Soil Temperature

Soil temperature is the main variable for solar heating effect, due to the above-cited critical or lethal accumulation of heat effects over a temperature threshold of about 37°C for most mesophylic organisms. Stapleton (1997) observed that the highest soil temperatures during solarization were achieved near the soil surface in the daytime, whereas temperature decreased by increasing depth and at night. Under a clear plastic mulch, temperatures higher than 50°C were recorded only in the top 5 cm soil, but literature reported temperatures of 40-50°C and 36-40°C down to 10-15 cm and 20-30 cm depth, respectively, during summer solarization in warm areas, whereas no lethal or sublethal thermal levels were generally found at deeper soil layers, where temperature increases by only 3-4°C (Porter and Merriman 1983; Stapleton and DeVay 1983; Greco et al. 1985; Chellemi et al. 1994). Stapleton and DeVay (1986) hypothesized that nematode population reduction found at 46-91 cm depth could be due to further suppressive factors, like releasing of volatile toxic compounds. Thermal levels originated by solarization were generally found much higher under closed greenhouse conditions or in containerized soil than in open field (Cartia 1998; Stapleton et al. 2000; Castronuovo et al. 2005).

Several types of models have been developed to predict temperatures of either bare or mulched soil during solarization (Mahrer 1979; Mahrer 1980). One- or two-dimensional numerical models of Mahrer and Katan (1981) and Mahrer et al. (1984) described soil temperature and moisture regimes of solarized soil on the base of environmental data, soil physical characteristics, and film optical properties, but their use was limited by the requirement of difficultly available weather data. On the same theoretical bases, Ten Berge (1990) and Horton and Chung (1991) developed models predictive of bare soil temperatures on the base of more easily available weather data, like solar radiation, air temperature, wind speed, and total rainfall. The one-dimensional model of Bristow and Campbell (1986) described both heat and moisture transfer through the soil, though with no consideration of mulch density and arrangement effects. Cenis (1989) proposed a site-specific model simulating daily sinusoidal change of temperature in a homogeneous soil, whereas the model of Sui et al. (1992) simulated soil temperature and moisture profiles under various type of mulches. Other models referred more specifically to the effect of mulch optical properties on soil heating (Ham and Kluitenberg 1994; Wu et al. 1996; Ruocco 2000), whereas Graefe (2005) proposed an energy balance model that was also applicable for a two-dimensional ridge surface partly covered by a plastic mulch.

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