UC Davis

Returning almond hulls and shells to the orchard soil as organic matter amendments can provide numerous soil, water, and plant benefits. Billions of pounds of almond hulls and shells accumulate at California almond processors annually and need a convenient and sustainable outlet. As hulls and shells contain substantial potassium (K) concentrations, surface-applying these materials as organic matter amendments onto nearby almond orchard soil could reduce K fertilizer costs for growers by around 80% by retaining plant K in the orchard. Prior studies have found many crop residues including nutshells release K readily under water application, can improve soil-plant water dynamics, and support higher levels of soil microbiology. However, field trials characterizing the effects of nutshell amendments on K cycling, soil-water dynamics, and soil microbial responses are scarce. Research is needed to evaluate K release from nutshells and impacts on soil K availability and tree K status in commercial orchards. The effects of surface-applied nutshell amendments on soil-plant water dynamics and impacts on soil microbial community composition over time merit further research to assess potential benefits to soil and plant function. Three field trials were conducted to evaluate the effects of this practice on: hull/shell K solubilization, soil K availability, plant nutrient status, yield, soil-plant water dynamics, decomposition rates, and soil microbial functional community composition. Research questions were investigated in field trials established in 2020 in mature commercial California almond orchards. All field trials are randomized complete block designs with each treatment applied to the entire row (at least 40 trees per experimental unit). Each site was approached as a case study with distinct research questions and treatments tailored to grower priorities and current research gaps. This research was funded through a Western SARE grant. Results indicate surface-applied almond hull and shell organic matter amendments can increase K cycling and plant K status, reduce soil surface evaporation, maintain higher moisture in the upper soil layer, moderate plant water stress during dry periods, and increase microbial biomass of many beneficial functional groups in the soil and the amendment layer including arbuscular mycorrhizal fungi. Off ground harvest machinery can be used to maintain the microbially rich amendment organic layer on the soil surface in the tree row and release a more complete percentage of total amendment K. Findings can be used to support grower decision-making, practice implementation, and future research across different orchard contexts and management approaches.