Guest contributors: Jaacov Katan and Abraham Gamliel
Soil solarization (also referred to as solar heating of the soil in early publications) is a new method for disinfestation of soil, first described in 1976 by Katan et al, for controlling soilborne pathogens and weeds, mostly as a pre-plant soil treatment. It was opened to scientific examination and criticism via a publication in Phytopathology in 1976.
Soil solarization is achieved by covering (mulching, tarping) the soil in a moist condition with transparent polyethylene during the hot season, thereby heating it and killing the pests. The soil is pasteurized but not sterilized. The 1976 publication described in detail the method, its principles and potential to control disease, insect pest, nematodes and weeds under field conditions.
The Origin of Soil Solarization and its Expanded Use
The concept of soil solarization could only have evolved following the advances in plasticulture development, especially in plastic mulch technology that began two decades earlier. The basic approach of using the sun to heat plastic-mulched soil for pest control has taken off since then and has been adopted based on fundamental research accompanied by extensive implementation under various cropping systems and in different regions. It has also been used to clean up diseases and weeds in gardens, golf putting greens, and many other applications.
Over the years, solarization has been assessed as a control method against a wide range of soilborne pests, including plant pathogenic fungi (such as Verticillium, Fusarium, Pythium), pathogenic bacteria, plant parasitic nematodes, weeds and arthropod pests. There are also cases in which effective control has not been achieved, possibly because of heat-tolerance or -resistance of the pathogen or weed seeds.
Adoption of solarization has increased significantly since 1976, despite the fact that this technology does not enjoy industry support. On the other hand, the methyl bromide crisis and its phase-out have initiated new opportunities, especially with intensive cropping practices. One outcome is that soil solarization (alone and especially when combined with other practices such as adding organic amendments, e.g. certain crop residues, oil meal products, compost, and animal manure) has become an important player in the soil disinfestation arena.
Other Methods of Disinfesting Soils
Soil solarization is the third approach for soil disinfestation; the two other main approaches – soil steaming and chemical fumigation -were developed at the end of the 19th century. Soil fumigation was identified for many years with the fumigant methyl bromide. See also: Why the Increased Growth and Yield Response of Crops to Soil Fumigation? It comes as no surprise, therefore, that the phase-out of methyl bromide along with extensive efforts to find effective chemical and nonchemical alternatives for control of soil-borne pests has further emphasized the limitations of using available fumigants and other disinfestation methods as stand-alone approaches.
The last decades have seen adoption of the integrated pest management (IPM) philosophy for soil disinfestation, realizing that, beyond killing the pests, soil disinfestation should encompass economic, social, legislative, and environmental aspects. IPM in soil disinfestation represents a continuous process rather than a single action, and brings together the application of various control measures and concepts. Combining solarization with other pest-control measures is one of the important tools for IPM implementation and improvement in soil disinfestation.
Soil Solarization Used World-Wide and is Focus of Extensive Research
The various strategies of combining soil solarization with pesticides, organic amendments or beneficial microorganisms, including the integration of soil solarization into cropping systems under real farming conditions, has been extensively researched. Today, solarization is explored or implemented in more than 70 countries, and studies have been documented in over 1,400 research papers, mostly in the hot regions of the world. The studies demonstrated the effectiveness of solarization with vegetables, field crops, ornamentals, nurseries, and fruit trees against many pathogens, weeds, and soil arthropods, and in various cropping systems, including organic gardening and farming.
In parallel, the biological, chemical, and physical changes taking place in the solarized soil during and after the solarization process, interactions with other methods of control, and many other topics have also been investigated. Long-term effects, biological control and increased growth response similar to the IGR reported for soil fumigation were demonstrated for different climatic regions and soils. See also: Why the Increased Growth and Yield Response of Crops to Soil Fumigation? The results of these studies point to the involvement of general mechanisms in solarization, and the wide applicability of this approach to soil disinfestation.
