UC Davis

Regenerative agricultural practices strive to produce food that have lower economic, environmental, and social impacts defined by the processes used (i.e. using cover crops, reducing tillage) or by proposed outcomes (i.e. improving soil health, improving biodiversity). In almond orchards the regeneration of ecosystem health and productivity can be achieved through practices including recycling of almond hull and shell resources as soil amendments, reducing dust production during harvest, and minimizing synthetic fertilizer inputs. The current and set standards of conventional almond production poses a unique challenge when proposing regenerative agriculture goals. To facilitate almond harvest and simplify irrigation and fertigation practices, almond orchards are typically managed with the orchard floor bare, and trees are grown in rows with irrigation and fertilization localized exclusively to the 3.6 meter (12-ft) wide soil strip below the tree canopy. The alleyways are often left unirrigated and bare to facilitate almond harvest. These conventional practices reduce soil health, whereas with the advent of off-ground harvesting, opportunities exist to incorporate cover crops (CC) and the use of organic matter amendments (OMA) including almond hulls and shells (AHS) to maintain a more permanent mulch layer to protect the soil. This thesis examined the changes in soil physical, chemical, and biological properties from the use of AHS and a CC mix of 60% oats, 35% spring peas, and 5% yellow mustard in a randomized complete block design field trial. Nutrient cycling and OMA decomposition dynamics was analyzed one and two years after practice adoption. Effects on soil aggregate stability, particulate organic matter (POM), mineral associated organic matter (MAOM), microbial biomass, and soil moisture in amended trees harvested using on-ground and off-ground harvest machines were determined. Results showed increased C and N in microaggregate and macroaggregate soil organic matter fractions, increased saprophytes and arbuscular mycorrhizal fungi, and improved soil moisture in amended soils. Exchangeable potassium (XK) and soil organic matter (SOM) increased in the amended on and off-ground harvest treatments and was highly correlated with microbial communities measured. We found that after 3 years of AHS OMA and catch-frame harvest, MAOM and soil aggregate stability increased due to this undisturbed forest litter layer which may allow more microbial processes to contribute to more MAOM over time. Cover crop biomass was highest in oats but peas contributed the highest soil N. 192 days after AHS application, an average of 38% of the AHS material decomposed and 90 days after the CC were seeded, an average of 69% of the CC material decomposed.