How to Rid Soil of Pests and Diseases

Soil-borne diseases can spell disaster for plants. In many cases, you can control disease organisms by mulching, rotating crops and stocking the soil with large helpings of organic matter. But when you're dealing with soil that's badly infected--especially if disease-bearing nematodes are at work--you need to wipe the slate clean. Solarizing is the way to go.

Instructions

Difficulty: Moderately challenging

Things You’ll Need:

• 3- To 6-mil Clear Plastic
• Garden Hose
• Rake
• Shovel Or Rototiller
• Soaker Hose
• Soil Or Stones
• Wooden blocks, stones or aluminum cans

1. Understand how solarizing works: It raises the soil temperature to around 150°F (65°C), a lethal level for garden culprits like fungi, nematodes and Colorado potato beetles.
2. To generate the necessary heat, treat your soil in the warmest, sunniest time of the year--July and August in most regions.
3. Dig or pull up all plants, and till the soil to a depth of 12 inches (30 cm). Remove rocks, twigs and other debris.
4. Rake the surface until it's smooth, and shape it into whatever rows, beds or hills in which you intend to plant. (You'll want to disturb the soil as little as possible after it's been treated.)
5. Water until the soil feels moist but not soggy, to a depth of 2 feet (60 cm). If you have very well drained soil, lay a soaker hose across the site (you may need to provide additional water during the treatment period).
6. Cover the plot with a sheet of 3- to 6-mil clear plastic, and pile soil or stones around the perimeter to seal the edges. If you're working with a raised bed, make sure the plastic drapes down over the sides. Leave some slack in the cover so it can puff up, rather than burst or blow away, as the heat intensifies.
7. Install a second layer of plastic if your climate is cool, cloudy or humid. First, place wooden blocks, large stones or aluminum cans every 4 to 6 feet (120 to 180 cm) across the first cover. Lay down the second sheet so that it floats on top of the first, moving the supports around if necessary. Then seal the edges with soil or stones.
8. If you laid down a soaker hose in step 5, pick up a corner of the plastic after a couple of weeks and feel the soil. If it's dried out, run the hose to replenish moisture, then replace the plastic.
9. In the Southwest, leave the plastic in place for a total of three to four weeks; elsewhere wait six to nine weeks. Then remove the plastic; the soil is ready for planting.

Tips & Warnings

• If you don't want to put your whole garden out of action at the height of summer, solarize one section at time, beginning with the most problem-ridden.
• If summer thunderstorms occur frequently in your area, try solarizing before or after the height of the stormy period; you'll stand a better chance of clear skies and intense sunlight.
• To boost the heat in cool climates, before you cover your plot, apply a 2-inch (5-cm) layer of fresh manure and mix it in well.
• Cultivate solarized ground as little as possible. If you till any deeper than 6 inches (15 cm), you'll bring up untreated soil.

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