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Historical review and principles of soil
solarization
Solarization for greenhouse crops in Japan
Soil solarization: yield and quality benefits
for potato in a temperate climate - short- and long-term effects
and integrated control
Historical review and principles of soil solarization
James E. DeVay, Department of Plant Pathology,
University of California, Davis, CA 95616 USA
Historical Review
Soil solarization is a hydrothermal process that occurs in moist soil which is covered by a transparent plastic film and exposed to sunlight during the warm summer months. During solarization, soil temperatures are achieved which arc lethal to many plant pathogens and pests and which also cause complex changes in the biological, physical, and chemical properties of soil that improve the growth and development of plants (Figure 1). Soil solarization is a mulching process that had its origin in early agriculture where covering soil and plants with organic or inorganic materials formed a protective barrier against frost or warmed soil to increase plant growth; mulching also was used to limit soil water evaporation, to control weeds, to improve soil filth and manage erosion (4, 8, 11, 14, 21, 30, 31, 39, 48, 49). When plastic mulches came into use, mulching soils with black polyethylene (PE) film reduced southern blight of tomato and dwarf bean caused by Selerotium rolfsii (16, 38), lettuce head drop caused by Sclerotina minor (18), and rots of lettuce caused by Rhizoctonia solani and bacteria (19).
The above uses of black PE film were effective for control of plant pathogens and weeds, but a landmark publication in 1976 by Yaacov Katan and coworkers on solar healing of soil (26) opened a new era and approach for the non-chemical control of plant diseases and pests using transparent PE film. Essentially what they did was to bury in field plots, samples of inoculum of Fusarium oxysporum f. sp. vasinfectum and Verticillium dahliae at 5 cm, 15 cm, and 25 em and then cover the moist soil with transparent PE film for 14 days. At 5 cm depth, population reduction of the pathogens was 94 to 100 percent while al 15 cm it varied between 67 and 100 percent, and at 25 cm, reductions of F. oxysporum f. sp. vasinfectum were between 54 and 74 percent. Average maximal temperatures of mulched soil were 50.7°C at 5 cm and 40.8°C at 15 cm while the average maximal temperatures of nonmulched soil were 37.6°C and 32.4°C, respectively. In other field experiments, Katan et al. (26) found that mulching for four to five weeks resulted in a significant reduction in both the incidence and severity of Verticillium wilt in eggplant. Additionally, the eggplants showed an increased growth response with an increase of 38 percent in plant height and a 26 percent increase in plant stand compared with non-mulched plots. They also observed almost complete control of several weed species in the mulched plots.
The process of solar heating of soil was designated soil solarization to encompass the complex of physical, chemical, and biological changes in soil associated with solar heating (37). Following the paper by Katan et al. (26), a series of papers appeared on the fundamental aspects of soil solarization (3, 23, 32-35, 37, 4042, 45). A proliferation of articles then occurred on the effectiveness of soil solarization for controlling many different pathogens and pests under a wide range of climates and different cropping systems (27). While the contributions of Katan and coworkers (23-25) in Israel and those of Pullman et al. (34-37), Stapleton (40-45), Elmore (34), and Ashworth (3) in California pioneered the uses of soil solarization under field conditions, Kodama and coworkers (28, 29) and Horiuchi (20) in Japan pioneered the use of solar heating of soil in closed vinyl houses.
The first uses of soil solarization were to disinfest soils of pathogens and pests before planting time; however, in an urgent situation in California to control Verticillium wilt in established pistachio orchards, Ashworth (3) pioneered the poll-plant use of soil solarization with much success. Subsequent work by Stapleton (44) and Tjamos et al. (47) confirmed the effectiveness of post-plant solarization of soils for managing soilborne pathogens in established orchards.
During the decade after the appearance of the article by Katan, et al. (26), over 173 articles by research workers in 24 or more countries were published (27) that dealt with all aspects of soil solarization. In a tangential way, several workers had data and knowledge on the effectiveness of solar heating of soil for pathogen and pest control before Katan et al. (26) published their paper, but Katan and his coworkers are recognized for their signal contribution which led to the spectacular and extensive development of soil solarization; they articulated the process, its effects and potential for the non-chemical control of pathogens and pests; they caught the attention of other scientists concerning the value of soil solarization as an alternative to the use of agricultural chemicals where agriculturists must work within the increasing constraints of public opinion and safety.
Principles of Soil Solarization
The success of soil solarization is based on the fact that most plant pathogens and pests are mesophylic, i.e. they are unable to grow at temperatures above 31° to 32°C; they are killed directly or indirectly by the temperatures achieved during the solar heating of moist soil under transparent plastic films which greatly restrict the escape of gasses and water vapour from the soil. Thermotolerant and thermophylie soilborne micro-organisms usually survive the soil solarization process (42). However, all soilborne organisms, if not directly inactivated by heat, may be weakened and become vulnerable to changes in the gas environment in solarizing soil or to changes in the populations of other organisms which may exert a form of biological control (25, 40). Soil solarization is a hydrothermal process and its success depends on moisture for maximum heat transfer to soilborne organisms; it is a function of time and temperature relationships. The thermal decline of soilborne organisms during solarization depends on both the soil temperature and exposure time, which are inversely related. The effectiveness of solarization for disinfesting soil and increasing plant growth and development depends on soil colour and structure, air temperature, soil moisture, length of day, intensity of sunlight, and the thickness and light transmittancy of the plastic film.
