solarize

Solarization is a non-chemical soil treatment in which the soil surface is covered with clear plastic film, typically 25 to 40 microns thick, and left exposed to direct sunlight for 4 to 6 weeks during the hottest part of the year. The trapped solar radiation heats the soil to temperatures of 40 to 60 degrees Celsius at shallow depths (5 to 15 centimeters), sufficient to kill or disable many fungal spores, bacterial pathogens, nematodes, and weed seeds that live in the upper soil profile.

The process works because the clear plastic acts as a greenhouse: shortwave solar radiation penetrates the film and heats the soil, while longwave thermal radiation cannot escape back through the plastic. The soil temperature rises highest at shallow depths where most pathogens and dormant seeds concentrate. Depth of effect depends on soil color, moisture content, and ambient temperature, which is why the technique works best in climates with strong summer sun and still air.

The soil must be moist before plastic installation, then wetted again if rain does not fall during the solarization period. The plastic is anchored at the edges by burying it, weighting it with soil, or fastening it to stakes. Drip lines can be installed beneath the plastic to maintain consistent moisture. The treatment works most reliably in Mediterranean, arid, and warm temperate climates; in cool or cloudy regions, the effect is marginal. Solarization is commonly used before planting high-value crops such as tomatoes, strawberries, and nursery stock where soil-borne diseases like Fusarium and Verticillium cause serious losses.

Effectiveness varies considerably with the pathogen. Root-knot nematodes and many fungal spores are substantially reduced; some bacterial wilt agents are less vulnerable. The technique does not sterilize soil completely, and re-infestation can occur if contaminated water, tools, or transplants are introduced afterward. Solarization also suppresses the soil microbiota, and recovery to a balanced microbial community takes several weeks to months after plastic removal.

The term derives from the use of solar energy rather than steam or chemicals. It is sometimes called soil solarisation in British usage. The technique gained commercial adoption in the 1970s as a chemical-free alternative to fumigants like methyl bromide, and remains valuable where synthetic pesticides are restricted or where disease pressure warrants the cost of plastic film and labor.