First and foremost, heat kills the pathogens, weed seeds and other pests during the solarizatoin process. However, biological mechanisms are also involved. Frequently, populations of fluorescent pseudomonad bacteria, Bacillus sp., the antagonistic fungus Talaromyces, and other antagonists are increased following soil solarization, either in the soil or in the rhizosphere of the plants subsequently grown in that soil. See also: The Rest of the Story: Take-all Decline with Continuous Wheat Monoculture and First Field Test in the U.S. Pacific Northwest with a Genetically Modified Organism. Frequently, the solarized soil becomes more suppressive to disease caused by pathogens that may survive or be reintroduced. Phenomena of increased availability of mineral nutrients, e.g., N, K and Ca, as well as improved growth of the plant in the solarized soil, even in the absence of known pathogens, have been reported. Induced resistance to pathogens in plants growing in solarized soils has also been reported.
New Applications and Innovations
The use of solarization in existing orchards (e.g. to control Verticillium in pistachio, olive or avocado plantations) first reported as early as 1979 was an important improvement and deviation from the standard pre-plant method. Other expanded applications and advances include solarizing the soil in a closed greenhouse, using a double-layer mulch, and special types of improved plastic films. Sprayable plastic material has also been developed. Solar heating is also being developed to sanitize greenhouse structures and agricultural tools, disinfest water, control animal and human pathogens, solarize piles of growth substrate material and nurseries.
Computerized simulation models for predicting temperatures of solarized soils are now available and can be used as guidelines by researchers and growers interested in using solarization but are not sure whether the ambient conditions in their place are suitable for solarization. In addition, simulation models and approaches for predicting rate of thermal killing of pathogens and rate of pathogen control by solarization are available.
Soil solarization as a method for disinfestation of soil or other habitats where pests and pathogens reside is a nonchemical method with no known endangerment to the user or the environment. It is also simple for application and less expensive than many fumigation methods. Its major limitation is its climate dependency, namely, the need for several weeks of intensive sunshine during which time that soil cannot be occupied with crops. Although no major negative side effects of this method have been reported, such a possibility should not be excluded. Therefore, solarized fields should be continuously monitored, as should be done with all the disinfested fields.
It should be emphasized that soil solarization is not a magic cure for every illness but rather, it is one more tool for pest management that, under the appropriate conditions and application, can become an important additional tool to our rather limited arsenal for management of soil-borne pests.
For further information, please visit the following websites:
About the Authors:
Jaacov Katan, PhD
The Buck Family Professor Emeritus of Plant Pathology
Department of Plant Pathology and Microbiology
The Hebrew University
Faculty of Agriculture, Food and Environment
Rehovot 76100, Israel
Abraham Gamliel, PhD
Head, Laboratory for Pest Management Research
Institute of Agricultural Engineering
ARO, The Volcani Center, Bet Dagan, 50250 Israel
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DeVay, J. E., Stapleton, J.J. & & Elmore, C. L. 1991. Soil Solarizaiton, Proceeding, First International Conference on Soil Solarizaton, Amman, Jordan. FAO Plant Production and Protection Paper 109. FAO Rome.
Elmore, C. L., Stapleton, J.J., Bell, C. E., DeVay, J. E., and Hart, W. H. 1984. Soil solarization: A non chemical method for controlling disease and pests. Cooperative Extension, Division of Agriclture and Natural Resources: Leaflet 21377, University of California, Oakland, 14 pages.
Eshel D., Gamliel, A., Grinstein, A., Di Primo, P., and Katan, J. 2000. Combineed soil treatments and sequence of application in improving the control of soilborne pathogens. Phytopathology 90:751:757
Gamliel, A., and Katan. 1991. Involvement of fluorescent pesudomonads and other microorganisms in increased growth response of plants in solarized soils. Phytopathology 81:494-502.
Gamliel, A., and Katan. 2009. Control of plant disease through soil solarization. Pages 196-220 in Disease control in Plants: Biologically and Environmetally friendly approaches. D. Walter, Ed. Wiley-Blackwell, Oxford.
Gamliel A., and Katan. J. 2012. Soil Solarization: Theory and Practice. APS press St. Paul MN.
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