Effects of non-NPK organic soil amendments on yield and quality of vegetable crops

Project Overview

Project Type: Partnership
Funds awarded in 2016: $10,197.00
Projected End Date: 12/31/2017
Grant Recipient: Organic Growers Supply (Fedco)
Region: Northeast
State: Maine
Project Leader:
John Paul Rietz
Organic Growers Supply (Fedco)

Annual Reports


  • Agronomic: potatoes


  • Crop Production: food product quality/safety
  • Education and Training: technical assistance, workshop
  • Farm Business Management: budgets/cost and returns
  • Soil Management: soil analysis, nutrient mineralization

    Proposal abstract:

    Many organic farmers use certain soil amendments that do not contain significant quantities of recognized necessary plant nutrients in an effort to increase yields or improve the quality of the food they grow. As one of the primary/leading retailers of soil amendments for farmers in the Northeast, Fedco/Organic Growers Supply would like to investigate how much these products really affect yield and quality, and publicize our findings to serve our region’s farmers.

    OGS will coordinate trials at four Maine certified organic vegetable farms to test five popular amendments that haven’t been sufficiently studied: Azomite, BrixBlend Basalt, Menefee Humates, Zeolites and Biochar. Each farm will fertilize with each amendment on one higher-value crop (spinach) and one lower-value crop (potatoes). Yields and Brix levels will be compared to controls. The goal is to determine what–if any–crop improvements there are for each amendment.

    The results of our study will be published on the Organic Growers Supply website, with the web address published in Fedco’s popular paper catalog. We will also organize a series of public presentations, which we will advertise through MOFGA, UMaine Cooperative Extension, and our own Facebook page.

    Project objectives from proposal:

    We wish to answer the question, do any of these specialized, non-NPK amendments (Azomite, Humates, BrixBlend Basalt, Zeolites, Biochar) justify the time and money that farmers spend to apply them, either by improving fertilizer availability, by improving yields, or by producing vegetables with higher Brix (a measure of eating quality and therefore a measure of crop value)? Can applying these amendments improve production, improve soil quality, or increase net farm income? In order to answer this, we will be testing if and how much each amendment influences soil nutrient levels and the yield and Brix level in two common annual vegetable crops: one higher-value (spinach), and one lower-value (potatoes).



    We will test the hypothesis that Azomite, BrixBlend Basalt, biochar, Menefee humates, and zeolites have a significant positive impact on crop yield and quality. If this is true, we expect to measure a significant increase in harvested weight and Brix in spinach and potatoes.

    This study will be conducted at four certified organic vegetable farms in Maine:

    1. Morning Dew Farm in Newcastle

    2. Bahner Farm in Belmont

    3. Crooked Door Farm in Whitefield

    4. Earth Dharma Farm in Jackson

    Each farm will select a total of eight ninety square foot replication plots, four for potatoes and four for spinach. Each replication plot will be divided into six 3'x5' treatment plots. Each farm will collect soil samples for pre-fertilization soil tests on each replication plot in early spring. Soil tests will be conducted by the University of Maine Soil Testing Service.

    Farmers will till in NPK fertilizers along with (at the same time as) the non-NPK amendments featured in this study. These NPK fertilizers will be applied at rates suitable for achieving the following NPK soil levels:




    Nitrogen (lbs/acre)



    Phosphorus (lbs/acre)



    Potassium (lbs/acre)



    Simultaneously, the non-NPK amendments being tested will be applied. Rates below are lbs. per 15 square foot plot (for both spinach and potatoes), based on application rates recommended by manufacturers or agricultural consultants:

    • Control (no amendment)

    • Azomite, 0.3 lb.

    • Biochar, 0.2 lb.

    • Brixblend Basalt, 3.75 lbs.

    • Humates, 0.12 lb.

    • Zeolites, 3 lbs.

    Immediately before planting, farmers will again collect soil samples from each replication plot, for soil nutrient testing by the University of Maine Soil Testing Service. Post-fertilization tests will show whether any of the special amendments influenced pre-planting NPK levels.

    Each farm will grow a total of 24 3'x5' plots of Avon spinach (six treatments, four replications per treatment per farm). Harvest will begin approximately six weeks after planting.

    Farmers will harvest spinach once each week for market, until plants stop producing quality leaves.

    For each spinach harvest day, yield from each treatment will be weighed each harvest. Spinach will be weighed before washing. Brix will be measured with a refractometer for each treatment.

    Each farm will grow a total of 24 3'x5' plots of Desiree potatoes (six treatments, four replications per treatment). Farmers will harvest potatoes all at once at the end of the season. Yield from each treatment will be weighed each harvest. All potatoes will be washed and dried before weighing. Brix will be measured with a refractometer for each treatment.



    All Dates are 2016 unless otherwise specified.

    Feb 21. Distribute project packages to cooperating farmers.

    April 4. Farmers conduct pre-fertilization soil tests. May be slightly earlier or later depending on when soil thaws. Farmers test and record NPK levels, then notify project organizer.

    April 21. Project organizer distributes NPK fertilizers to each farm, based on demonstrated fertility needs for spinach and potatoes.

    April 21-May 1. Farmers till in NPK fertilizers and non-NPK amendments being tested.

    May 1. Farmers conduct post-fertilization soil tests and notify project organizer. Farmers plant spinach.

    May 9. Farmers plant potatoes.

    June 7. Organizer contacts farmers to schedule first spinach harvest.

    Week of June 15. Farmers' first spinach harvest, attended by project organizer. Brix and yield measurement taken on harvest days.

    Week of June 21. 2nd spinach harvest.

    Week of July 1. 3rd spinach harvest.

    Week of July 7. 4th spinach harvest, if needed.

    July 10. Farmers send all spinach yield and brix data to project organizer.

    July 19-21. Organizer performs statistical tests for spinach data.

    August 15. Organizer contacts farmers to schedule potato harvest. If necessary, potato foliage is flamed or mown to allow tubers to cure prior to harvest.

    August 22. Farmers harvest potatoes, with project organizer in attendance. Farmers clean, weigh, and measure brix levels within 1 week of harvest.

    Sept 6-8. Organizer performs statistical tests for potato data.



    The project organizer will present the results of this study at the following events:

    1. MOFGA’s Common Ground Country Fair (September 23-25 2016).

    2. Fedco's Bulb Sale (October 2016).

    3. MOFGA's Farmer to Farmer Conference (November 2016).

    4. NOFA-NY Winter Conference (December 2017).

    On the Fedco Website: We will publish an engaging summary, with a link to the full report on the SARE website, in the “Grower's Library” section of the Organic Growers Supply website. The web address for this summary will be published in the next paper catalog (printed in November 2017), which is mailed to about 50,000 customers.

    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.