2017 ASHS Annual Conference

Anaerobic soil disinfestation (ASD) in an increasingly common non-fumigant soil treatment technology in plasticulture strawberry production in California. It relies on 3-4 weeks of anaerobic degradation of added labile carbon source mixed into soil and covered by plastic mulch. Following ASD, the holes in plastic are punched, soil is aerated and strawberry planting can begin. Changes in microbiological, chemical and physical soil environment during and after ASD have consistently improved strawberry fruit yields and suppressed pathogens such as Verticillium dahliae. High costs of rice bran, a standard carbon source for ASD prompted investigation of more affordable locally available alternatives for ASD in southern California. In trials at Santa Paula (2015) we evaluated ASD with coffee grounds, brewery waste and grass clippings incorporated to soil at 9 t (dry weight) /acre. All of these carbon sources are by-products of local industries and are available within 20 miles of application sites in large quantities. All carbon sources provided expected anaerobic conditions, and fresh grass clippings generated strongest anaerobic conditions surpassing 120,000 cumulative Eh mV hours of anaerobiosis, more than twice the threshold for suppression of V. dahliae. Nitrate nitrogen availability in plots with grass clippings was also greatest among treatments. Plants in all ASD plots (no pre-plant fertilizer) grew more rapidly than in untreated control (pre-plant fertilized with 500 lbs of 18-6-8) and fresh market fruit yields were improved 47% with grass clippings, 51% with brewery waste and 83% with coffee grounds. Due to ease of application and availability of coffee grounds from a different supplier at no cost, we evaluated ASD with coffee grounds in 2016 on a larger scale. Even though anaerobic conditions were sufficient, strawberry plants were stunted and grew poorly in soil with coffee grounds. Analyses showed that the nitrate nitrogen was immobilized by these coffee grounds from soil and severe nitrogen deficiency in strawberry leaf tissues. This was opposite of the effect of grounds used in 2015 which provided nitrate nitrogen during strawberry growing season similar to synthetic fertilizer. Additionally, laboratory tests of coffee grounds used in 2016 showed twice water holding capacity and ash content compared to those used in 2015, while all other parameters were similar among the two roasting suppliers. This work shows that consistencies in both physical and chemical characteristics are essential in reliably successful carbon sources for ASD.