Plastic films. - Low density PE (10) is widely used for agricultural mulch because of its flexibility, tensile strength, and resistance to puncture and tearing. Films as thin as 1 mil (25 millimicrons) are commonly used in broadcast situations for soil fumigation with methyl bromide and are left on fields for about 48 hours. Surprisingly, these thin PE films, even without ultraviolet stablilizers, have been maintained for up to 9 weeks for soil solarization without major deterioration (36, 37). PE is an ideal film for solar heating of soil because it is essentially transparent to solar radiation (280 to 2 500 nary), extending to the far infra-red, but much less transparent to terrestrial radiation (5 000 to 35 000 nary), and thus reducing the escape of heat from the soil. Among the plastic films used in agriculture, such as polyvinyl chloride and ethylene vinyl acetate, the chemical and physical characteristics of PE have made it most useful in soil solarizalion. PE is a petro-chemical and its cost is directly related to its thickness. Thicker PE films (2, 4 and 6 mil) have been used effectively in soil solarization, but the thinner films (1 and 1.5 mil) are more effective in soil heating and are more cost effective (43). Compared with transparent PE films, black PE containing carbon black, absorbs solar radiation and thus reduces the heating of soil by several degrees C, however, it is more stable and lasts considerably longer under field conditions (2, 13, 17).
Improvements in the technology of soil solarization under field conditions have included the use of double layers of PE (6, 9) which simulate solarization of soil under glasshouse conditions (15) and causes a 3 to 10 degree increase in soil temperatures compared to that of soil under a single layer of PE film (Figure 2). The air layer (7.5 cm or more) between the two layers of plastic film provides insulation against the escape of both heat and moisture from the soil.
Soil Moisture. - Soil moisture is a critical variable in soil solarization since the transfer of heat to weed seeds and plants and micro-organisms in soil is greatly increased by moisture. Soil solarization is a hydrothermal process and its success depends on moisture for maximum heat transfer. Moreover, the temperature maxima of soils increase with increasing soil moisture (33). Cellular activities of seeds and the growth of soilborne micro-organisms are favoured by soil moisture, making them more vulnerable to the lethal effects of high soil temperatures associated with soil solarization.
Soil Temperature. - Soil temperature of moist soil is the main variable in the process of soil solarization For mesophylic organisms a temperature threshold of about 37 C is critical; the accumulation of heat effects at this or higher temperatures over time is lethal. With increasing temperature, less time is required to reach a lethal combination of time and temperature. For example, at 37 C, a killing temperature (ED90) for many mesophilic fungi, exposure may require from two to four weeks, whereas at 47 C, one to six hours exposure will result in an ED90 (Figure 3). The sensitivity of organisms to high temperatures is related to small differences in macromolecules which lead to increased intramolecular bonding involving slight changes in hydrogen bonds, ionic bonds, and disulfide bonds (7). Organisms sensitive to high soil temperatures which occur during soil solarization, have a greater amount of unsaturated cellular lipids than thermotolerant or thermophylic organisms. Thus, mesophylic organisms which do not survive the high temperatures in solarized soil, have lower melting fatty acids in their membrane lipids and lower phase transition temperatures for the lipids (46). Present evidence suggests that the heat sensitivity of organisms is related to an upper limit in the fluidity of membranes, beyond which membrane function is reduced (46).
Effect of on Soil Moisture. - A phenomenon which often is apparent during soil solarization but which lacks experimental verification is the cycling of moisture in soil during solarization. The upper layers of soil (5 cm) have a marked diurnal fluctuation in temperature; cooling at night and heating to high temperatures during the sunlight hours (Figure 4). This diurnal fluctuation in temperature causes moisture in the upper zones in soil to move downward during the day due to the solar radiation, while at night the soil surface cools causing an upward migration of moisture. As the solarization process deepens in the soil, the movement of moisture becomes more pronounced, changing the distribution of salts and improving the filth of the soil. A reduction of soil salinity resulting from solarization has been reported by Abdel-Rahim et al.(1). To maximize this effect in soil, pre-irrigation of soil or irrigation of soil under plastic sheeting at the beginning of soil solarization should reach to a depth of at least 60 to 75 cm.
Length of Day and Intensity of Sunlight. - The success of soil solarization is affected by the intensify and length of exposure to sunlight (5). The times of the year corresponding to these conditions occur during the warm summer months. Soil solarization has been attempted at other times of the year but with limited success. Also, climates with cool air temperatures are less likely to provide adequate weather conditions conducive to the success of soil solarization. However, unexpected successes with soil solarization have been achieved in England (50), Quebec, Canada (22), in the southwest area of Oregon by Pacific Bulb Growers (L. Riddle, personal communication) and in Idaho (12) in the USA. It is generally known that plant pathogenic fungi in warm climates are adapted to higher temperatures than the same species in cooler climates. This difference in heat sensitivity is believed to contribute to the control of plant pathogens by soil solarization in climatic regions where the rise in temperature of solarized soil is less than that in warmer climates (12).
Increased Growth Response. - Although the heat generated in soil by solar radiation and the resultant death of plant pathogens and pests encompass the major principles of soil solarization, the increase in available plant nutrients and relative increase in populations of rhizosphere competent bacteria, such as Bacillus spp. (42) which contribute to the marked increase in the growth, development, and yield of plants grown in solarized soil, are major components of the soil solarization process (25, 43). The changes in populations of soilborne micro-organisms also constitute the basis for biological control of plant pathogens and in some cases the development of disease suppressive soils (25). Following solarization, the complex of physical, chemical, and biological changes which occur in solarized soil may persist for at least two years (25, 37).
Conclusions
In the short history of soil solarization, its use and potential for the non-chemical control of plant pathogens and pests and as a cultural practice for improving the growth and development of crop plants have been widely recognized and accepted. Soil solarization embodies the major thrust of integrated pest management in that it is an effective alternative to the use of agricultural chemicals and provides a favourable base for a natural system of biological control of plant diseases and pests.
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