Chapter 13

Weed Management for Organic Crops

Soil Solarization for Weed Control

Soil solarization is a non-chemical soil treatment that utilizes solar radiation and a thin film of transparent mulch, usually of polyethylene, to heat the soil to a range of 100 to 122 degrees F (38 to 50°C) to a depth of about four to eight inches (10 to 20 cm) for soil pasteurization (See Figure 14.28).  In addition to pasteurization, it has been known to also control weeds as a pre-emergent control in terminating weed seeds in the ground before and during germination. The degree of weed suppression achieved with solarization varies with weed species, depth of seed in the soil, and length of solarization. In general, solarization is more effective against annual weed species and less effective against perennial weeds. Perennial weeds are more difficult to control than annual weeds because their underground vegetative structures—rhizomes, tubers, or bulbs—allow them to survive most non-chemical controls, including cutting, cultivation, and mowing. The drawbacks of solarization include the use of plastics in agriculture and their associated disposal problems (though sheets may be re-used if they are not used as in-season mulch), and the fact that land is taken out of production during the summer.

Mechanism of Soil Solarization

Soil solarization is conduction of heat by entrapment of solar irradiation through the greenhouse effect. Very thin (25 to 50 µm) transparent polyethylene (PE) mulch is commonly used to trap solar heat because it is permeable to the short-wavelength solar radiation, but does not transmit longer-wavelength radiation (heat) from the ground back into the atmosphere.  Although the thin PE mulches are less durable and are susceptible to wind and animal damage, it is more effective in raising the soil temperature and controlling weeds than the thicker PE mulches of 50 to 100 µm. The second mechanism of soil solarization is through a solar heating process by soil moisture.

Time of Soil Solarization

Generally, soil solarization is conducted for a minimum of four to six weeks during the warmer time of year, when there is high solar irradiation and minimal cloud and precipitation, thus its effect is climate dependent. It has been attempted in the spring and fall, but may not be as reliable then because temperatures are cooler. In addition, depending on soil texture, solarization heat penetrates to different soil depths.

Methods of Soil Solarization

Broadcast Solarization

Broadcast solarization and strip solarization are the two main methods of soil solarization. Broadcast solarization consists of completely covering a whole field with PE.

Strip Solarization

Strip solarization is simply applying the sheets of PE without attaching them together. A mulch-laying machine is commonly used to install 4 m-wide PE sheets.

Solarization Practices for Controlling Weeds

To use solarization successfully, organic farmers should rely on these practices:

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