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Soil Solarization
by Julie Ferraro
by Julie Ferraro

November 14, 1999

Soil solarization is a process that produces very high levels of heat in the soil. This process pasteurizes the soil, destroying harmful bacteria, fungi, nematodes larvae and weed seeds near the surface yet leaving most beneficial organisms unharmed. The beneficial organisms are more heat tolerant than harmful soil pathogens, so they recolonize faster. Solarization also has the added benefit of making nutrients such as nitrogen, calcium, and magnesium more readily available to plants.

The drawbacks are that you can only use it on a new or empty bed (as the high heat would kill the existing plants in a garden) and that it is most effective during the growing season. The best time to solarize soil is during July and August when temperatures are highest and days are sunny. But it is worthwhile if your garden is infected with a soilborne disease especially if it is recurring. To use this method follow the steps below.

•Loosen the top foot or so of soil (add organic matter or fertilizers as needed) with a garden fork or tiller.

•Water the soil heavily so that it is soaking wet - wetter than if you were simply watering your plants. Let the bed sit overnight.

•The next day, cover the bed with 1 to 3 mil clear plastic. The plastic must be clear to produce the desired greenhouse effect.

•Seal the plastic along the edges with soil, rocks, bricks or boards and keep the soil tightly covered for 4 to 6 weeks.

The plastic will trap the sun's heat in the covered area, causing the soil temperature to rise as high as 160°F.

Another soil solarization method is to use two layers of plastic. The second top layer provides additional insulation, reducing the loss of heat at night and creates a thermal barrier. There is some disagreement whether to use clear or black plastic as the lower layer. The thinking is that if black plastic is used that the heat that is trapped between the two layers will be absorbed by the black and transmitted to the soil below. When light strikes a darker substance it loses some energy. Some of that energy is absorbed, some is reflected. Much of the reflected energy is too weak to pass back through the top clear plastic layer and is trapped, causing the temperature buildup in the soil. To use the two-layer method, follow the above steps, adding a second layer of plastic over spacers (rocks, wood, PVC piping) and stretch it just enough (not too tightly) so that the two layers are not touching.

If it rains during the solarization period, sweep off the puddles from the plastic with a broom because they reduce the effect of the sunlight hitting the plastic. After the solarization period, the area is ready for planting as long as it is sufficiently dry. Try not to disturb the soil very much when you plant. The weed seeds near the surface have been killed but the seeds four or more inches down could still germinate if brought to the surface.

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