Soil Solarization
By David A. Bainbridge
Associate Professor
United States International College of Business
Alliant International University
San Diego, CA 92131
Solar energy can be used to kill weeds, weed seeds, and pathogens in
garden and restoration sites. Solarization is often as effective as
herbicides, fumigants, and other hazardous and expensive pest control
methods. Soil solarization is ideal for a wide range of applications in
locations with high summer air temperatures (greater than 85°F during the
mid afternoon is preferred but not essential) and plenty of sun.
A single layer of clear plastic laid on the soil surface can greatly
increase solar soil heating and provide good control of weeds and
pathogens. Recent studies at San Diego State University and Tuskegee
University have demonstrated the increased temperatures can be reached
with a second layer of plastic (bubble pack worked well).
The influence of these high temperatures on weeds, seeds, and pathogens is
complex and not fully understood. It often provides an added growth
response on top of weed and disease control. Deep rooted, heat tolerant
weeds with rhizomes are usually suppressed but may not be killed by a
single treatment. There may also be some surprises. One group we know
found many old clover seeds germinated after solarization. Control can be
improved by repeating the solarization process after retilling the soil.
Adding compost and other soil amendments may improve control of the more
resistant weedy species by increasing microbiological activity in the
soil.
Solarization also reduces or eliminates many common plant pathogens such
as Fusarium, Verticillium, and root rots. This effect has
led to most of the research on soil solarization. Control of some insect
pests has also been noted.
Soil solarization is well suited for preparing seed beds in open areas. In
hotter areas it may be effective in open woodlands and between shrubs with
moderate canopy cover. Solarization is also excellent for preparing soil
mixes and seed beds for nurseries. By placing soil mixes on an insulated
surface it is possible to generate much higher temperatures. Soil should
be solarized during the hottest part of the year and treatment should
continue for as long as possible (3-8 weeks). Soil temperatures over
100°F for several weeks are desirable. Solarization may be effective in
cooler coastal areas with intermittent cloud cover if the plastic film is
left in place for a longer time or if a double layer is used.
Soil thermometers are inexpensive and two or three at different depths
will provide a good indication of how well the soil is heating. If soil
temperatures are not reaching 95-105°F a small test plot (20 sq ft.) of
doubled plastic can be monitored to determine the potential soil
temperature rise. Small bags of dirt or rounded rocks used to hold the
first sheet down (see below) can support a second sheet 1-2 inches above
the bottom sheet. Bubble pack plastic also works well and is more
resistant to tearing.
Begin by cultivating the area thoroughly, then level the surface and
minimize clods, stubble, sticks, and stones that can tear the polyethylene
plastic sheeting. For dry soils apply 1 inch of irrigation water just
before laying the polyethylene sheeting to improve heat capacity and heat
transfer. Fertilizer and soil amendments may be applied before the plastic
is laid. Some soil amendments have improved the weed and pathogen killing
effect of solarization.
Apply sheets of clear (not black or colored) 2 mil polyethylene (4-6 mil
in windy areas) when it is least windy. The plastic doesn't have to lay
flat on the ground for good heating, but it may flap less if it is in
contact with the surface. Thinner plastic lets more solar energy through,
but is more fragile. Use wide sheets to minimize joints and place the
edges of adjacent polyethylene sheets in furrows and cover them with soil.
Bury the free edges, and compact the soil around the sheets to reduce the
loss of heated air and moisture. Place weights 2-3 m apart on the sheeting
to prevent the plastic from flapping and tearing in the wind. Rounded
river stones or small soil or sand filled plastic bags (fist-sized) can be
gently placed on the plastic. The heating is less intense at the edges so
overlap the planting area by a meter or two if you can. When planning the
layout leave sufficient space for access to other areas and drainage,
either to a drain channel or to other plantings. Solarization may provide
double benefits when it is used to concentrate the runoff from the plastic
for crops or trees.
Inspect the site regularly. If holes develop in the polyethylene sheeting
they should be patched with wide clear plastic tape. You can find holes by
looking for areas without condensed moisture on the bottom of the plastic.
Avoid walking on the plastic unless repairs are needed and then use socks
or bare feet.
Although farmers in the developed countries often plant into holes punched
in the sheet, stronger plastic can be removed and reused for several
seasons. If the soil is too wet when the plastic is removed let it dry to
a workable texture before planting. If you cultivate after treatment, keep
cultivation shallow (preferably less than 5 cm) to avoid moving viable
weed seeds from the deep soil to the surface.
Soil solarization is not perfect. It does not work against all weeds and
pathogens, requires the use of chemicals and energy to make the
polyethylene, and leaves a plastic waste for disposal. But it is much
cleaner and safer than hazardous herbicides and fungicides and often as
effective.
Further Reading:
Bainbridge, D.A. 1990. Soil solarization for restorationists.
Restoration and
Management Notes 8(2):96-97.
Katan, J., Grinstein, A., Greenberger, A., Yarden, O., and J.E. DeVay.
1987. The
first decade (1976-1986) of soil solarization
(solar heating): a chronological
bibliography (173 citations). Phytoparisitica.
15(3):229-255.
Pullman, G.S., DeVay, J.E., Elmore, C.L. and W.H. Hart. 1984. Soil
Solarization: A
Non-chemical Method for Controlling Diseases and
Pests. UC Cooperative
Extension, Berkeley, CA 8 p.
Stapleton, J.I., Quick, J., and DeVay, I.E. 1985. Soil solarization:
Effects on soil
properties, crop fertilization and plant growth.
Soil Biology and Biochemistry.
17:369-373.
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