Third-year field trials are under way. We found that organic growing techniques did not do well in soils that had been fumigated in successive years, cultivated, left unfumigated, and planted with the disease-susceptible strawberry varieties commonly used in California. This scenario spells disaster for farmers regardless of how much they want to adopt nonchemical growing practices. Sances tested alternative chemical treatments as well as promising organic mixtures of broccoli, high rates of compost, and then applied mycorrhizal bacteria to treated soil. The conventional alternatives obtained high fruit yields, he says, but the organic material could not regenerate the soil to the extent needed to suppress root pathogens and achieve commercially acceptable yields.
Soil reconditioning takes time.
Repeated treatments with fumigants kill harmful, disease-causing organisms in the soil. But they also eliminate beneficial microorganisms and species complexity, and cause the soil to lose some of its ability to retain nutrients and water and resist erosion. For these reasons, Sances recommends adding additional organic matter and beneficial microorganisms to both fumigated and organic-supplemented fields. Local strawberry growers cooperate in Sances' project to incorporate broccoli residues, mushroom compost, and a combination of both into the soil. In fields treated with organic residues, weeding costs were five times more than in those chemically treated.