Function of a plastic film in the solarizing process is to increase soil temperature by allowing passage of solar radiation while reducing energetic radiative and convective losses (Papadakis et al. 2000). Transmission of solar radiation by plastic films was found related to their radiometric properties and mainly transmissivity, as higher values of solar transmissivity coefficient resulted in higher temperature rises under the mulch (Scarascia-Mugnozza et al. 2004; Vox et al. 2005). However, other radiometric properties, i.e., reflectivity, absorptivity, and emissivity, were also involved in the thermal effectiveness of a plastic film (Papadakis et al. 2000). Radiometric properties of a large variety of plastic film were thoroughly documented either in laboratory or field tests (Pearson et al. 1995; Papadakis et al. 2000; Heissner et al. 2005; Vox et al. 2005) (Fig. 9.2).
A wide range of different plastic materials were reported as applicable to soil solarization (Lamberti and Basile 1991), though best solarizing properties were shown by polyethylene and its low-density or vynilacetate-coextruded formulations, either alone or added with inorganic salts, and by copolymer ethylene vynilacetate and polyvinylchloride films (Stevens et al. 1991a). In field and laboratory comparative trials on the radiometric properties of different plastic films, an ethylene-tetrafluoroethylene copolymer film showed better spectrora-diometric characteristics compared to ethylene-co-vinyl alcohol, polyethylene, photo-selective red colored and UV-absorbing films, as achieving higher soil temperature and thus improving solarization efficacy (Cascone et al. 2005). Under greenhouse conditions, soil solarization with an ethlylene-tetrafluoroeth-lylene film, though inducing a lower thermal regime than ethlylene vynilacetate, provided a better management of several important soilborne pathogens (Polizzi et al. 2003). Malathrakis and Loulakis (1989) reported that solarization with polyethylene and polyvinylidene chloride sheets were similarly effective against S. rolfsii, but only the latter film significantly reduced cucumber (Cucumis sativus L.)
root rot and vascular browning caused by Acremonium spp., Chase et al. (1999a) found a clear thermal-infrared absorbing film as consistently more effective in increasing soil temperature than low-density polyethylene. Numerical and field studies of Al-Kaysi and Al-Karaghouli (2002) showed that mulching soil with a paraffin-wax emulsion film, rather than transparent polyethylene, resulted in more effective soil heating and a faster killing of pathogenic soil fungi.
A high suitability of polyethylene to solar heating was generally stated, due to its high solar radiation transparency, flexibility, tensile strength, and resistance properties (Espí et al. 2006). Thinner polyethylene films were generally found more effective, as less expensive and producing higher temperature increases than thicker ones (Stapleton and DeVay 1986; Abu-Irmaileh, 1991a,b). Use of a double polyethylene layer was documented as more effective than a single layer film, due to a 3-10°C soil temperature increase and then an improved effect on target pests and soil microflora (Ben-Yephet et al. 1987; Cenis 1987; Mahmoud 1996), thus resulting particularly suitable to nursery conditions and in cloudy climate areas (Annesi and Motta 1994; Stevens et al. 1999; Rodríguez Pérez et al. 2004).
The color of solarizing plastics was also investigated by a number of authors, as determining energy-radiating behavior of mulches and their influence on soil water content and microclimate around the plant (Lamont 1993; Streck et al. 1995; El-Keblawy et al. 2006). Haynes (1987) reported that soil temperature can be differentially affected by mulch color, generally following the order: transparent mulch > black mulch > white mulch. Barakat (1987) found that the use of an opaque black film, as blocking the passage of most solar radiation, reduced soil temperatures by several degrees compared to a transparent film, but clear and black plastic mulches resulted in similar soil temperature regimes in other studies (Hasing et al. 2004). High temperatures were found to last consistently longer under the clear film, whereas black polyethylene provided a higher stability and a longer durability under field conditions and an enhanced weed suppression (Dubois 1978; Hancock 1988; Hasing et al. 2004). Solarization with transparent, black, or other color mulches was investigated with mixed results in many experiments (Kadman-Zahavi et al. 1986; Abu-Gharbieh et al. 1991; Ham et al. 1993; Chase et al. 1999; Campiglia et al. 2000; Rieger et al. 2001; Hasing et al. 2004). Black plastic was suggested under special conditions, as in nursery beds or established tree orchards (Stapleton and Garza-Lopez 1988; Abu-Gharbieh et al. 1991; Stapleton 1997). Mulching soil of a newly established fruit orchards with a black polyethylene film resulted in higher weed suppression and lower root-knot nematode galling, when compared to the nonmulched control (Duncan et al. 1992). Some studies reported that solarization with black polyethylene film also reduced populations of many soil phytopathogenic fungi and bacteria and was more effective for preventing weeds (Reynolds 1970; Hawthorne 1975; Lamont 1993). In other experiments, black plastic mulch resulted in higher early yields but reduced total crop yield compared to transparent film (Schonbeck and Evanylo 1998).
Application of plastic films for soil solarization requires specific equipments and time-spending procedures, as well as the plastic disposal at crop end can be expensive and represent a serious environmental problem (Parish et al. 2000).
A number of studies focused on the potential responses provided to these problems by the improvement of plastic technology (Spreich et al. 1990; Doran 2002; Zheng et al. 2005). Mulches of sprayable degradable polymers, applied on soil surface at an appropriate amount, were found to be a feasible and cost-effective alternative to traditional plastic mulch for soil solarization (Gamliel et al. 2001). Soil solarization with sprayable mulches, though soil temperatures were generally lower than under traditional plastic film, was reported to be effective in controlling potato scab and peanut (Arachis hypogaea L.) pod wart, or in increasing eggplant (S. melongena L.) yield and fruit weight (Mahmoudpour and Stapleton 1997; Gamliel et al. 2001). Mulches based on biodegradable and renewable agricultural raw materials were also tested as a sustainable alternative to the conventional plastic films, due to their easy disposal in the soil or composting plants (Chandra and Rustgi 1998; Narayan 2001; Malinconico et al. 2002; Kirikou 2007) (Fig. 9.3). Starch-based biodegradable films were largely studied for their degradation and morphological behavior (Bastioli 1998; Briassoulis 2004; Heissner et al. 2005; Vox et al. 2005; Scarascia-Mugnozza et al. 2006). These materials generally were found to produce lower temperature levels and for a shorter period, compared to the traditional low-density polyethylene and ethylene vinyl acetate films (Candido et al. 2005; Russo et al. 2005). However, several field tests documented also their successful application in many crops (Chandra and Rustgi 1998; Manera et al. 2002). Under greenhouse and field conditions, soil solarization with a corn starch-based biodegradable film resulted effective for the control of root-knot nematodes (Melodogyne spp.) on melon (Cucumis melo L.), corky root disease caused by Pyrenochaeta lycopersici
Schneider and Gerlach in tomato (Lycopersicon esculentum L.), S. minor on lettuce (Lactuca sativa L.) and weeds, improving also crop yield and quality (Panattoni et al. 2004; Cascone et al. 2005; Castronuovo et al. 2005).
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