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

Final report for ONE16-270

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)
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Project Information


For organic growers, there are now numerous soil amendments on the market that do not contain significant quantities of recognized necessary plant nutrients. Despite the fact that such amendments do not provide any primary nutrients (nitrogen, phosphorus, potassium–NPK), they continue to be employed by many farmers who believe that these amendments have alternative benefits. As one of the leading soil amendment retailers in the Northeast, Fedco Organic Growers Supply wanted to determine whether some of its best-selling amendments had any significant effect on crop yield or Brix content (a measure of soluble solids, which relates to flavor and nutrient density). The amendments we included in the study are a representative sampling of popular non-NPK products and of their different modes of action; the five chosen were: Azomite (micronized), RICHMix Biochar, BrixBlend Basalt, Meneffee Humates, and Zeolites. We chose yield and Brix as our two metrics because those are two of the most important metrics for market growers, but please note that there are other very important metrics we didn’t use, such as cation exchange capacity (CEC), soil porosity, and soil life. We chose to test these amendments on two crops: spinach and potatoes.

Due to severe drought conditions, the 2016 growing season was particularly challenging for farms in central Maine. Of course, farmers did not know this ahead of time, so they planted their spring crops how they always do, and for many this meant planting without an irrigation system in place, since spring in Maine usually comes with ample rainfall. Due to the drought (and unseasonably high temperatures in the spring), two of the four cooperating farms in our study experienced complete crop failure on both their spinach and potatoes, which severely compromised our study. Thankfully we got useable yield (weight) data from both of the remaining cooperating farms. We were not able to obtain useable Brix data because the analog refractometers we budgeted for were severely imprecise; we would have needed expensive refractometers with digital readouts.  

We performed statistical analysis tests on the weight data, to determine how the crops yielded with each amendment, compared to the control. Every test we performed indicated that there was no significant difference in yield compared to the control. Though this indicates that none of the amendments can be expected to perform like a silver bullet, there was not enough data to make conclusions about the usefulness of any of the amendments studied.

We have publicized the findings of this study via the Fedco newsletter, and at two large events, NOFA’s 2016 Summer Conference, and MOFGA’s 2016 Common Ground Country Fair.


Researchers have tested many soil amendments and evaluated their impacts on crop growth. In most cases, however, the “soil amendment” in question is some type of organic matter: compost, manure, or other agricultural byproduct. Relatively few studies have investigated soil amendments that are mined from natural deposits, and
even fewer studies have tested products that contain no significant quantities of recognized plant nutrients. Regarding such products, most of the existing “research” is produced or sponsored by the manufacturers themselves. Manufacturer-driven research has its place, but must be substantiated by further independent studies.
A number of SARE projects have involved treatments with one or more trace-mineral/ soil-conditioning amendments. What distinguishes our project is that it is focused on isolating a handful of specialized products, and seeking to determine if they significantly improve yields or nutritional content. The SARE project that most
relates to our research goals is FW15-024 (still in progress), which is testing the influence of soil and foliar-applied minerals on the nutrition of blueberries. We look forward to reading the results of that study, and we think our project will add significantly to the findings by testing on vegetable crops, testing other trace-mineral amendments, and testing yields. Another SARE project that relates to our proposal is FNE14-792, which compared a “nutrient-dense” fertilization regime to a traditional nitrogen-based regime. The primary difference is that our proposal is to test specific amendment products, whereas FNE14-792 evaluated the usefulness of a fertilization system.
Non-NPK amendments are popularly promoted in organic farming, but there is insufficient evidence that these products are actually helpful. We intend for this study to inspire many other studies on effects of these specialized products; in other words, whatever our findings are, they cannot be considered definitive until they are
substantiated by research across the country. We also will not be measuring all possible benefits of these amendments; for example, further research could investigate their impact on soil biota, soil porosity, long-term retention of soil nutrients, or levels of specific vitamins and minerals in the produce.
If the collective research shows that these products produce no significant difference in yield or nutrition, then all parties in the organic farming sector would be impacted. Agricultural consultants may need to identify alternative techniques for improving yields and produce quality, suppliers like Fedco may need to modify their product lines,
and farmers would need to alter their soil-building plans.
On the other hand, if our findings indicate that these specialized amendments do improve crops, these products ought to be publicized and distributed widely, because any farmers that haven’t heard of them will be missing an opportunity to grow better crops.
Project Objectives:

The objective of this study was to test the hypothesis that Azomite, BrixBlend Basalt, biochar, Menefee humates, and zeolites have a significant positive impact on crop yield and quality.

