INTRODUCTION
Prior to the ready availability of pesticides in the late 1940s, soil disinfestation carried out with the use of heat, steam or hot water was an old and well-known practice used for controlling soil pests (Newhall, 1955; Baker, 1962). Raising the oil temperature to 60°C by aerated steam for 30 minutes has been a standard recommendation in methods used in the control of soil pests (Brazelton, 1968).
Soil solarization is a term that refers to disinfestation of soil by the heat generated from trapped solar energy. Trapping solar energy for the purpose of elevating soil temperature to control soil pests is an old venture. Grooshevoy (1939) experimented with soil solarization in the Caucasus in 1938. He obtained effective control of soil pathogenic organisms by trapping solar energy under cold frames subjected to direct sunlight prior to planting for periods sufficient to raise the temperature of the top layer of soil (to a depth of 10 cm) to between 40° and 60°C, resulting in the control of the black root rot of tobacco seedlings caused by Thielaviopsis basicola.
Soil solarization is a hydrothermal process that takes place in moist soil which is covered by plastic film and exposed to sunlight during the warm months. The process of solar heating of the soil is known as soil solarization, and encompasses the entire complex of physical, chemical, and biological changes in soil associated with solar heating and has value as an alternative to the use of certain agricultural chemicals that will be phased out of agricultural usage. Soil solarization is a mulching process that had its origins in early agriculture where the practice was used to cover soil and plants with organic and inorganic materials so as to form a protective barrier against frost. The warmed soil was used to increase plant growth, and mulching was also used to limit soil water evaporation, to control weeds, to improve soil tilth and to combat soil erosion (Lai, (1974); Waggoner et al.(1960); Burrows and Larson, (1962).
When plastic mulches first came into use, polyethylene was an ideal film for solar heating of soil because it is essentially transparent to solar radiation (280-2 500 nm), extending to the far infrared, but much less transparent to terrestrial radiation (5 000-35 000 nm), thus reducing the escape of heat from the soil. Polyethylene is a petrochemical and its cost is directly related to its thickness. Polyethylene films have been used effectively in soil solarization.
CONVERSION OF SOLAR RADIATION INTO HEAT UNDER MULCH
During soil solarization, incoming solar radiation penetrates the plastic film and gets absorbed by the soil. Most of the absorbed radiation is converted into heat. Since all objects above absolute zero emit radiation, both quantity and the quality of radiant energy emitted from the soil are dependent upon soil temperature. According to Stefan’s law; the quantity of radiation emitted is a function of the fourth power of the absolute temperature.
Q= e dT4
{Q= quantity of energy radiated in calories,
d = Stefan-Boltzman constant (8.132 × 10-11 cal/cm2/min.deg K4)
T= the absolute temperature in Kelvin}
The wavelength of the emitted radiation from any object is also influenced by temperature. According to Wein’s law, the wavelength of the emitted radiation is inversely proportional to the object temperature (ë á 1/T). Thus, solar radiation is emitted at shorter wavelengths compared to that emitted from earth (99 percent of solar radiation lies between 150-4 000 nm, while terrestrial radiation is emitted at about 10 000 nm, 99 percent of earth radiation is emitted between 400-100 000 nm (long IR) (Salisbury and Ross, 1980). Therefore, solar radiation can easily penetrate the plastic mulch, but the emitted radiation from soil (normally at long wave length) cannot penetrate through the plastic cover. Consequently, most radiation would be trapped under the plastic mulch. During the process, soil temperature could be raised to levels lethal to many soil-borne organisms, including plant pathogens and pests
SELECTIVE ACTION AGAINST SOIL ORGANISMS
The efficiency of soil solarization in controlling soil-borne pests is a function of time and temperature relationships, and is based on the fact that most plant pathogens, weeds and other pests are mesophylic. 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 (LD90 for many mesophylic fungi), exposure may require from 2-4 weeks, whereas at 47°C, 1-6 hours’ exposure will result in an LD90 (DeVay, 1990).
Temperatures commonly reached by soil solarization are 35-60°C, depending on soil depth, reaching higher than 45°C in the upper 15 cm in the San Joaquin valley during summer (Stapleton, 1990). Many soil pests are adequately controlled by 4-8 weeks of soil solarization in the upper 10-30 cm (rooting zone).
