By: Tim Marshall

It is a useful technique for organic growers, as it offers a thermal substitute for pre-emergent herbicides, nematicides and soil fumigants.

It is particularly useful for control of pathogens in potting soils and is employed by many large scale wholesale nurseries as an alternative to chemical sterilisation.

The technique uses clear plastic sheet to cover the soil. Polyethylene sheet 40 - 100 um thick is ideal. The edges must be secure to prevent heat leakage, usually by trenching in or hilling up at the edges of the treated area. The sheeting is reusable if carefully treated. Soil must be moist before covering with plastic, as the thermal properties of moist soil are much better for this purpose than dry soil and because weed seeds may survive treatment in dry soil.

For the best results the soil surface must be free of vegetation. Organic mulch or living vegetation will lift the sheet above the soil and reduce the temperature and the penetration of heat into the profile. Most soils will benefit from rotary hoeing or otherwise loosening and irrigation prior to treatment. A smooth surface will provide the best contact between sheet and soil and result in deeper treatment.

It is usually recommended that the treatment remain in place for a minimum of five to six weeks, although this may be reduced for potting soils if the minimum daily temperature is monitored during treatment. With potting soils a shallower pile will heat more quickly to a sufficiently high temperature, but a minimum treatment time would be seven to ten days.

Daytime temperatures of 55 - 56 degrees are possible in summer.

Some weed seeds are capable of withstanding very high temperatures, but most seeds are killed by solarisation.

Seasonality and climate are major limiting factors on the usefulness of solarisation for much of Australia. However solarisation has an important role to play in Australian horticulture and plant hygiene as a replacement for methyl bromide.

The bromine content of methyl bromide has been revealed as an ozone -destroying chemical. The Montreal Protocol on Substances that Deplete the Ozone Layer requires Australia and 148 other signatories to phase out methyl bromide by 2005.

Solarisation has an important role to play in alternative agriculture and non-chemical farming systems, because it is effective against plant pathogens that are difficult to treat directly with other methods, Solarisation should not be used frequently because it will severely reduce populations of desirable soil micro-organisms too, but it does have a role in an integrated pest management program, in combination with selection for plant resistance, steam/hot water and flame treatments, uv light, companion planting and crop rotation.

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