Evaluating Locally Available Organic Amendments to Enhance Soil Health Indicators for Highbush Blueberry Production East of the Cascades in the U.S. Pacific Northwest

Project Director: Scott Lukas, Oregon State University

Project Overview

Blueberries are a popular and important berry crop in the United States, especially in the Pacific Northwest. According to the USDA Economic Research Service, upwards of 32,000 acres of blueberries are currently cultivated in Oregon and Washington alone, contributing more than a third of the national harvested acreage and 62% of total national blueberry production. Much of this production takes place to the west of the Cascade Mountain Range, owing to the region’s optimal blueberry growing conditions and soil properties (high organic matter/SOM and soil acidity). Soils to the east of the Cascade Mountains tend to be sandier and lower in organic matter (less ideal for blueberry production); however, soil requirements in this region can be met by applying organic amendments. Presently, the organic amendments available for blueberry production are costly, and alternatives are needed that are inexpensive, locally-available, and effective.

Two organic soil amendments that may be of interest to blueberry producers in this region include grape pomace and biochar. Grape pomace is a by-product of wine-making and includes the grape skins, seeds, pulp, stems, and leaves left behind after the grape juice has been extracted. It is acidic, has a high carbon to nitrogen (C:N) ratio, and contains an array of important nutrients including K, P, and Mg, as well as biologically-active compounds (i.e., polyphenols). It is also an inexpensive and readily-available organic amendment in the Pacific Northwest, owing to the ubiquitous PNW wine industry. Biochar is another inexpensive organic soil amendment, recognized for its ability to improve SOM and alter physical soil properties, including water-holding capacity and nutrient availability.

This study, conducted at Oregon State University’s Hermiston Agricultural Research and Extension Center (Hermiston, OR), evaluated the impact of four organic soil amendments (grape pomace, biochar, grape pomace co-composted with biochar, and woodchips) on several soil health indicators and soil microbial community structure to determine viability for organic blueberry production east of the Cascade Mountain Range.

Farmer Takeaways

Incorporating composted grape pomace into sandy, permeable soils can significantly increase SOM content and nutrient availability, including total N, ammonium, nitrate, and available P and K.
Grape pomace compost may be an effective, locally-available soil amendment for organic blueberry producers operating on sandier soils, but longer-term studies are needed to understand how this amendment impacts blueberry crop yield and performance.
Additional research is needed on how to manage resource-use efficiency in these systems to prevent excess nutrient leaching.

Project Objectives and Approach

To investigate the role of composted grape pomace, biochar, and grape pomace co-composted with biochar on several soil health indicators and soil microbial community dynamics of permeable soils under blueberry production

A field experiment was set up at Oregon State University’s Hermiston Agricultural Research and Extension Center. The soil at this site is classified as Atkins fine sandy loam, and was acidified to adjust soil pH to recommended levels for blueberry production through a split application of elemental sulfur.
- - Organic ‘Draper’ northern highbush blueberries were established at the site in rows of raised beds covered with polypropylene landscape fabric for weed suppression. Berry plants were irrigated and fertilized through a drip system with liquid pea-protein hydrolysate.
Experimental treatments included three organic soil amendments (and a control), which were either banded on the soil surface (1m-wide band for each row) or incorporated to a depth of 0.2m prior to shaping the beds. All treatments were applied at a rate of ~10% by soil volume in early April.
Treatments included:
- 1. Banded Grape Pomace Compost
  2. Incorporated Grape Pomace Compost
  3. Banded Biochar
  4. Incorporated Biochar
  5. Banded Biochar Co-Composted with Grape Pomace
  6. Incorporated Biochar Co-Composted with Grape Pomace
  7. Apple Woodchips (Control; standard regional practice)
Soil samples were collected from each treatment plot in September. A subsample from each treatment was sent to a lab for DNA extraction and sequencing, and the remaining samples were analyzed for soil pH, EC, ammonium (NH4+), nitrate (NO3-), available P and K, total N and soil organic matter (SOM), extractable organic C, microbial biomass C, enzyme activity, and microbial respiration.

Key Findings

Soil Organic Matter and Nutrient Availability

Soil organic matter (SOM) was significantly influenced by the organic amendments. After two years, SOM was highest in the incorporated grape pomace treatment and lowest in the banded grape pomace treatment (19 and 12g/kg, respectively). Of the treatments, only the incorporated grape pomace treatment resulted in SOM levels statistically higher than those in the woodchip control.
- - This suggests that both amendment type and application method impact SOM, and that incorporated grape pomace may be a promising organic amendment for PNW organic blueberry producers wanting to build SOM.
All treatments, including the woodchip control, increased SOM levels over the two years, with the smallest increase observed in the banded grape pomace treatment (+2g/kg).
Soil pH and EC were within the recommended range for northern highbush blueberry, but were unaffected by the organic amendments. Note: The researchers postulate that this could be due to the fact that the experimental fields were acidified prior to planting and irrigated with acidified water.
Soil nutrients, including total N, ammonium, nitrate, and available P and K, were significantly higher in the incorporated grape pomace treatment than in the woodchip control.
Extractable organic C, microbial biomass C, active C, and microbial respiration were significantly higher in the incorporated grape pomace treatment than in the woodchip control during both years of the study, suggesting high microbial activity.

Microbial Community Structure

There were no significant differences between treatments for any of the bacterial or fungal diversity indices; however, the dominant bacterial species changed considerably over the two years. The fungal community remained similar between years.

Blueberry Plant Response and Yield

In a related study at the same research site (see linked article below), researchers did not find any differences in blueberry plant height, fruit set, yield, or fruit quality between treatments. This suggests that, despite the noticeable two-year improvement in SOM, nutrient availability, and microbial activity in the incorporated grape pomace treatment, blueberry crop responses may be longer-term, with longer-term studies needed.

Resources

Singh, S., Lukas, S. B., Retano, A., & Bryla, D. R. (2025). Evaluating locally available organic amendments to enhance soil health indicators for highbush blueberry production east of the Cascades in the U.S. Pacific Northwest. Scientific Reports, 15(1), 20933.

Retano, A., Bryla, D.R., Lukas, S.B., Singh, S. and Orr, S.T. (2025). Alternative soil amendments for cultivation of northern highbush blueberry in marginal soils. Acta Hortic. 1440, 469-474

Location

Oregon

Collaborators

Shikha Singh, Washington State University
Andrea Retano, Oregon State University
David Bryla, USDA-ARS

Region

Northwest

Topic

Soil Health, Crop Nutrient Management

Year Published

2025