Temperatures achieved during soil solarization are considered mild compared to soil steaming (Baker, 1962; Stapleton and DeVay, 1986). Thus, soil solarization is more selective towards thermophilic and thermotolerant biota, actinomycetes which may survive and even flourish under soil solarization (Gamliel et al. 1989; Stapleton, 1981). Lethal effects of soil solarization are more pronounced on micro-organisms which are not good soil competitors. Many pathogens fall into this group, since they tend to have specialized physiological requirements which are more adapted to coexistence with the host plants (Stapleton and DeVay, 1986), resulting in a population shift favouring thermotolerant species, including Bacillus species, fluorescent Pseudomonads, and thermotolerant fungi (Gamliel et al. (1989); Staplelton (1981), and may suppress pathogen, allowing re-colonization (after possible initial declines in their population) by micro-organisms which are more competitive and often are antagonistic of plant pathogens and pests. The antagonistic fungi Trichederma harzianum aggressively colonized solarized soils (Katan (1981). Most of the solarization-tolerant micro-organisms have been implicated as biological control or plant-growth stimulating agents (Baker and Cook, 1974).
PROPOSED MODE OF ACTION
Although soil solarization presents itself as a very simple method, the mode of action is complex, and can be described by physical, chemical and biological effects.
Soil-borne organisms are killed directly or indirectly by the temperatures achieved during the solar heating of moist soil under polyethylene films which greatly restrict the escape of gasses and water vapour from the soil. The sensitivity of organisms to high temperature is related to small differences in macromolecules which lead to increased intra-molecular bonding involving slight changes in H-bonds, ionic bonds and disulfide bonds (Brock, 1978). Unsaturated lipids (having a lower melting point) in the membranes of mesophylic organisms make them more sensitive to high temperature during soil solarization than the thermo-tolerant. Heat sensitivity of organisms is related to an upper limit in the fluidity of membranes, beyond which the membrane function is reduced (Sundarum, 1986). Thermal death curve for fungal pathogens was shown to be logarithmic in nature (Pullman et al. 1981). Thermotolerant and thermophylic soil-borne organisms usually survive the soil solarization process (Brock, 1978; Stapleton and DeVay, 1984).
The thermal decline of soil-borne organisms during solarization depends on soil moisture, soil temperature and exposure time, which are inversely related. Soil moisture is a critical variable in soil solarization. Moisture not only makes organisms more sensitive to heat, but the transfer of heat to weed seeds and other living organisms in soil is greatly increased by moisture. As soil solarization is a hydrothermal process, and its success depends on moisture for maximum heat transfer, the heat maxima of soils increase with increasing soil moisture (Mahrer, 1979). Cellular activities of seeds and the growth of soil-borne micro-organisms are favoured by soil moisture, making them more vulnerable to the lethal effects of high soil temperatures associated with soil solarization.
The interaction between temperatures on soil moisture brings about cycling of water in soil during soil solarization. The upper soil layers (upper 5 cm) have a marked diurnal fluctuation in temperature, cooling at night and heating to high temperature during sunlight hours,. This diurnal fluctuation causes moisture in the upper zones in soil to move downward during the day as a result of solar radiation, while at night the soil surface cools causing an upward migration of moisture. As the soil solarization deepens in the soil, the movement of moisture becomes more pronounced, changing the distribution of salts and improving the tilth of the soil. A reduction in soil salinity resulting from soil solarization was reported (Abdel-Rahim et al. 1988). To maximize this effect in soil, pre-irrigation at the beginning of soil solarization should reach to a depth of at least 60-75 cm. In addition, efficiency of solarization is also influenced by soil type, soil colour and structure, soil moisture, thickness and light transmittance of the mulching material (plastic film), organic matter content, air temperature, length of day, intensity of sunlight, extent of heating, sensitivity of pathogens and pest species to heat, cropping history, and other components of soil ecology (Katan, 1987; Stapleton and DeVay, 1986).
The heat generated in soil by solar radiation and the resultant death of pests encompass the major principles of soil solarization. However, the increase in available plant nutrients and relative increase in populations of rhizosphere competent bacteria, such as Bacillus spp. (Stapleton and DeVay, 1984), which contribute to the marked increase in the growth, development, and yield of plants grown in solarized soil, are major components of soil solarization (Katan, 1985; Stapleton, and DeVay, 1986).
The increased availability of mineral nutrients following soil solarization are particularly those tied up in organic fraction, such as NH4-N, NO3-N, P, Ca, and Mg, as a result of the death of the microbiota (Baker 1962; Chen and Katan 1980; Stapleton et al. 1990; Stapleton et al. 1985). Extractable P, K, and Ca, Mg sometimes have been found in greater amounts after soil solarization (Stapleton et al. 1985). The liberation of N compounds (vapour and liquid) is a component of the mode of action, increased concentration of reduced N would then nitrify after termination of soil solarization to provide NO3 for increased crop growth (Stapleton et al. 1990).
