Soil solarization heats the soil by covering it with clear polyethylene sheeting during hot periods to control soil borne diseases and weeds. The technique has been commercially exploited for growing high-value crops in diseased soils when maximum daily air temperatures regularly exceed 35°C. Farmers of the Deccan plateau in India have long exploited a form of solar heating of soil, by plowing the soil to expose the subsoil when maximum daily air temperatures usually exceed 40°C and soil temperature may reach 70°C. Studies have showed that 50°C will kill almost all weed seeds.
Using clear plastic makes it possible to heat the soil to 80°C to achieve better control of weeds and pathogens. The difference in soil heating between solarization and a control is shown in table 1. Moistening the soil improves heat transfer and improves weed and pathogen kill. Midsummer to fall has the highest performance.
In a preliminary test I found that bubble-pack sheet could help provide high temperatures with lower air temperatures. Double layers of plastic can also increase soil temperatures. Machines have been developed to lay plastic on large areas.
Table 1. Effects of solarization on maximum soil temperatures in °C (ICRISAT).
Soil depth Mean over Days
Treatment (cm) High test period >45°C
Solarization 5 54.1 49.9 48
Control 5 47.7 43.7 22
Although the major benefit of solarization is reduction of soil borne pathogens, especially verticillium and fusarium diseases, it also can control many weed and insect pests. A massive success in the Middle East has been the control of parasitic weeds like broomrape, that can devastate crops like tomatoes and cucumbers.
Studies have shown that solarizing soil effectively kills these seeds and can boost yields up to 98% In general. winter annual grasses seem to be especially sensitive to solarization, with control often evident for more than a year after treatment. More tropical weeds such as Bermuda grass and nutsedges are only partially controlled. Summer annuals, purslane and crabgrass, are also more resistant. Solarization can be used to promote germination and growth of weeds before cultivating during cool seasons. Adding compost and other soil amendments may improve control of the more resistant species.
Plants often grow faster and produce more (quality, size, and appearance) when grown in solarized compared to untreated soil, and has been found to increase plant growth and yield in following crops. This effect is greater than the benefits provided by pathogen and weed control, and is not fully understood.
Availability of nutrients may be increased substantially. Nitrate nitrogen NO3-N almost tripled in the top 0-15 cm soil and doubled in the soil from 15-30 cm deep in an experiment at ICRISAT. Beneficial microorganisms (mycorrhizal fungi, actinomycetes, etc may survive the solarization process or recolonize rapidly and provide biological control of pathogens and pests or improve plant nutrition.
Soil solarization has been used in commercial planting. Soil solarization is well suited for a kitchen garden or raised beds. It can also be used to sterilize planting mix. Spread moist soil mix on a hard surface or tarp, cover with clear plastic sheeting during the hottest months.
Solarization can be used to prepare areas for wildflower planting around field borders. Archaeological sites represent challenging requirements for weed management. Soil solarization has been tested and found to be helpful. In experimental plots in Greece the soil solarization treatments significantly reduced weed growth during the winter weed flush period. The highest level of weed control was achieved with clear plastic during the summer solarization period, that had the highest recorded temperatures for the largest sum of hours (238 hours with T ≥ 40 °C).
For effective solarization, consider the following:
- Solarization should be conducted for at least 4 and preferably 6 weeks during the hottest part of the year, longer with lower air temperatures.
- The area to be solarized should be cultivated and leveled to minimize clods, stubble, and stones that might tear the polyethylene sheeting. Fertilizer and soil amendments can also be applied and tilled in before putting down the plastic.
- Use clear, not black or colored sheets, of 1-2 mil (4 mil in very windy areas). Thinner plastic is more effective in heating the soil, but thickness should be balanced against durability. Wide sheeting to minimize joints is preferred. A roll of 2 mil 122m x 3m can cost as little as $25.
- Plastics designed for large-scale solarization are often treated with an ultraviolet (UV) inhibitor so they will not break down as quickly in sunlight. Some will last for 4 years. When plastic with UV inhibitors is used the plastic can often be lifted and reused or left in place as a mulch during the growing season.
- If possible 2-3 cm of irrigation should be applied just before laying the polyethylene sheeting. The moisture improves the heat capacity and heat transfer in the soil. It may also make pests or seeds more vulnerable to heating. 6. It is best to apply the sheeting at dawn, when it is least windy. The edges of adjacent polyethylene sheets should be inserted in the furrows. All free edges should be buried and the soil around them should be compacted to prevent escape of heated air or soil moisture. To prevent flapping and tearing of the sheeting in the wind weights should be placed on the sheeting. Plastic bags filled with soil work well. Bigger bags in windy areas where haboobs might develop. Consider where runoff will go in a summer rainstorm. Perhaps it can lead to rainwater harvesting.
- It is desirable to have a buffer zone (about 1 m) around the area to be solarized because there is less heating near the edges. Sufficient space should also be provided between solarized areas for access, equipment operation, and drainage.
- Holes in the polyethylene sheeting should be sealed at the earliest opportunity. Tape can be used. Holes can be recognized by absence of condensed moisture on the underside of the plastic.
- Avoid walking on the plastic. Use bare feet or smooth-soled shoes while making repairs.
- Remove the plastic when finished and let soil dry to a workable texture and cool off. The soil can be planted to a fall or winter crop or left fallow until spring. Removing potential weed seed sources around the field is desirable if the field will be fallow. If the soil is to be cultivated before planting, the cultivation should be shallow (less than 5 cm) to avoid moving viable weed seeds to the surface.
Soil solarization is not perfect. It does not work against all weeds and pathogens and requires the use of energy and petrochemicals to make the polyethylene. But it is clean, safe and often as effective as many dangerous herbicides and fungicides. In the weed and pathogen infested student garden at UC Riverside soil solarization worked better than the highly toxic METAM (Methyl Isothiocyanate (MITC) gas, a very toxic soil sterilant (banned in France) that is used to eliminate nematodes, fungal pathogens, germinating weed seeds, and soil insects.
Recommended Reading
Dahlquist, R.M., Prather, T.S., and Stapleton. J.J. 2007. Time and temperature requirements for weed seed thermal death. Weed Science. 55:619–625.
Greenberger, A., Alan, H., and Grinstein, A. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soil borne pathogens. Phytopathology. 66:683-688.
Kanellou, E., Papafotiou, M., Economou, G. and Ntoulas, N. 2023. Soil solarization as an alternative weed control method for archaeological sites in the Mediterranean Region. Sustainability. 15:11324.
Pullman, G.S., DeVay, I.E., and Garber, R.H. 1981. Soil solarization and thermal death: A logarithmic relationship between time and temperature for four soil borne plant pathogens. Phytopathology. 71: 959-964.
Stapleton, J.J., Wilen, C., Molinar, R.H. 2019. Soil solarization for gardens and landscapes. Pest Notes, Publication 74145. ANR Publications, University of California, Oakland. https://ipm.ucanr.edu/home-and-landscape/soil-solarization-for-gardens-landscapes/#gsc.tab=0
Stapleton, J.J., Molinar, R.H., Lynn-Patterson, K., McFeeters, S.K., Shrestha, A. 2005. Soil solarization provides weed control for limited-resource and organic growers in warmer climates. California Agriculture. 59: 84–89.
Rubin, B., Gamliel, A. 2018. Soil solarization: A sustainable method for weed management. In Integrated Weed Management for Sustainable Agriculture; Burleigh Dodds: Cambridge, UK, 2018; pp. 303–318.