There are a number of ways in which farmers would benefit from our study and its findings:

  1. Seeing beyond NPK: If these amendments impact crops, farmers ought to be informed so they can improve their harvest. If no impact is observed, this study will nevertheless introduce some farmers to the concept of trace minerals and soil conditioners.
  2. More informed cost-benefit analyses: Farmers will know whether to expect significant yield and Brix improvements in the first year, and can thus decided if it is worth spending money on or not.
  3. Establishing an experimental model: Farmers can use our model to test whatever amendments they want.



Click linked name(s) to expand/collapse or show everyone's info
  • Christa & Mike Bahner
  • Suzanne Balbo
  • David McDaniel
  • Brendan McQuillen


Materials and methods:
We tested the hypothesis that Azomite, BrixBlend Basalt, Biochar, Menefee humates, and Zeolites have a significant positive impact on crop yield and quality. If true, we expected to measure a significant increase in harvested weight and Brix in spinach and potatoes.
This study was 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 selected a total of eight ninety square foot replication plots, four for potatoes and four for spinach.
Each replication plot was divided into six 3'x5' treatment plots. Each farm collected soil samples for pre-fertilization soil tests on each replication plot in early spring. Soil tests were conducted by the University of Maine Soil Testing Service.
Farmers tilled in NPK fertilizers along with (at the same time as) the non-NPK amendments featured in this study. Fedco supplied blood meal for available N (12% soluble N), bone char for available P (16% soluble P), and K-Mag for available K (22% soluble K). These NPK fertilizers were applied at rates suitable for achieving the following NPK soil levels, based on levels recommended by university extensions for these crops:
Nitrogen (lbs/acre)
Spinach: 85
Potatoes: 150
Phosphorus (lbs/acre)
Spinach: 75
Potatoes: 200
Potassium (lbs/acre)
Spinach: 85
Potatoes: 200
Simultaneously, the non-NPK amendments being tested were 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.
Each farm grew a total of 24 3'x5' plots of Avon spinach (six treatments, four replications per treatment per farm). Spinach seedlings were grown indoors and then transplanted into the beds at 6” in-row, 12” spacing between rows. Harvest began ~ six weeks after planting.
Farmers harvested spinach leaves when they grew to a marketable size, until plants stopped producing quality leaves.
For each spinach harvest day, yield from each treatment was weighed. Spinach was weighed before washing for the convenience of the farmers. Many attempts were made to measure Brix of the spinach leaf juice, but the refractometers we budgeted for were severely inadequate.
Each farm grew a total of 24 3'x5' plots of Yukon Gem potatoes (six treatments, four replications per treatment).
One whole (uncut) seed potato was planted per square foot, 6” below the soil surface, and hilled as necessary.
Farmers harvested potatoes all at once at the end of the season. Yield from each treatment was weighed at harvest. All potatoes were washed and dried before weighing. Many attempts were made to measure Brix of the potato tuber juice, but the refractometers we budgeted for were severely inadequate.
The following materials were given to each farm for Brix measurement:
--knives and cutting board for sample preparation
--“Magic Bullet Mini” blender to pulverize samples
--Brix press modified vice grips for squeezing juice out
--pipettes to collect juice from sample pulp
--delicate task wipe to filter samples and clean refractometer without scratching
--data sheet and clipboard
In addition, the following materials were given to each farm for measuring weight of yield:
--digital, battery-powered scale (with tare function)
--plastic bags for spinach
--paper bags for potatoes
--data sheet
Farmers returned datasheets to Fedco via Google Drive (shared files managed by project organizer).
For each crop at each farm, we performed statistical analysis (specifically, a paired t-test) to determine if any yields and/or Brix readings are statistically "significantly different."
Research results and discussion:

Our primary objective was to determine if the addition of any non-NPK amendment led to a significant difference in yield (measured in pounds) or and/or brix content (measured with refractometer), in either spinach or potatoes. Due to the severe drought of 2016, 2 of the 4 participating farms had to drop out of the study due to crop failure, so our data set was limited fairly early on. Regarding the Brix testing, we made numerous attempts to measure the brix of juice we extracted from spinach leaves and potato tubers, but the refractometers we had were not adequate for this purpose. No matter how we treated the juice, the refractometer readings were always blurry and thus not scientifically useful. Apparently, the only way to gather useable Brix data is to use an expensive digital refractometer (hundreds of dollars), which was not written into our budget proposal. On the other hand, weight data from the two farms was adequate enough to use for statistical testing.

It wouldn't be scientifically prudent to draw any conclusions from a data set that is only half the size it was designed to be, but we have nevertheless created 2 tables to summarize the results of our statistical tests. For each statistical test, the yield data from a given crop at a given farm with a given amendment were compared to the control (that same crop at that same farm without any non-NPK amendment). The type of statistical test we used was the paired t-test. The purpose of the paired t-test is to determine whether there is statistical evidence that the mean difference between paired observations on a particular outcome is significantly different from zero. In this type of test, the "P value" indicates whether or not the mean difference is considered to be not statistically significant. By conventional criteria, if the P value is below 0.05, it indicates the means are statistically different. Note that most of our resulting P values were quite far from 0.05; none indicated significant difference.