The relative concentration of each is a function of soil pH, nitrifying micro-organisms (Hasson et al. 1977). High temperature during soil solarization in soil high in organic matter may kill much of the microbiota, including nitrifying microorganisms, thus favouring the accumulation of NH4-N. On the other hand, low temperature in soil with little organic matter would allow the survival of soil biota and promote aerobic conditions, with minimal liberation of nitrogenous compounds, resulting in nitrification and loss of N from the soil, as NO3 is easily leached from the soil.
SOLARIZATION AND BIOFUMIGATION
Soil solarization also includes changes in soil volatile compounds (Stapleton and DeVay, 1986). Different types of organic matter, such as animal manure and crop residues could be combined with soil solarization to perform biofumigation so as to increase soil temperature resulting from the heat of decomposition of these materials in the soil and to increase the heat-carrying capacity of the soil (Gamliel and Stapleton, 1993). During the process of solarization, biotoxic volatile compounds are released when organic matter is heated (Stapleton, 1997). Organic amendments, particularly plant residues and animal manure, augment the biocidal activity of the biofumigation through the production of biotoxic volatile compounds emanating from the decomposition of organic materials (Gamliel and Stapleton, 1993; Gamliel and Stapleton, 1997). Many volatile biotoxic compounds have been produced during the decomposition of cabbage residues, specifically during the first three weeks of soil solarization (Gamliel and Stapleton, 1993a).
Treatment of soil with organic and inorganic NH4- based fertilizers and /or soil solarization was active against natural populations of Pythium ultimum, and Meloidogyne incognita in the soil. Combining fertilizer with soil solarization on reduced Verticillium dahliae in some instances. Composted chicken manure alone at 5381 kg/ha significantly reduced Pythium sp., and when combined with heat (42°C), the Pythium population was eradicated (Stapleton et al. 1990).
SOIL SOLARIZATION TECHNIQUE, OPPORTUNITIES AND LIMITATIONS
Soil solarization is normally carried out during the hottest period of the year - meaning the summer months in the Northern Hemisphere. It includes proper soil preparation for normal cropping, i.e. the soil should be irrigated, and then ploughed whenever the soil tilth allows. Large clods should be broken up, and rocks, weeds, debris or any other objects that will raise or puncture the mulch should be removed. The soil surface should be smoothed and very well- levelled prior to mulching. Plastic sheets of the desired specifications are then laid (by hand or machine) with the plastic edges anchored firmly by burying in trenches surrounding the treated area. Plastic is laid either in complete coverage, or strip coverage where the planting rows only are to be treated. If heavy machinery is used to lay plastic, soil must be dry enough to avoid compaction. Additional irrigations are required every 2-3 weeks to keep the soil moist during the period of soil solarization (a 6-week period is recommended in many places, including Jordan). Post-mulch irrigation may be carried out via installed drip lines or via furrows which are made prior to laying the plastic. Farmers in the Central Jordan Valley usually irrigate twice weekly, but only in very small amounts. In total, only about 103 m3 are applied on average per plastic house of 500 m2 (2000 m3 per ha) for the entire period of about 40 days. This is almost the same amount needed for methyl bomide application, where the farmers use about 100 m3 on average (Barakat et al. 2001). The plastic sheets may be removed before planting, or the plastic may be left on the soil as mulch for the following crop by planting/ transplanting through openings perforated in the plastic. In this case, solarization with black plastic is used.
Soil solarization is a non-pesticidal method, not hazardous to the user, and does not transmit toxic residues to the consumer. It is easy to train farmers to apply it. The produce will be pesticide-free (if no other pesticides are used), and may command a high market price. It can be integrated with IPM and provide control of many soil-borne pests. Soil solarization can be applied manually or by machines, thus it is suitable for both developed and developing countries.
The cost-effectiveness of soil solarization should take into account the short- and long-term effects of the treatment on the agro-ecosystem (soil-borne pest management, yield increase, improvement of soil nutrient levels and other soil characteristics), as well as the opportunities that soil solarization can offer for economic pest control. Some of these opportunities are outlined by Elmore (1990) as the following:
in a crop where there is no available pesticide because of lack of registration, availability, crop tolerance, hazard of application from a pesticide or expense;
in a crop where there are pest problems that do not allow control by other means;
in cases where more than one pest problem can be solved with soil solarization;
where a crop is grown organically;
where soil solarization can change the cropping sequence or culture to increase yield on the same area or maintain yields on smaller areas;
in a crop where early seedling vigour and rapid growth is an advantage;
competition in markets, where ‘organic’ foods compete with conventionally produced pesticide treated products;
better environmental impact.
