The Positive Influence of Compost and Cover Crops on Key Soil Health Indicators in Nut Orchards

Alexia Cooper , Rebecca Ryals, Jeffrey Borum

Sustainable management practices in nut orchards are crucial for enhancing soil health and minimizing environmental impacts associated with conventional production. Over three years, this study evaluated the effects of stacked soil health practices in five nut orchards (almonds and walnuts) in California. Experimental plots received annual applications of 19 Mg/ha of yard waste and manure compost, along with cover crops at a rate of 13.8 kg/ha, while conventionally managed plots served as controls. Compost and cover crops significantly improved all soil health indicators by year three. Microbial biomass carbon (MBC) increased significantly by 573 ± 98% in alleys and 307 ± 100% in berms, while microbial biomass nitrogen (MBN) showed significant increases of 487 ± 95% in alleys and 55 ± 14% in berms. Permanganate oxidizable carbon (POXC) rose significantly by 133 ± 23%. Short-term carbon mineralization rates were 70 ± 5% higher, reflecting enhanced soil biological activity likely from increased POXC. Cation Exchange Capacity (CEC) significantly increased by 25.7 ± 14.5% in alleys and 48 ± 16.9% in berms. Soil extractable ammonium (NH4+) levels increased rapidly following compost application, remaining significantly elevated for five months before aligning with control plots, whereas soil extractable nitrate (NO3-) levels did not significantly change. By the end of the study, the average percent increase in soil organic carbon (SOC) storage with the stacked practices was 102 ± 21%, resulting in an average SOC content of 60.7 Mg C ha⁻¹ across sites. The pH in compost + cover crops plots decreased slightly from 7.1 in 2021 to 6.8 in 2024, with an average change of -0.3 ± 0.3. Electrical conductivity (EC) increased significantly with an average rise of 445.3 ± 311.2 µS/cm. Improvements in wet aggregate stability were also significantly improved, with large macroaggregates increasing by 2.1 ± 0.5%, medium macroaggregates by 1.3 ± 0.2%, and small macroaggregates by 8.4 ± 2%. These findings underscore the critical role of compost and cover crops in enhancing soil health within conventional nut orchard management, independently from site factors such as soil type and management. The increase in SOC storage highlights the potential of orchard soils to serve as a significant carbon sink while supporting agricultural sustainability and resilience to climate change.