain beneficial organisms
are not harmed.  Jim DeVay, chairman of the plant pathology department at
the University of California at Davis, is quoted as saying: "While many
fungi, bacteria and other pathogens are killed, certain fungi that play an
important role in utilization of plant nutrients and crop development
withstand the heat and survive."

The best time to solarize soil is during  July and August when temperatures
are highest and days are sunny.  The procedure for solarizing all or part of
a bed is fairly straightforward.  Simply follow these steps:

1. Loosen up the top 12 inches or so of soil with a fork or tiller.
2. Water the soil heavily so that it is soaking wet-wetter than if you were
simply watering your plants,  then let the bed sit overnight.
3. The next day, cover the bed, or part of the bed, with 3 to 6-mil clear
plastic film. Don't use black plastic because it will not produce the
greenhouse effect.
4. Seal the plastic film along all edges with soil, and keep the soil
covered for 4 to 6 weeks.

It is likely to rain during the 4 to 6 week solarization period,  leaving
puddles on the plastic. Take a broom and wipe the puddles away, because they
reduce the effect of the sunlight striking the film.

Do not punch holes in the plastic to drain the water,  because that will let
the heat escape.

After the solarization process is finished you can plant a vegetable crop
for late fall harvest or a cover crop. Try not to disturb the soil very much
when you put in the new crop. The weed seeds near the surface have been
killed,  but the seeds  further down could still germinate if brought to the
surface.

Solarization offers a wonderful opportunity to strike back at some of the
soil diseases that have foiled gardeners up until now. Although research has
not definitively determined how long the effects of solarization last,  4 to
5 years is probably a good estimate.

In summary; to solarize soil:  clear plastic is spread over an empty bed the
high heat and humidity produced under the plastic effectively destroy
harmful fungi and bacteria that are in the soil.  Midsummer is the best time
to solarize soil to kill soilborne insects and disease organisms, especially
in the North.  Cultivate and remove crop residues from the soil, rake it
smooth, and water it if it is dry. Dig a trench several inches deep around
the bed, and spread  thin clear plastic film (3-6 mils) over the bed.  Press
the plastic into close contact with the soil, and seal the edges by tilling
the trench with soil.  Leave in place for 1-2 months,  then remove plastic.


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