Exploring Low-Phosphorous Wool Pellets as Fertilizer and Soil Conditioner for Vegetables

Progress report for ONE20-358

Project Type: Partnership
Funds awarded in 2020: $29,329.00
Projected End Date: 05/31/2022
Grant Recipient: University of Vermont
Region: Northeast
State: Vermont
Project Leader:
Dr. Terence Bradshaw
University of Vermont
Laura Johnson
UVM Center for Sustainable Agriculture
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Project Information


This project addresses Northeastern vegetable farmers’ needs for sufficient production and yield, careful nutrient management and climate resiliency, by using a natural and potentially locally-produced item made from a waste product from sheep farms: pelletized raw wool.  We propose a two-year study of how wool pellets affect nitrogen and phosphorus interactions with plants and soils, and how wool pellets are affecting soil moisture content.

The experimental design will be a completely randomized design with four replications and three treatments. The trials will be conducted in two Vermont locations, one at the UVM Horticulture Research and Education Center in Burlington and the other at Cedar Circle Farm and Education Center in East Thetford. 

Incremental and final findings will be shared through field walks, print and web-based materials, video and social media with vegetable growers, shepherds, agricultural service professionals and other colleagues throughout the region.


Project Objectives:

This project seeks to: 

  1. Determine if by using wool pellets as fertilizer providing N and K, spinach and tomato plants will utilize existing P in the soil.
  2. Determine whether the N in the pellets releases quickly enough for plants to utilize within the growing season.
  3. Understand whether wool pellets hygroscopic property can ameliorate the fluctuating extremes of precipitation on soil moisture levels during the growing season 

These are important questions for vegetable farmers.  If we find that by using wool pellets for N and K, plants utilize existing soil P, that means less leaching into nearby waterways – a win for producers and the community. And if we find the pellets release N slowly enough so there is minimal, if any, runoff of excess, that too, is a win for both producers and the community.  And if we find the hygroscopic properties of the pellets shows they can help absorb the moisture in excessive precipitation events, and then release in drier periods, this could help eliminate much of the need for irrigation and running electrical pumps, both promoting resilience and saving money for  producers.



This project plans to explore two agricultural issues: nutrient management on vegetable farms and utilization of wool, currently a waste product and cost to sheep producers. We are specifically interested in how well pelletized raw wool performs for plant and soil needs of nitrogen (N) and phosphorous (P), and soil moisture content over time. 

Vermont vegetable growers must carefully consider fertilizer use on their crops, seeking to balance productivity, costs, regulatory limits of N and P, and needs of their sites and soil type(s). Commonly used products, such as compost and chicken manure, often applied at rates that result in higher concentration of P than a crop’s ability to utilize it, result in water quality issues.  
Meanwhile, Vermont shepherds contend with costs that currently do not contribute to their revenue.  Shearing sheep is a necessary step in the husbandry of this small ruminant. Whether raised for meat or milk, an annual “haircut” is required for their health. Since much of the clip produced in the northeast is coarse wool, not suitable for value-added products that require softness like yarn or clothing, it has little value.  Since 1994, the market price for raw wool has been dropping, and is now below the cost of shearing and the transport of itA small percentage has the fine softness used for many small batch cottage industries, but generally, if not sold to the local wool pool collection, many simply pile it in a corner of their barn or haul it out to the woods to dump it.  

Findings from a USDA Rural Development-funded project conducted from 2016 – 2019 indicate an opportunity to address these issues for both vegetable growers and shepherds with this natural, locally produced item. Analysis of wool pellets, reveals an NPK profile average of 9-0-4, supplying nitrogen, virtually no phosphorus, and contributing small amount of potassium. The nitrogen slowly releases due to physical properties of fibrous wool pellet’s slow breakdown. For many vegetable farmers in Vermont this is an ideal combination. Many are over their regulatory limit for P, and need a source of N that supplies the nutrient at a time a crop needs it.  A slower N-release rate helps to avoid the risk of nitrate leaching and runoff before plants can utilize the nutrient.   Additionally, fifty percent of the weight of wool is carbon and thus pellets may also provide an opportunity for farmers to sequester carbon through their choice of fertilizer by incorporating the wool pellets into the soil.

Trials conducted with three Vermont vegetable farms in 2019 indicated that wool pellets may, in some ways, have superior qualities to the commonly used peanut meal or compost Wool’s hygroscopic quality means an ability to ameliorate climate/weather extremes that contribute to excessive or limited soil moisture content because it can absorb, hold and release moisture as well as nutrients, over time. 



Click linked name(s) to expand
  • Nic Cook - Producer
  • Kimberly Hagen
  • Cheryl Herrick


Materials and methods:

Statistical Analysis 

The experimental design will be a completely randomized design with four replications of  four treatments, including: non-treated control; grower-standard fertilization; wool pellet fertilization at 25lbs wool pellets per 100ft; and wool pellet fertilization standardized to match N input of grower standard treatment. The experimental plots will be ten-foot sections of a single field bed, totaling 160 bed feet. Data will be collected from four-foot sections within the center of each plot to minimize edge effects between treatments.  All data will be analyzed for mean differences using one-way ANOVA (SAS v.9.3, Cary, NC) with fertilization treatment effects separately analyzed for each crop and site. 