SPINACH Azomite Basalt Biochar Humates Zeolites
Earth Dharma Farm P=0.6929 P=0.2317 P=0.7132 P=0.1114 P=0.0709
Crooked Door Farm P=0.7137 P=0.8995 P=0.5499 P=0.9768 P=0.5368


POTATOES Azomite Basalt Biochar Humates Zeolites
Earth Dharma Farm P=0.8822 P=0.9854 P=0.5304 P=0.5244 P=0.0820
Crooked Door Farm P=0.7212 P=0.1979 P=0.1723 P=0.8685 P=0.5473
Research conclusions:

Despite the obstacles we faced (namely, severe drought & 2 of 4 farms having to drop out due to crop failure), we were able, nevertheless, to draw some preliminary conclusions. In every single statistical test that compared the yield of plots treated with a given amendment with the yield of the control plots, there was no statistically significant difference found.

Although we had a limited data set to work with, we could tell that no amendment was proving to be a “silver bullet” (i.e. an amendment that miraculously results in increased yield within the same season of application). That, alone, is a fairly important finding, because there are growers who purchase these non-NPK amendments and expect them to give them an immediate payback. That being said, some experts on the subject of non-NPK amendments–such as Dan Kittredge of the Bionutrient Food Association–point out that without ample soil moisture, such amendments cannot be expected to perform as intended.

While crop yield and brix are very important to growers, they are not the only important considerations for sustainable agriculture, nor are they the only factors of the farm system that may be affected by the application of non-NPK amendments. For example, the use of non-NPK amendments may change certain characteristics of the soil, such as CEC, porosity, or the quantity & quality of soil microbes. We at Fedco have noticed recently that the farming community has become more interested in soil microbial life and how to improve it. As it so happens, of the five amendments we studied (Azomite, Biochar, BrixBlend Basalt, Menefee Humates, and Zeolites), each is said to promote beneficial microbe activity in some way.

We would like to investigate how soil life may be influenced by these five amendments. For such a study, we would specifically choose to avoid using vegetable crops as the medium; instead, we would opt to test the soil life in areas that were simply planted to a cover crop, so that the success of the plant growth wouldn’t be so dependent on water, and we would choose a heat-loving cover crop (BMR Sorghum/Sudangrass) so that extraordinarily hot growing conditions wouldn’t be a problem. We very much look forward to doing further research with non-NPK amendments.

Participation Summary
8 Farmers participating in research

Education & Outreach Activities and Participation Summary

1 Curricula, factsheets or educational tools
2 Published press articles, newsletters
2 Webinars / talks / presentations

Participation Summary:

50 Farmers participated
1 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

Via the Fedco newsletter, we published two articles about the research design and results.

At NOFA's Summer Conference (Aug 2016), we presented our preliminary findings, though we had yet to receive all the data and perform all the statistical analysis tests.

At MOFGA's Common Ground Country Fair (Sept 2016), I lead a 1-hour discussion about this study, including its implications for farmers and home gardeners. At this presentation, I distributed a handout I created that explained the supposed benefits of the amendments in this study, along with a limited cost-benefit analysis.

Learning Outcomes

20 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Influence of soil moisture content on dispersal of amendments in the soil; Brix as a measure of nutrient content.

Project Outcomes

Assessment of Project Approach and Areas of Further Study:

This study was too small and too limited in scope to determine anything definitive about the effects of any of these amendments. These amendments are not really intended to be fast-acting yield- or brix-increasers. Their supposed benefits are slow to materialize; numerous growing seasons would be required to assess if steady progress is happening. Also, results may have been more apparent with numerous seasons of re-application of these non-NPK amendments. Furthermore, more precise tools would be necessary to determine subtle measurements like brix (note that brix is only a basic measurement of nutrients; it would be much more useful to test for specific vitamins and minerals). Perhaps using entirely different metrics would yield results that showed whether these amendments give any significant benefits. For example, one could test the soil microbiological activity (by measuring CO2 respiration) in amended versus control plots; or test the long-term retention of key plant nutrients, or test the impacts of non-NPK amendments on long-term soil porosity.

From Fedco's perspective, there are many important studies left to conduct on the subject of non-NPK amendments. Independent research must at some point determine if these amendments are worth the purchase price and the effort needed to transport and apply them. Those who stand to gain the most from this research are small farmers who struggle to produce crops on small plots of marginal farmland.

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.