In a nutshell: almond hull and shell organic matter amendments increase soil and tree potassium status

Background Crop residues used as organic matter amendments have been shown to release potassium (K) into the soil, promoting K cycling in agronomic systems. Orchard field trials are needed to evaluate K dynamics under almond hull and shell amendments, which contain high K concentrations. Methods Three field trials in commercial almond orchards were conducted to assess the effects of surface-applied almond hull and shell amendments on K cycling within plant and soil systems. Amendment K concentrations over time, soil exchangeable K, and tree K status were measured as well as decomposition rate and crop yield. Results Hulls and shells released K rapidly under irrigation and rainfall, significantly increasing soil exchangeable K in the upper 0–10 cm soil within 2–7 weeks. Amendments increased tree leaf K status within the first 1–3 years to varying degrees depending on site. Initial amendment K concentrations decreased by at least half by dry weight within the first 25.4 cm (10 inches) of water (irrigation and precipitation) within the irrigated zone. Conclusions Almond hulls and shells can increase soil and plant K status when used as amendments on the soil surface. This practice can address byproduct utilization issues, recycle potassium (K), and reduce orchard K fertilizer demand by replacing the majority of tree K demand. Growers can tailor application rates to meet orchard-specific K management goals. Off-ground harvest preserved the hull/shell organic layer over time and maximized K cycling. Hull/shell amendments applied on the soil surface cover more soil area within the irrigated wetted zone compared to banded K fertilizer. This practice can reduce reliance on K fertilizers and reduce associated costs while providing a convenient outlet for hulls and shells.