A four-year field trial in Iran’s semi-arid almond-growing regions has found that wood vinegar offered the best value for money in improving soil health and nut production under rainfed conditions, delivering substantial benefits at a fraction of the cost of multi-component treatments.
Published in Scientia Horticulturae (2026), the study by Tadayon and colleagues addressed a practical constraint facing many rainfed almond growers: compost availability is often limited in rural areas, and transport costs can be prohibitive for mature orchards operating on tight margins. The researchers tested whether wood vinegar, either alone or combined with locally-produced biochar and microbial inoculants, could offer a viable alternative for rehabilitating degraded orchard soils.
The trial was conducted in a mature rainfed almond (Prunus dulcis Mill.) orchard from 2022 to 2025. Researchers evaluated wood vinegar application both as a standalone treatment and as part of a combined treatment that also included almond shell biochar at 2.5 tonnes per hectare, arbuscular mycorrhizal fungi (AMF), and plant growth-promoting rhizobacteria, comparing both against standard mineral fertilization alone.
Wood vinegar application was associated with marked improvements in soil microbial activity compared to mineral fertilization alone: soil respiration increased by 71% in the top 30 cm, microbial biomass carbon increased by 86%, and AMF colonization increased by 85%. Rhizobacteria populations increased threefold. These soil improvements were accompanied by substantial gains in tree physiology: chlorophyll content increased by 38%, stomatal conductance by 63%, and relative water content by 20%. Leaf temperature decreased by 5.3°C and electrolyte leakage—an indicator of drought stress—decreased by 45%.
Nut yields under wood vinegar treatment increased by 48% compared to mineral fertilization alone. Kernel weight increased by 18%, shelling percentage by 15%, and oil content by 10%. Shoot length increased by 52%, shoot diameter by 38%, and leaf number per shoot by 50%. Wood vinegar’s cost-effectiveness makes these yield improvements particularly relevant for growers facing tight margins in rainfed systems.
The researchers also tested wood vinegar as part of a combined treatment with biochar (produced locally from almond shell waste), AMF, and rhizobacteria, which showed additional improvements but at higher input costs.
The study wasn’t designed to isolate wood vinegar’s mechanism of action. The researchers acknowledge that further investigation is needed to determine whether wood vinegar’s effects operate independently or through interactions with soil microbial communities and plant physiology.
The findings suggest that wood vinegar offers a cost-effective option for improving soil function and yield stability in rainfed almond orchards under semi-arid conditions, particularly where compost-based rehabilitation faces logistical or economic barriers.
Full citation: Tadayon, M.S., Mousavi, S.M., Hosseini, S.M., & Sadeghi, S. (2026). Enhancing drought resilience and kernel yield in rainfed almond orchards: Biochar-based management strategies. Scientia Horticulturae, 366, 115084.
Full paper: https://doi.org/10.1016/j.scienta.2026.115084
