Physical Soil Health and Water Storage Dynamics of a Novel, Perennial Cover Crop in California Almond Orchards

This project builds on an NRCS Conservation Innovation Grant titled “Climate-Smart Irrigation for Drought, Fertility, & Structural Resilience in Almond Systems,” which established on-farm trials of Oakville Bluegrass across multiple commercial orchards in California’s Central Valley. Project team members—Kelsey Fenn, Dr. Mallika Nocco, Dr. Cristina Lazcano, and Jeff Borum—are collaborators on both the leveraged project and this grant, contributing to grower recruitment and trial implementation.

Research activities under this project are being conducted at three almond orchards participating in the Oakville Bluegrass trials. These sites span representative fine- and coarse-textured soils commonly found in the region (see Table 1 for site details). In fall 2023, Oakville Bluegrass was sown in the orchard alleys (tractor rows) at a seeding rate of 150 lbs per seeded acre. A control treatment—conventional bare ground management—was maintained in adjacent alley sections, reflecting grower-standard practices such as herbicide application and mowing.

Two of the orchard trials follow a randomized complete block design, with three to four replicate blocks of both the cover crop and control treatments. The third site was implemented as a split-orchard design (cover crop vs. control sections), with multiple observations per treatment to allow for pseudoreplication. Growers provided input on trial layout and sampling logistics to ensure compatibility with site-specific management and operations.

Table 1. Orchard site information for participating orchards in this study.

Research Objective 1: Evaluate the impact of perennial cover cropping on structural and hydrologic soil properties compared to bare ground management. 

Two replicate samples were collected and subsampled at two depths (0–15 cm and 15–30 cm) within each treatment plot. Bulk density was determined using the intact core method¹. Gravimetric water content and aggregate stability were analyzed on bulk samples. Aggregate stability was assessed using two complementary approaches: (1) a standard wet-sieving method for water-stable aggregates², and (2) a slaking index quantified via image analysis³—a method endorsed by the Soil Health Institute for its accessibility and comparable sensitivity to management practices.

Surface and subsurface compaction were measured using a handheld cone penetrometer and corrected for field soil moisture⁴. These metrics directly assess whether Oakville Bluegrass contributes to improved soil structure—by increasing aggregation, reducing surface hardness, and enhancing bulk density through greater root density and groundcover.

To evaluate soil water storage dynamics, plant available water (PAW) was measured using pressure plate extractors on the intact cores⁵. PAW was calculated as the volumetric water content difference between field capacity (-33 kPa) and permanent wilting point (-1500 kPa).

In fall 2024, water infiltration was measured using a tension disk infiltrometer at two locations per treatment block at each orchard. These infiltration tests were conducted prior to the onset of winter rainfall to assess treatment effects on saturated hydraulic conductivity and water movement. These physical and hydrologic measurements collectively address whether cover cropping improves soil water retention, infiltration, and overall plant-available water capacity compared to bare ground management.

To assess treatment effects, ANOVA was used to test for differences in individual soil health indicators between cover crop and control plots, followed by post hoc pairwise comparisons. Given that orchard soil properties evolve over time due to perennial crop management, we focus on comparing treatment differences within each sampling year rather than referencing static baseline measurements. We also tested for site-level interactions to account for variability in soil type and grower practices.

In spring 2025, the second year of sampling was completed across all three orchard sites using the same field methods as the previous year. Laboratory and statistical analyses are currently underway.

Figure 1. A diagram of the physical soil measurements and plant samples taken per block of each participating orchard. Measurements and samples will be collected in the middle of the alley at least 4 feet from the berm.

Research Objective 2: Quantify changes in soil carbon pools from cover cropping and evaluate carbon inputs from cover crop biomass, ground coverage, and plant C:N. 

During the spring 2024 field sampling events described under Research Objective 1, 80 plant biomass samples were collected from orchard alleys at peak Oakville Bluegrass growth to assess above- and belowground organic matter inputs. In each plot across the three orchards, a 0.1 m² quadrat was used at two sampling points to collect aboveground cover crop biomass or any existing vegetative groundcover. This method was recommended by the technical team at Vitidore, Inc. To quantify the area of groundcover, photographs were taken at each sampling point and processed using Canopeo, a mobile application that calculates percent canopy coverage from digital imagery⁶. Belowground biomass was collected by compositing four soil cores (0–30 cm) per aboveground biomass location, capturing the soil depth where most Oakville Bluegrass roots are concentrated. Roots were extracted using a root washing protocol, and fresh and dry weights were recorded separately for above- and belowground biomass.

To characterize the biochemical properties of the plant material, a subsample of aboveground biomass was ground and analyzed for total carbon and nitrogen content. These analyses were performed at the UC Davis Analytical Lab using combustion methods. Because plant C:N ratios reflect physiological traits unlikely to vary significantly across years, these measurements were only conducted in Year 1.

Bulk soil samples collected for Objective 1 were also used to assess changes in soil carbon pools. Subsamples were ground and analyzed for total carbon and nitrogen via elemental combustion analysis⁷. With another subsample, soil organic matter was fractionated into particulate organic matter (POM) and mineral-associated organic matter (MAOM) using physical separation via wet sieving⁸. These fractions were also analyzed for carbon and nitrogen content to provide insight into the size and composition of soil organic matter pools.

This approach enables differentiation between labile (POM) and stable (MAOM) carbon pools. While MAOM is relatively slow to respond to management changes, POM—primarily composed of plant residues—is more sensitive to inputs from cover cropping. Changes in the POM fraction over the first two years of cover crop establishment may signal early indicators of carbon sequestration potential under perennial cover cropping.

Research Objective 3: Examine the effect of cover cropping on soil moisture and plant water response of almond trees.

Before the start of the second growing season of Oakville Bluegrass (early Fall 2024), continuous soil moisture monitoring was initiated to evaluate how cover cropping influences soil water dynamics under orchard alleys. TEROS 12 soil moisture sensors were installed at two project orchards in both cover crop and conventional plots, positioned at 15, 30, and 45 cm depths within the center of the alley rows. These sensors measure soil moisture and temperature via dielectric permittivity and were connected to in-field data loggers programmed to record data hourly.

At the third orchard site, sensor installation was not feasible due to high clay content and subsurface rock, which prevented proper sensor placement. However, this grower has existing soil moisture monitoring infrastructure installed by a third-party vendor. These data will be used to supplement baseline soil moisture for this location.

Sensor placement was coordinated with each grower to avoid interference with orchard operations and ensure long-term data reliability. The resulting data stream is now being processed and will be used to calculate a cover crop soil moisture ratio—a metric comparing volumetric water content under cover crop versus bare ground treatments across time. This ratio will identify when and how often soil moisture levels differ between management strategies.

In addition, fractional soil moisture will be calculated at the start of the almond growing season, defined as the proportion of moisture remaining in the soil relative to peak winter saturation. This metric is especially relevant for growers, as it provides a basis for timing the initiation of irrigation based on residual soil water storage. Together, these analyses aim to quantify the water-use implications of perennial cover cropping in almond systems and inform conservation-oriented irrigation management.

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