Almond processing residues as a source of organic acid biopesticides during biosolarization

•Feasibility of almond by-products was demonstrated for use in biosolarization.•Treated soils experienced pH depression and organic acid accumulation.•Residue application to soil resulted in 84–100% nematode inactivation.•Both endogenous and fermentative organic acids likely contribute to efficacy. Biosolarization utilizes organic amendments to produce biopesticide compounds in soil that can work in tandem with other stresses to inactivate agricultural pests. The prospect of using by-products from industrial almond processing as amendments for biosolarization was assessed. Soil mesocosms were used to simulate biosolarization using various almond by-products, application rates, and incubation times. Several potentially biopesticidal organic acids were identified and quantified in the soil, and the toxicity of soil extracts was evaluated for the root lesion nematode (Pratylenchus vulnus). It was determined that both almond hulls and a mixture of hulls and shells harbored several acids, the concentration of which was enhanced 1–7 fold via fermentation by native soil microbes. Organic acid concentration in the soil showed a significant linear relationship with the quantity of waste biomass amended. Extracts from soils containing at least 2.5% incorporated biomass by dry weight showed a 84–100% mortality of nematodes, which corresponded to acid concentrations 0.75 mg/g (2.0 g/L) or greater. This study showed that almond processing by-products – hulls and a hull and shell mixture – were suitable amendments for control of P. vulnus and potentially other soil agricultural pests in the context of biosolarization.