Organic Farming

Abstract: Soil solarization using clear plastic is a promising weed management strategy for organic farms in the Northeast USA. Based on grower concerns that the practice might negatively affect beneficial soil microbiota, we conducted experiments to measure the effects of 2 and 4 weeks of solarization in a field and a closed greenhouse. Soil microbial communities were assayed by dilution plating on semi-selective agar media. Populations of general bacteria, general fungi, bacilli, and florescent pseudomonads were unaffected by field solarization, but fluorescent pseudomonads were reduced following greenhouse solarization. At plastic removal, soil biological activity was reduced non-significantly in the field and by 45% in the green- house. Soil biological activity fluctuated following field solarization, being significantly suppressed at 5 but not 14 days after plastic removal. In the greenhouse, biological activity remained suppressed up to 28 days after plastic removal. Solarization increased available nitrogen in the field and greenhouse. Four weeks of solarization reduced viability of buried weed seeds by 64% in the field and 98% in the greenhouse, indicating that the practice can cause substantial weed seed mortality. Maximum soil temperatures, measured at 10 cm depth under solarization, were 44◦ C in the field and 50◦ C in the greenhouse; temperatures were theoretically sufficient for the reduction of some soil borne pathogens. A subsequent experiment measured the effects of solarization and tarping (black plastic) on soil biological activity. During mulching, biological activity was unaffected by treatment, but 14 days after plastic removal, biological activity was reduced in the solarized treatment as compared with the control. Overall, these results suggest that solarization can deplete the weed seedbank. Although soil biological activity was reduced by solarization, it may bounce back after a period. Greenhouse solarization achieved higher temperatures and was more lethal to weed seeds and some microbiota than field solarization.