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Saying Goodbye to Soil Fumigants In Strawberry Fields

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2 Saying Goodbye to Fumigants In Strawberry Fields 800.961.1220 With methyl bromide no longer an option, strawberry growers have moved to fumigation options that include combinations of chloropicrin (a toxic air contaminant that has mandated buffer zones, waiting periods for tarp removal, and monitoring requirements), 1,3-dichloropropene, and metam sodium. One commonly used combination product has a label that includes 45 pages of regulations, use restrictions, and safety precautions. Because these chemicals are less volatile than methyl bromide, they can be applied through drip irrigations systems—an approach used on over 55% of California strawberry acreage. Applying the chemicals via irrigation eliminates the need for workers to be out in the fields while the chemicals are being applied, but it does require more specific soil preparation and application management to ensure thorough coverage. There is also the risk that the chemicals can damage irrigation PVC pipe if not thoroughly flushed after application. Strawberry growers are willing to go through this complicated and dangerous process because of the promise of greater yields, and it is easy to understand why. Strawberry growers saw their yields nearly quadruple in the decades after fumigants were first used. Many growers attributed this tremendous yield success to fumigant use, and they remain convinced that they cannot achieve commercially competitive yields without it. Advances in Understanding Soil Microbiology In addition to the long-known negative health effects of soil fumigants on humans and other living things, more recent developments in the understanding of the importance of biodiversity in crop soil has led many to believe that the use of soil fumigants damages nutrient cycling and the long-term sustainability of soil fertility. Fumigants kill almost everything in the soil, including the beneficial bacteria, fungi, and other micro- and macro-organisms that keep the soil healthy and fertile. In recent years, soil scientists have developed a much better understanding of the plant-microbial interaction: in exchange for what they want, some soil microorganisms break down nutrients and provide them to plants in a form that the plants can easily use; others help protect plants from many types of diseases and predators, as well as create a soil structure that is beneficial to the absorption and flow of necessary oxygen, carbon dioxide, and water—improving plant respiration and hydration. When soil fumigants sterilize the soil, they not only kill off all the beneficial microorganisms, they also set up conditions that make the soil and crops more vulnerable to future pest invasions. In healthy soils, biodiversity helps to control damaging pests by giving them competitors that "out-eat" and "out-survive" them—a process referred to as "competitive exclusion." When the good competitors are killed off in fumigation, the door is left wide open for the damaging pests to come back stronger than ever; hence, the need for year- after-year fumigation applications. Also, without the beneficial microorganisms being available in sufficient numbers to provide nutrients to plants in a form that they can easily consume, growers are forced to increase the amount of fertilizers they use to sustain previous yield levels—increasing inputs but decreasing productivity. Despite the concerted effort to eliminate disease, strawberry pathogens have continued to evolve. In the past decade or so California has seen a devastating rise in two soil-borne fungi pathogens that were previously not problematic for strawberry growers: Macrophomina phaseolina and Fusarium oxysporum f. sp. fragariae. (More about these two later.) Sustainable Alternatives Soil fumigants have been tantalizingly easy for growers to use because one application process has so many good immediate outcomes—such as weed control, nematode control, disease suppression, etc. So, with fewer effective soil fumigants available and costs rising to meet the restrictions associated with fumigant application and use, what alternatives do responsible growers have if they want to grow premium crops in a cost-effective and sustainable way? They begin by providing sound soil stewardship that puts the soil microorganism ecology, or "microbiome," back in balance.

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