may-jun-2020

compounds — like methyl bromide — are coming under increased regulatory scrutiny. Ultimately, soil fumigation alternatives are needed, not just for almonds but for many other tree nuts, fruits and vegetables. Enter anaerobic soil disinfestation (ASD), also called biosolarization. This practice originated in Japan and the Netherlands, and in the United States one of the first assessments of its utility took place in the strawberry industry. “I was at a conference nine years ago when I first heard about ASD. It was the coolest thing I learned at that event,” said Gabriele Ludwig, director of Sustainability and Environmental Affairs at ABC. Substrates, Saturation and a Tarp Here’s how ASD works: After an old orchard and its roots are removed, and the ground is ripped, tilled, leveled, etc., a source of readily available carbon (via a substrate) is tilled into the soil. Immediately afterwards, a clear tarp cover is applied over the orchard and the entire soil profile is saturated with water. Typically, drip irrigation is used, and Browne recommends up to six irrigation lines per row, with emitters spaced about one to two feet apart. High soil moisture is then maintained by delivering additional water to the soil several times per week for several weeks. The carbon and associated nutrients in the soil provide a food base for microorganisms, while the wet soil and the heat-and-vapor-retaining tarp accelerate microbial activity, consuming oxygen and creating anaerobic conditions. Under these conditions, microbes generate organic acids, volatiles and additional conditions toxic to plant pathogens. “You’re essentially removing the oxygen and changing the soil ecosystem, temporarily,” Ludwig explained. Browne’s lab has tested several ASD carbon substrate sources, with particular focus on rice bran and ground almond hulls and shells, using a typical substrate rate of nine tons per acre. Among Browne’s ASD trials at multiple sites in the San Joaquin Valley, the best and most consistent results were obtained by tilling rice bran substrate into the soil and applying water under a tarp for four to six weeks. That said, in some trials the ground almond hull and shell mixture worked just as well as rice bran. In fact, research led by Dr. Chris Simmons’ lab at UC Davis demonstrated, both in the lab and in the orchard, that ASD with almond hull and shell substrate can generate organic acids that contribute to control of replant pests. Also, in certain trials the rice bran substrate incorporated in the soil stimulated orchard growth equally both with and without tarp and water applied. In addition, each ASD substrate varies in nitrogen, potassium, phosphorus and other nutrient contents. This reality, along with other factors such as treatment timing, native soil nutrient levels and the presence of Whole Orchard Recycling residues, likely impact ASD results at any given site. As a next step to this research, Browne is working with colleagues Brent Holtz, Mohammad Yaghmour, Phoebe Gordon, Amisha Poret-Peterson and others to learn more about how soil fertility impacts PRD and its remediation with ASD. Team members hope to improve the reliability of ground almond hull and shell coproducts as ASD substrates due to their wide availability and relatively low cost. 3 5 M A Y – J U N E 2 0 2 0