Soil Solarization, How to Kill Pests in the Soil
Soil Solarization-What's it good for?
Sometimes pests populations in the soil explodes and ruins anything you plant in that area.
Have you got an area of your garden in which you wish you could just sterilize the soil and start over? Maybe you have not been able to rotate crops properly and nematodes have built up, or Squash Vine Borers are destroying your pumpkin patch? Maybe the Japanes beetle grubs are munching and destroying anything you plant there.
There are a number of pests that have a stage of life that is spent in the soil, either as a cocoon, or larvae or as a worm or that lives exclusively in the soil. These are difficult to control with organic methods.
Some are quite difficult to control, such as the Wireworms, which are the larvae of click beetles (Family Elateridae). They are hard grubs about an inch long, a beige brown colour. They attack potatoes, carrots and parsnips in my garden. They probably eat other roots too but I don't get to see roots very often.
There might be a particularly difficult weed that has invaded a patch of ground or a mould causing trouble.
Soil Solarization might be a solution, particularly if the area is relatively small.
It is a non pesticide-herbicide method that is useful in organic gardening. No harmful residues are produced. By covering an area with a clear plastic sheet and exposing it to sun, temperatures as high as 140°F can be generated at depth of 5-6 inches. Temperature gradually decreases at greater depth but there can be a significant effect up to 18 inches.
The tarp needs to stay on for several weeks for maximum effect and heat build-up, 4-6 weeks is recommended but even after a week or 2, reduced beneficial effects can be seen.
Soil Solarization is most effective against organisms that are found in the areas of greatest temperature, 5-6 inches deep. The benefits gradually lessen as the temperature goes down at greater depth.
Organisms that are mobile, such as earth-worms move down as the heat increases and are not harmed.
Soil Solarization is useful in killing weed seeds but gradually lessens as the temperature gets lower with depth. Any plant that has deep roots or soil structures such as rhyzomes are not affected as much.
Besides reducing or killing the soil pests and pathogens, it increases the availability of various nutrients including nitrogen. The higher temperatures allow for faster breakdown of organic matter and the resulting increase of available nutrients. Plants that are grown in soil that has been solarized usually grow better and stronger because of reduced disease, improved nutrition, fewer organisms attacking and eating them, and lessened competition from weeds.
What pests can I expect to be killed or slowed down.
Soil Solarization is most effective in controlling annual seeds that are closest to the surface. Perennials are tougher and require the maximum temperature to die. All conditions need to be at their best to kill some plants. High level of sunshine, longer solarization period, enough moisture in soil, well placed plastic tarp that does not allow heat to escape.
Bacteria and Fungus
Solarizing your soil can significantly reduce some soil-borne pathogens. Damping off, fusarium wilt, tomato canker, crown gall, vermicillium wilt are good candidates.
Airborne bacteria and fungus will not be affected.
Nematodes are sensitive to solarization of the soil and can be very effective particularly in a raised bed or in containers. Because they are relatively mobile creatures they can re colonize the area. Kill off is most effective in the top 10 or so inches. Below this they can survive and attack roots that go deeper.
Japanese Beetles, Squash Vine Borer and other Insects
In order to be effective for insect pests, you have to time your soil solarization properly.
Squash Vine Borer cocoons hatch at the end of June / Beginning of July when the mature insects emerges. It is useless to solarize the soil expecting to kill them if you wait till mid July, that's closing the barn door after the horse has escaped. You have to time your attack properly and solarize before the emergence of the bugs.
Controlling Japanese beetles using soil solarization faces the same problem. The grubs are quite deep early in the year, then come up much closer to the surface and eat what they can when the weather improves. When the weather gets colder they burrow deeper and are not so likely to be killed by solarization. They are most easily killed when they are close to the surface in summer.
What about the Good Soil Organisms
Good Guys such as mycorrhizal fungi, and fungus and bacteria that parasitize and attack the bad guys re colonize quite quickly. In fact there is often an increase in population of the good guys after solarization. Many garden store sell mycorrhizal spores or beneficial nematodes and you can speed up re-colonization by inoculating your soil with beneficial organisms.
Earth worms just move down and come back up when conditions are good.
How to Solarize Soil
It is a simple procedure but each step has to be done with some care.
- Prepare your area by turning the soil over and smoothing it as free from bumps as possible. Highest temperature are achieved when the plastic sheet touches the soil with minimum air space under the plastic.
- Water the area thoroughly. Hot moist conditions are the best to kill pathogens and other bad guys.
- Choose a period of high sunshine. Since this is a solar heating project you need the most sun possible.
- Choose clear plastic instead of dark. This allows the most amount of sun through and results in maximum temperature of the soil. If you use a dark tarp the tarp gets very hot but the heat is not transferred to the soil as much as with a clear tarp. Thinner is better than thicker plastic because less solar radiation is blocked. I have tried dollar store painting plastic, very thin and inexpensive.
- Adding a second layer of plastic with an airspace in between the 2 can really insulate the installation and increase the temperature in cooler areas or at times of year when the sun is reduced.
- Make a shallow trench around the area to be solarized and put the edge of the plastic in the trench. Put soil on the edge to seal in the heat. Avoid folds and pull the tarp so there are no folds or loose plastic, contact with the soil improves the temperature. Sealing in the edges also keeps moisture in improving the process.
- Let it bake for 4-8 weeks depending on where you are, how much sun there is and how warm it is. Keep an eye on your bed to make sure the plastic does not get torn or pulled up by hopeful animals, or moved by the wind.
- At the end of the Solarization period, remove the plastic and plant whatever you are intending for this spot. Avoid bringing up soil from deeper. It might have weed seeds or pathogens that have survived. If possible recycle the used plastic sheets.
I try to be accurate and check my information, but mistakes happen.email me if you find mistakes, I'll fix them and we'll all benefit: Christine