I. Effects of Solarization

A. Use of Sunlight to Heat the Soil

The principal of soil solarization is that the soil lying beneath a single or double layer of plastic mulch will be heated by solar energy to temperatures sufficient to kill or otherwise inactivate soilborne pathogens and pests such as fungi, bacteria, nematodes, and weeds. The edge(s) of the plastic mulch must be tucked into the soil, giving airtight borders and thereby trapping solar energy beneath. This plastic mulch is made of transparent polyethylene, which is permeable to the short-wavelength solar radiation, but does not transmit longer-wavelength radiation from the ground back into the atmosphere (McGovern and McSorley, 1997).

Studies conducted in during the summer showed that a double layer of mulch is more effective in raising soil temperatures than a single layer (McGovern et al., 2002). Adding a second sheet of plastic to create a layer of air between the first and second sheet increased in soil temperature and reduced population densities of Fusarium spp., Pythium spp., and Rhizoctonia solani (McGovern et al., 2002).

Soil can reach temperatures of 35 to 60 °C (86 to 140 °C) when the double-layer system is used during the summer. However, raised temperatures do not extend much below soil depths of 10 to 20 cm (3.9 to 7.8 in), and are therefore most effective against pathogens in the topsoil (Katan, 1981, 1987; Stapleton, 1991; Stapleton and DeVay, 1986).

B. Effects Against Soilborne Pathogens

There are several ways in which solarization may reduce soilborne pathogen populations. These methods include:

1. Direct killing of pathogens.

2. Weakening of pathogens by various means.

3. Competition with antagonistic species.

4. Alteration of soil physical and chemical properties.

C. Increases in Plant Yield and Vigor

Reasons for increased plant vigor by solarization are numerous and complex. Perhaps the most obvious reason why plant health would increase in solarized soil is the removal of root-damaging organisms. Another reason involves the availability of soluble nutrients. Some elements and compounds are released from soil organic matter as decomposition increases at high temperatures. The removal of weeds and microorganisms that compete with the plant for nutrient uptake is another reason for better plant growth in solarized soil. Once beneficial organisms recolonize the soil, they breakdown organic molecules into plant-usable soluble compounds. Many phytopathogenic organisms are not well-adapted for survival in the soil. As a result, they are killed by solarization and are slow to rebuild populations. Beneficials microorganisms, on the other hand, are less affected by raised temperatures or show more rapid population growth and are thus bountiful and available to convert many organic molecules into their inorganic, soluble forms (Katan, 1987; Stapleton and DeVay, 1995; Gamliel and Stapleton, 1993b).

It has been observed in many studies that plants will grow more rapidly following either solarization or chemical fumigation techniques. The absence of soilborne pathogens allows plants to devote more energy to biomass production rather than to staving off pathogen attacks or competing with weeds. Correct use of solarization has been noted to produce a limited period of reduced-pathogen soil (usually lasting for about one season). Pathogens that are suppressed include: Phytophthora cinnamomi, Pyrenochaeta lycopersici, Verticillium dahliae, and Roselinia necatrix (Katan et al., 1983; Kassaby, 1985; Abdel-Rahim et al., 1988; Morgan et al., 1991; Tjamos and Paplomatas, 1988; Tjamos et al., 1991; Sztenjnberg et al., 1987).

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