Field Description 

This trial will be conducted in two Vermont locations, one at the UVM Horticulture Research and Education Center in Burlington and Cedar Circle Farm and Education Center in East Thetford. The soil types in these locations are Adams and Windsor loamy sand and Adams and Windsor sandy loam, respectively. The experimental design will be a randomized complete block design with four replications and four treatments for both crops, spinach and tomatoes 

Spinach (cvr, “Space”) will be direct seeded at a seeding rate for full-size leaf harvest, which will be 10 seeds/foot, ½ inch deep with 12 inches between rows. The crop will be managed using each grower’s standard practices for cultivation, weeding, and irrigation. 

Spacing between tomato (cvr. “Defiant PhR"), plants will be 12 inches or 10 plants per 10 feet. Field prep will include tillage with grower standard plot management. Tomato transplants will be seeded in in a soilless mix in a greenhouse, with seeds sown 1/4 inch-deep in 72-cell plug flats then transplanted into a potting soil starter mix in 4-inch pots. Tomatoes will be transplanted outdoors at 7-8 weeks and receive drip irrigation and cultivation following growers’ standard practices.  

Each treatment plot will receive 4 fertilizer treatments. These treatments will be determined based on crop nitrogen need, which is 100 lbs/ac for spinach and 150 lbs/ac for tomatoes as cited by the New England Vegetable Management Guide. Fertilizer treatments will consist of wool pellets applied and grower standard fertilizers. Treatments will include the following: non-treated control; grower-standard fertilization; wool pellet fertilization at 25lbs wool pellets per 100ft; and wool pellet fertilization standardized to match N input of grower standard treatment. 

Harvest and evaluation protocol 

Spinach and tomatoes will be harvested per the grower standard, and marketable yields will be recorded by weight in pounds 

Agronomic sampling (soil, water, and fertilizer sources) 

Pre-season and post-harvest soil samples will be taken at 6-8" depths plus one pre-sidedress soil nitrate sample at 12” depths until harvest. Soil samples will be submitted to the University of Vermont Extension soil analysis lab. Temperature and soil moisture will be measured weekly using a soil thermometer and TDR soil moisture probe. Wool pellets, from Utah, have been submitted to the University of Maine soil analysis lab and are found to have a 9-0-4 nutrient profile. 

Analysis, summary, interpretation and presentation of any data gathered or program results, whether qualitative or quantitative. 

Quantitative data will include the following: 

  • Spinach will be harvested at 5-6 weeks after sowing or at marketable full leaf maturity. All leaves will be cut from the plant and be separated into two categories, mature and cull. Total yields (mature and cull) will be recorded and marketable yields will be recorded by weight in pounds.  
  • Tomatoes will be harvested weekly starting at 10 weeks or when the majority of the fruit is at marketable ripeness. All red tomato fruit will be stripped from the plant and separated into two categories, red and cull. Total yields (red and cull) and marketable fruit yields (red) will be recorded by weight in pounds.  
  • Two standard soil tests will be taken at each field trial location to measure pre and post season nutrient profiles of soil NPK and soil organic matter (SOM) 
  • One pre-sidedress nitrate test at each field location will be taken to measure nitrate levels mid-season. 
  • Soil moisture content will be measured and recorded bi-weekly throughout the crop growing season (May-October/November). 
  • Soil temperature will be measured and recorded bi-weekly throughout the crop growing season (May-October/November). 

Statistical analysis of data using the following: 

  • All data will be analyzed for mean differences using one-way ANOVA (SAS v.9.3, Cary, NC) with fertilization treatment effects separately analyzed for each crop and site.

Data interpretation:  

  • Data will be formatted to fit factsheets, videos, presentations, and other outreach platforms to target audiences. 
Research results and discussion:

To-date (through January, 2021), we have planned the project procedures and lined up farms for deployment in 2021 growing season. No active research has occurred yet.


Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

The UVM Center for Sustainable Agriculture is well-positioned as a trusted resource for information, with robust engagement with farmers and regional agricultural service providers, and as a part of UVM Extension.  We will seek to broadly share our findings within our networks - indeed it is the enthusiasm and interest of these networks that are inspiring our pursuit of this work's next phases. 

This research is being done for the benefit of the region's vegetable growers and livestock farmers, and the outreach and communications will be offered with those audiences in mind. Kimberly Hagen, with support from Center for Sustainable Ag. colleagues, will take the lead on outreach by: 

  • Arranging for at least one Across the Fence segment to air on television and online 
  • Offering two field walks for fruit and vegetable growers and for sheep farmers during the two growing seasons that will take place within the project's timeline. (reaching a projected 100 farmers in-person) 
  • Leading three workshops for fruit and vegetable growers, sheep farmers and Extension colleagues during the winter season once the research concludes.  (For example, at the VVBGA annual meeting, Vermont Grazing & Livestock Conference, Vermont Sheep & Goat Association annual meeting, and/or NOFA-VT winter conference. (reaching a projected 60 farmers in-person) 
  • Producing two fact sheets to distribute via web and print, targeted to vegetable growers and shepherds: one to describe preliminary observations, and a one to convey eventual conclusions and recommendations. (we will print and distribute these at public events, and track downloads from our website, reaching a projected 500 farmers). Information will be shared on the Veg and Berry listserv, the VT Grass Farmers Association listserv, the VT Healthy Soils Coalition listserv, the Center For Sustainable Agriculture listserv, and with CRWFA membership. 


All activities will be concurrent with and following field research in 2021 season.


Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.