However, soil solarization involves limitations and difficulties. It can only be used at certain times and in hot climates when the soil has to be crop-free for the soil solarization period. It may be less effective in cooler regions and it could be expensive. Its applicability is limited to certain systems, i.e. irrigated vegetables and orchards. It is not applicable for pest control in agronomic crops grown in large areas that are rainfed in arid or semi-arid environments.
Other limitations include:
the need for crop-free fields for relatively long periods (1-2 months) during summer months;
the shortage of supplementary irrigation water during soil solarization;
the survival of pathogens at deeper layers in the soil;
possible pollution from plastic residues after the treatment termination;
lack of adequate machinery for large scale soil mulching in developing countries.
some pests are not controlled or difficult to control by this treatment;
no pest control in the furrows between the solarized strips;
high wind and animals may tear the plastic.
SOLARIZATION AND PEST MANAGEMENT
Various studies have been conducted on the effectiveness of trapping solar energy by polyethylene mulching of moist soils during the periods of highest air temperature and bright sunlight in order to increase the soil temperature sufficiently to kill soil pests (Katan et al. 1976; Braun, 1987; Abu-Irmaileh, 1991; Chen and Katan, 1980). Soil solarization proved effective, environmentally safe, and applicable to various agricultural situations for the control of various soil-borne pests, including phytopathogens and weeds. Two international conferences have been held on soil solarization; and the proceedings cover various aspects of soil solarization, applications and limitations (DeVay, Stapleton and Elmore (eds.), 1991; Stapleton, DeVay and Elmore (eds.), 1997).
Verticillium and Fusarium wilts of several crops, as well as other plant diseases, have been controlled successfully by soil solarization. However, the success has been poor for the control of some pathogens caused by other fungi including some species of Pythium, Fusarium, Sclerotium roflsii, and some heat-tolerant pathogens (Stapleton and DeVay, 1986). Post-plant soil solarization controlled Verticillium wilt of Pistacio (Ashworth and Gaona, 1982).
Populations of soil-borne nematodes were reduced by soil solarization to varying degrees (Stapleton and DeVay, 1986; Abu Gharbieh et al. 1990). Populations of Pratylenchus thornei were greatly suppressed by soil solarization (Greco et al. 1990). Control of Meloydogyne spp. by soil solarization was increased when the soil was treated with systemic nematicides in sandy soils (Osman, 1990).
Compared with clear polyethylene, black polyethylene containing carbon black, absorbs solar radiation, and thus reduces the heating of soil by several degrees. The average maximum temperatures over the entire period of soil solarization at 10 cm depth were 46.2°C and 45.7°C under 0.06 mm thick CPE and BPE, respectively, and 41.8° C in the un-mulched soil (Barakat, 1987). Thinner films are more effective in soil heating and are more cost effective (Stapleton and DeVay 1986). In soils covered with clear polyethylene (CPE) mulch, the highest temperature at 10 cm depth was 52.4°C under 0.04 mm thick CPE, and 47.9°C under 0.08 mm thick CPE during the period August 12 to October 16, 1986 in the Jordan Valley (Abu-Irmaileh, 1991a, 1991b). However, black polyethylene mulch is more stable and lasts longer under field conditions (Anonymous, 1984; Dubois, 1978; Hancock, 1988; DeVay, 1990). Mulching soils with black polyethylene film reduced the population of many soil phyto-pathogenic fungi, such as southern blight of tomato and dwarf bean caused by Sclerotium rolfsii (Reynolds, 1970); lettuce head drop caused by Sclerotinia minor (Hawthorn, 1975) and rots of lettuce caused by Rhizoctonia solani and bacteria (Hillborn et al. 1957). The complex of changes which occur in solarized soil may persist for at least two years (Pullman et al. 1981).
