Introduction
- Soil
solarization is a practice used to manage weeds, nematodes, diseases, and
insects in soil. The soil surface is covered with clear plastic, which allows
sunlight to pass through and heat up the soil to temperatures that are lethal
to many of these pests. If effective, solarization can reduce population levels
of these pests for 3-4 months, sometimes longer.
Installation
- The
area to be solarized must first be cleared of existing weeds and debris.
Tilling the site is helpful to increase penetration of heat into the top 6
inches of soil. Sticks, old roots, and other debris should be removed so they
do not poke holes in the plastic.
Water helps to conduct heat, so best results occur
if soil is moist but not waterlogged or muddy. If the soil is very dry and
dusty, the solarization will not work as well. On sandy soils in Florida, the
best conditions are when the soil received rain or irrigation the day before
plastic is applied. If rain or irrigation occur just a short time before
applying plastic, the soil can be heavy, muddy, or otherwise difficult to work
with, and the clear plastic can get dirty.
At present, there are no recommendations about type
or brand of clear plastic to use. Do not use black plastic or opaque plastic,
which are readily available at the box hardware stores, because the soil will
not reach as high a temperature as under clear plastic. Some people think
thinner plastic is better, but maybe the main consideration is that the plastic
should be strong enough to last for 6 weeks in the summer sun in Florida
without breaking up.
The plastic should be stretched tight and the edges
sealed completely by burying in soil. If edges are not completely sealed, heat
will leak out and problems may result in these cooler areas. Some people
recommend using two layers of clear plastic separated by spacers such as PVC
pipe. This will create a buffer zone between the outer air and soil which keep
soil temperatures higher, longer than a single sheet.
If the edges of the plastic come out of the ground,
heat will leak out. It is important to re-bury or re-seal any exposed edges as
soon as possible. Removal of a soil seal or damage to plastic will not ruin the
solarization if it is caught quickly and repaired soon after it happens,
preferably on the same day that the damage occurs.
The plastic should be left in place with all edges
buried for at least 6 weeks. After that, the plastic can be removed, and if the
procedure was successful, weeds and soil pests should be reduced for 3-4 months.
Do not plant anything until the plastic is removed because the heat under clear
plastic will kill seeds and plants! Disposal of used plastic can be a problem,
especially if the plastic is not strong and breaks apart before or during
removal.
Other
considerations - A few small holes may not have much impact
on solarization. However, duct tape can be used to seal small holes (< 1″
diameter). A small patch of duct tape will not affect heat conduction. Animals
walking on plastic or birds pecking at the plastic can cause small holes that
also can be sealed with duct tape. If the damage is extensive, the plastic may
need to be replaced.
Suppose you want to cover a 10-ft-wide area and have
strips of plastic only 5-6 ft wide. One possibility might be to lay the two
strips side by side, overlapping in the center. However, overlap of plastic not
sealed into the ground would allow a weak spot where heat could leak out. It is
unknown if the two plastic strips could be sealed together successfully with
tape, especially over a 6-week time period. A more reliable approach may be to
seal both strips into the soil in order to have two solarized strips (each 4-5
ft wide) next to each other. But this would leave an untreated strip of soil
between the two solarized strips where the edges of the plastic are buried
Raised beds are more work to prepare, but can be
solarized. If water pools in low spots on the plastic, it should not be a
problem as long as the water is clear, since light will pass through. But if
there is dirt in the puddle it will block light, so dirt or soil in the puddle
should be washed away or removed, especially if the area is large.
The edges of the bed may not receive direct sunlight
if beds are positioned in an east-west direction. If the bed is positioned in a
north-south direction, the east edge of the bed will receive direct sunlight in
the morning and the west side will receive direct intense sunlight in the
afternoon. On a bed that is positioned east-west, the top of the bed will
receive direct sunlight through much of the day, but the edges may not receive
much direct sunlight. As a result, the edges of the beds are cooler, and weeds
may grow on the edges, particularly on the north side of the bed.
Solarization can greatly increase temperatures in
the upper 4-6″ of soil. Deeper soil usually does not heat up enough to kill
pests that are located there. So pests such as nematodes that are living deep
in the soil will survive and eventually can move up into the solarized area.
This re-invasion of the solarized soil usually takes about 3-4 months, so after
that time the effects of solarization diminish. Solarization is best used for
short-season crops. Pests may be reduced in a vegetable or ornamental crop
planted right after solarization, but if another crop is planted immediately
after that (double-cropping), the recolonizing pests and weeds may damage the
second crop.
Do not bury drip irrigation tape or other irrigation
plastic near the soil surface in a site to be solarized. The soil temperatures
in excess of 120°F near the surface may melt some kinds of drip tape.
Temperatures cool as we move deeper into the soil, so lines buried 6″ deep
probably won’t be affected.
The temperatures that are lethal to pest organisms
will kill beneficial organisms as well. Fortunately, beneficial bacteria and
fungi seem to recolonize solarized soil quickly, so they can still break down
organic materials and recycle nutrients if organic fertilizers are used.
