Solarizing Unhealthy Garden Soil
On this page, find how to reduce populations of harmful nematodes and other garden soil pests in a vegetable garden’s soil, including:
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- Soil pests and problems affecting home vegetable gardens.
- Harmful soil nematodes and how they cause damage to vegetables.
- Soil pest prevention and the benefits of soil sterilization.
- Why soil sterilization is difficult to accomplish in a home vegetable garden.
- A step-by-step demonstration of how to sterilize a bed by trapping solar heat from the sun.
Harmful Soil Pests
Sometimes garden soils become infested with harmful nematodes, weed seeds, bacteria, and other organisms, so remove them by using a simple solarization technique to rid it of pests, sterilize it, and have healthy vegetable plants.
Nematodes are microscopic works that make their home in soil. There are beneficial and harmful nematodes. Destructive nematodes damage vegetable and other plants by feeding on their root systems, stunting their growth, and spreading infections and viruses.
Both types of nematodes have preferred plants that attract them, called hosts. Some of these hosts are the vegetables we grow, while others are native weeds and grasses.
Soils planted with the same crops season after season tend to build up colonies of harmful nematodes, so rotation is important to preventing garden soils from becoming infested.
Other harmful pests and weed seeds also attack vegetable plants or compete with them for water, nutrients and sunlight. They, too, build up in garden soils over time.
No completely effective and safe method exists for completely eliminating nematodes, viruses, bacteria, fungi, and weed seeds other than soil sterilization.
Fortunately, there’s an easy way to sterilize small plots of soil in home vegetable gardens: Solarization.
When to Solarize Garden Soil
Solarize at Start of Summer
About Sterilizing Soil by Solarizing
Preplanting treatment of infested soils in vegetable beds by solarization—heat sterilization using sunlight—is an effective control of most harmful soil pests, including nematodes and most plant diseases.
Commercial methods of sterilization are not safe or practical in home settings. Solarization does the same work as commercial fumigants, provided that treated soil reaches a high temperature—130°F (55°C)—for enough time—at least four hours—at sufficient depth to kill the pests it contains.
The problem with effective solarization is getting soil hot at depth. Soil, because of the air it contains, is an excellent insulator.
Even desert soils exposed day after day in summer to temperatures of 120°F (49°C) or are just 60°F (16°C) a few feet below the surface. Similarly, soils in arctic and antarctic regions have much warmer soil a short depth below the surface.
Most nematodes, weed seeds, and harmful organisms live in the top 12–14 in. (30–35 cm) of garden soils. To be effective, solarization need to heat all the soil in the planting profile, about 18–24 in. (45–60 cm) deep. This means deep-turning the soil and heating the surface several times to sterilize all its layers.
Step-By-Step Instructions
When vegetables such as onion, garlic, broccoli, cabbage, or potatoes fail and their roots have knots or rot away, it’s time to solarize. The best time of the year is during summer, when long daylight hours occur and the sun’s heat and ultraviolet rays are strongest.
At the time of the summer solstice—around June 21 in the northern hemisphere or December 21 below the equator—gardens receive about 14–16 hours of sunlight per day, more in the higher latitudes [see Solarize at Start of Summer diagram, above left].
Gather shovels, hoes, rakes, clear plastic film of at least 6 mil density, stakes, and a mallet, and follow these instructions to solarize the soil in a vegetable garden bed:
1. Prepare the Bed.
As soon as days turn sunny and hot, and at least two weeks of hot temperatures are forecast ahead, thoroughly work the soil of the infested bed by double digging it [see: The Double Digging Process].
When you finish turning the soil at depth, rake it smooth, break up all clods, remove any stones or other debris, and pull out any plant stems or roots.
2. Cover the Bed with Clear Plastic.
Carefully lay clear plastic film lengthwise on the bed. If more than one strip is required, overlap each seam by at least 12 in. (30 cm).
Stake or weight the plastic and turn soil over the edges to create a seal, stop air from escaping, and prevent wind from lifting or tearing the plastic.
3. Measure Temperature Under Plastic.
Solar heat will quickly raise the soil temperature beneath the plastic film to temperatures high enough to kill the nematodes, other pests, weed seeds, and disease spores.
After two days, test the temperature at least 10 in. (25 cm) below the surface.
The best time to test is about 4 p.m. in the afternoon, when air temperatures reach their maximum. The soil must test 130°F (55°C) or higher, and stay at that temperature for 4–6 hours. When complete, proceed to the next step, typically in about 4–5 days.
4. Turn Soil and Repeat
Remove the stakes or weights, pull back the plastic film, and turn the soil by double-digging again, moving the top 8–10 in. (20–25 cm) of soil under the layer below it.
Recover the bed as described, and continue solarizing a second cycle.
Because of the heat trapped in the buried soil, it may not take as long for this second and a third cycle to reach sterilization temperatures.
5. Turn Soil and Sterilize Third Time.
Repeat a final cycle, then remove the film and implements. The bed’s soil is sterilized and nearly ready for preparation for planting.
Remember that all living organisms have been killed by the process. Add fresh garden compost to seed the soil with beneficial bacteria and spores and double dig a final time to work it into the bed.
Worms and other soil creatures will quickly repopulate the soil after you plant.