One of the visible results of soil solarization is the control of a wide spectrum of weeds. Hence soil solarization is suggested as a method for weed control. However weed responses to soil solarization have varied. Soil solarization effectively reduced the rate of weed-spread early in the season, but gradually the effect declined towards the end of the season. Soil solarization controlled effectively both annual weeds and the parasitic weed Orobanche. However, perennial weeds were more tolerant to soil solarization. Many tolerant weeds were not controlled by solarization with CPE without further mulching. Solarization with CPE or BPE followed by soil mulching with BPE were the best treatment for controlling weeds. Soil solarization using BPE mulch is recommended for almost complete weed control through the rest of the growing season. In this case, the same mulch is not removed, but perforated at the required distance. Planting crop seedlings could be done through the perforations in the mulch. Soil disturbance after soil solarization reduced the level of weed control (Abu-Irmaileh, B.E. 1991a). Post-plant soil solarization using BPE controlled weeds in newly-established fruit trees and increased seedling growth of almonds, olives and grapes, but soil solarization with CPE brought about severe injuries to grape seedlings (Abu-Irmaileh, 1994). Soil solarization for 2-4 weeks almost completely prevented the emergence of many annual weeds (Digitaria sanguinalis, Malva, Echinochloa, Chenopodium, Amaranthus retroflexus, Solanum nigrum up to 4 cm (Elmore, 1983). Weeds seed germination after soil solarization was much decreased in the top layer and was increased with sample depths (Horowitz et al. 1983). Heat sensitive weeds were killed with shorter period of solarization, and weed seeds were killed at deeper soil layers, compared to heat tolerant weeds (standifer et al. 1984). Conyza and Malva were relatively more tolerant to soil solarization (Horowitz et al. 1983). Dormant weed seeds (Egley, 1983) and seeds buried at deeper layers (Horowitz et al. 1983; Rubin and Benjamin, 1984, Standifer et al. 1984) escaped the soil solarization effect. Bindweed, (Convolvulus arvensis L.) emerged in plots that were solarized by BPE. Purple nutsedge, C. rotundus L. survived 80°C for 30 min, while the rhizomes of Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers. were sensitive (Rubin and Benjamin, 1984). Soil solarization effectively controlled broomrapes, Orobanche spp. (Jacobson et al. 1980; Abu-Irmaileh, 1991b), but Cuscuta species were tolerant (Abu-Irmaileh and Thahabi, 1997). Soil solarization reduced dodder seed germination laid on soil surface (Haidar and Iskandarani, 1977). Weed seed bank in the soil was tremendously reduced by soil solarization. The effect could be through one or a combination of the following mechanisms: direct killing of weed seeds by heat, indirect microbial killing of seeds weakened by sub lethal heating, killing of seeds stimulated to germinate in the moistened mulched soil, and killing of germinating seeds whose dormancy was broken.
SOIL SOLARIZATION AS A COMPONENT OF INTEGRATED PEST MANAGEMENT (IPM)
In addition to the lethal effect of radiant energy on weed seeds and other pests in the soil, soil solarization, is an ecofriendly method of weed management, and may be considered as a replacement to soil fumigants such as Methyl Bromide (MeBr), which is toxic, expensive, and causes pollution to the environment (Saghir, 1997). With the impending prohibition on the use of MeBr as a pre-plant fumigant, increasing emphasis will be placed on the system of integrated pest management (IPM) programmes for the management of pathogens, nematodes and weeds, which will foster additional opportunities to utilize soil solarization. The factors which determine the utilization of soil solarization in IPM include its compatibility with standard production practices and other pest management tactics, efficacy against selected pests, cost effectiveness, and synergistic interactions with other pest management tactics (Chellemi, 1997).
Soil solarization has been effectively combined with biological control agents including Talaromyces flavus, Trichoderma harzianum Rifai, and the vesicular orbuscular (VO) mycorrhizal fungus Glomus fasciculatum, to control plant diseases (Eldad et al. 1980; Tjamos and Fravel, 1995). Synergistic interactions have also been observed among soil solarization and biological control agents (combining sublethal heating in the laboratory with the application of Trichoderma harzianum improved the control of Rosellinia necatrix in soil and in apple orchards), organic amendments, and chemical fumigants. Acceptance of soil solarization as a pest management with site- and pest- specific activity will facilitate its integration into IPM systems.
Soil solarization began in Jordan as early as 1978 as a research topic for graduate students (Al-Raddad, 1979). Several research topics proved its effectiveness as an environmentally safe means of management of various soil-borne pests, including phytopathogens, nematodes, parasitic flowering plants and weeds (Abu-Irmaileh, 1991a and 1991b; Barakat, 1987; Abu-Irmaileh, 1994; Abu-Irmaileh and Thahabi, 1997). As the technique proved to be applicable at the farm level, the transfer of this technique started in the southern Jordan Valley during the late 1980s for the control of Orobanche and weeds in vegetable crops. At present, the estimated adoption of this technique covers about 40 percent of the cropped area in Jordan valley, especially where drip irrigation and mulching with black plastic is available. It is replacing methyl bromide in protected agriculture, i.e. in plastic houses and tunnels.
Since 1998, Jordan has received support from the Multilateral Fund for the Protection of the Ozone Layer, under the Montreal Protocol. Soil solarization was recognized then as a viable alternative to MeBr. Its adoption was promoted strongly in the Central Jordan Valley, where the use of MeBr was most intensive. The technology has been readily adopted by farmers, and even in a drought year such as 2001, approximately 75 percent of farmers applied soil solarization (Hasse, 2001).
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