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 conducted a 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 is a completely randomized design with four replications and three treatments. The trials were 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 explores two agricultural issues: nutrient management on vegetable farms and utilization of wool, currently a waste product and cost for sheep producers to dispose of. 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 it.  A 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/collapse or show everyone's info
  • Nic Cook - Producer
  • Kimberly Hagen
  • Cheryl Herrick


Materials and methods:

Experimental Design

The experimental design is 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 as recommended by the manufacturer; and wool pellet fertilization standardized to match N input of grower standard treatment. The experimental plots are 3.3 m sections of a single field bed, totaling 160 bed feet. Data were collected from 1 m  sections within the center of each plot to minimize edge effects between treatments.  All data are being  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 was 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 Hartland silt  loam, respectively. The experimental design is a randomized complete block design with four replications and four treatments for both crops, spinach and tomatoes.  

At each site, one bed each of spinach (Spinacia oleracea, cvr. ‘Kolibri’) and tomato (Solanum lycopersicum, cvr. ‘Skyway’) were grown for use in the trial. Both crops were seeded in the greenhouse for later transplanting into the research plot. Spinach was grown with three rows per bed with 0.3 m between rows and 0.3 m between plants within the row. The crop was managed using each grower’s standard practices for cultivation, weeding, and irrigation. 

Tomatoes were planted in one single row per bed with 0.6 m between plants. Spinach was transplanted on 4 May and 26 April for CCF and HREC sites, respectively. Tomatoes were transplanted on 24 May at CCF, and on 23 June at HREC, with the later date selected in order to better align with the farm’s fall markets. In midsummer, tomatoes at CCF were infested with Colorado potato beetle (Leptinotarsa decemlineata) and organic pesticide treatments applied twice. Despite treatment, the plants were decimated by the insects, and no further work was done at that site with the crop.

The primary variable in this study was the type and/or amount or fertilizer used to provide supplemental nutrition to each crop beyond that supplied by the soil. Wool pellets were from Wild Valley Farm (Croyden, UT).  Commercial fertilizer used in comparison treatments was Pro-Booster 10-0-0 (North Country Organics, Bradford, VT). Pro-Booster is a mixed, pelleted fertilizer blend certified for use in organic production systems. In addition to soil building through cover cropping and prior-season compost application, farm managers at both research sites use this product as their standard fertilizer to meet crop nitrogen needs beyond that provided through decomposition of soil organic matter (SOM).

Table 1. Experimental fertilizer treatments applied to tomato and spinach plots at two locations.











% N





Target N, kg * ha-1





kg material * plot-1





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 were taken at 6-8" depths. Soil samples were submitted to the University of Vermont Extension soil analysis lab. Temperature and soil moisture were measured weekly using a soil thermometer and TDR soil moisture probe. Wool pellets were submitted to the University of Maine soil analysis lab and are found to have a 9-0-4 (NPK) mineral nutrient profile. 

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

Quantitative data included the following: 

  • Spinach was harvested at marketable full leaf maturity. All leaves were cut from the plant. Total yields were recorded by weight in grams.  A subset of five plants per replicate was harvested, included in the overall weight, then dried for later calculation of dry matter weight and for evaluation of plant tissue mineral elements.
  • Tomatoes were harvested weekly starting at 10 weeks and continued until the threat of frost. All red tomato fruit were 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) 
  • 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.


Research results and discussion:

Due to a shift in personnel after one key personnel left UVM and another retired (but stayed on the project part-time), this project has shifted into an undergraduate student-centered service learning project. Data in fall 2021 were collected by a team of students and summarized in December 2021. In January-February 2022, a team of student interns are completing full statistical analysis of all project data for publication in a scientific journal and a project fact sheet in spring. Preliminary analysis shows great promise for wool as an alternative fertilizer treatment compared to standard, commercial organic fertilizers:


Figure 1: Harvest weight of 1m x 1m plot of spinach grown with three fertilizer treatments at UVM Horticulture Research & Education Center. NTC = non-treated control; GS= grower's standard commercial pelletized organic fertilizer; WL25= wool pellets at manufacturer’s recommended rate; WLGS= wool rate adjusted to equal N rate of GS treatment. Data analysis is preliminary and not for publication beyond this report.

Figure 2: Total harvest weight of fruit per plant from tomatoes grown with three fertilizer treatments at UVM Horticulture Research & Education Center. NTC = non-treated control; GS= grower's standard commercial pelletized organic fertilizer; WL25= wool pellets at manufacturer’s recommended rate; WLGS= wool rate adjusted to equal N rate of GS treatment. Data analysis is preliminary and not for publication beyond this report.

Data are being analyzed by our student research interns. However, this preliminary analysis of crop yield suggests that wool pellets at the full manufacturer’s suggested rate yielded greater produce than the non-treated control. This is useful, but does not indicate whether or not the high rate led to reduced fertilizer efficiency and potentially greater runoff or leaching. Equally important, the wool pellets applied to a standardized equivalent rate of nitrogen as the grower’s standard, commercial fertilizer rate was no different from the grower’s standard. This suggests that wool pellets may be a viable alternative to the fertilizer programs presently used on commercial organic vegetable farms in the Northeast.


Participation Summary
2 Farmers participating in research

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. 


Outreach conducted to-date:

  • 2020 UVM Center for Sustainable Agriculture Annual Report feature – distributed print copies to 2000 people
  • Video story from 2020 Annual Report  - 191 views https://www.youtube.com/watch?v=uUxjhqyFess
  • Update featured in CSA newsletter – 94 views
  • Newsletter featuring Wool Pellets update – 2424 sent, 873 opened
  • Wool & Other Natural Fibers webpage – 688 views

And undetermined number of contacts in the outreach around our successful crowdfunding campaign to help support a test run of pelletizing Northeastern wool



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.