Soil solarization uses solar radiation to disinfest the soil from pests detrimental to crop plants (Katan, 1981; Stapleton, 2000). Soil solarization is used most in areas with high solar radiation and temperatures during the summer season (Gill et al., 2009; Stapleton, 2000). It is used as an alternative to soil fumigation for pathogen and pest control either alone or in combination with fumigants (Elmore et al., 1997; Hartz et al., 1993). Disinfestation is achieved by using solar radiation to passively heat moist soil covered with clear plastic sheeting for a period of a few weeks to increase soil temperatures to the point where they are lethal to soilborne organisms (Katan, 1981; Pullman et al., 1981; Stapleton, 2000). Higher soil moisture during solarization can increase the soil heat conductivity resulting in higher temperatures compared with soil that is less moist (Katan et al., 1976). Low-density PE film and ethylene vinyl acetate film have the best solarizing properties for increasing temperatures in production beds (D’Addabbo et al., 2010). Solarization is a strategy used to reduce the soilborne pests and diseases such as Verticillium dahliae (verticillium wilt), Fusarium spp. (fusarium wilt), Phytophthora cinnamomi (Phytophthora root rot), Meloidogyne incognita (root knot nematodes) and weed species (Dahlquist et al., 2007; Stapleton et al., 2000). The elevated soil temperatures are also beneficial for soil physical and chemical structure, accelerating the release of minerals from decomposition and increasing soil aggregation (Elmore et al., 1997; Stapleton, 2000).
The effectiveness of solarization is based on the actual maximum soil temperature achieved under the plastic cover and the amount of time that this temperature can be sustained (Chase et al., 1999). Soil temperatures of 37 °C can be effective in controlling some pests and pathogens if they are maintained for 4 to 6 weeks (Elmore et al., 1997). The optimal temperature for the solarization process depends on which organisms are present in the soil and their susceptibility to high temperatures. Some organisms and their thermal thresholds for inactivation are listed in Table 1. Studies have shown a logarithmic relationship between temperature and pathogen mortality where less time is required to kill pests and disease-causing organisms as soil temperature increases (McLean et al., 1999; Pullman et al., 1981). In laboratory studies, pathogens such as Verticillium dahliae, Pythium ultimum, and Thielaviopsis basicola required 29, 18, and 33 d to kill 90% of propagules at 37 °C, respectively (Pullman et al., 1981). However, at 50 °C those times were reduced to 23, 33, and 68 min. Temperatures of 50 °C or greater dramatically reduce the amount of time required to inactivate various pest organisms (Dahlquist et al., 2007; Mihail and Alcorn, 1984; Pullman et al., 1981; Stapleton et al., 2000).
Temperature and time period required for the inactivation of some pests.
Solarization in a greenhouse or high tunnel structure will yield higher soil temperatures than in fields or gardens (Elmore et al., 1997). Using PE mulch inside a greenhouse will further increase the effectiveness of the soil solarization process (Mahrer et al., 1987), especially in cooler coastal climates (Larson, 2007). Although temperatures in soils covered with PE mulch will be highest in a glass house, a structure with PE glazing will also be effective in raising soil temperatures above mulched soil exposed to open air conditions. Soil solarizaion in a Mediterranean climate using PE mulch for 7- to 9-week intervals was successful in raising soil temperatures at 15-cm depth between 37 and 50 °C in beds used for strawberry (Fragaria xananassa Duch.) cultivation under plastic tunnel conditions (Iapichino et al., 2008). Solarization in a high tunnel in Costa Rica increased the soil temperature of covered soil to ≈60 °C compared with 30 °C for uncovered soil (Santos et al., 2008). A double-tent system within a greenhouse facilitated structural solarization by raising temperatures to 60 °C and resulted in effective inactivation of Fusarium sp. (Shlevin et al., 2004). The same temperature was effective in killing weed species in containers filled with soil and exposed to solarization in a double tent (Stapleton et al., 2002). At present there are no studies for the semiarid southwestern United States region comparing solarization inside and outside of a high tunnel. One goal of this study was to provide growers using high tunnels with information that may be beneficial to protecting their crops from weed and other pest populations.
The semiarid climate in southern Arizona is ideal for soil solarization as a result of high solar radiation. June is the optimal time for solarization because the daily solar radiation can reach 30 MJ·m−2 and temperatures are higher for this month than others [Arizona Meteorological Network (AZMET), 2013; Mihail and Alcorn, 1984]. In Tucson, AZ, daily fluctuations in temperature are typically 8 to 17 °C with June average highs and lows of 38 °C and 21 to 27 °C, respectively (AZMET). Although these are averages, temperatures can reach extreme highs such as 47 °C in June of 1990. Low relative humidity during early summer months combined with high temperatures and high solar radiation provides growers with an opportunity for using solarization when production areas are fallow. The objective of this study was to determine the efficacy of clear PE mulch on the soil surface and glazing on a high tunnel to raise soil temperatures to solarize the soil during the hottest time of the year in the semiarid southwestern United States.
Arizona Meteorological Network (AZMET) 2013 20 July 2013. <http://ag.arizona.edu/azmet/>
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