Investigating effects of beneficial microbial inoculants on potatoes

Final Report for FNE11-723

Project Type: Farmer
Funds awarded in 2011: $3,798.00
Projected End Date: 12/31/2011
Region: Northeast
State: New York
Project Leader:
Marina Michahelles
Shoving Leopard Farm
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Project Information


We proposed to study the effects of beneficial microbial inoculants on Red Norland potatoes. Previous research shows that symbiotic relationships exist between certain plants and various soil microbes, including some nitrogen-fixing bacteria and fungal mycorrhizae. The Real Food Campaign in the Northeast has been promoting the use of several microbial inoculants as part of healthy and sustainable soil fertility management.

We compared the emergence, growth, leaf sap and tuber Brix measurements, soil conductivity, plant longevity, yield, as well as tuber flavor and texture of potato plants treated with a mixture of beneficial microbial inoculants against an untreated control. A positive correlation between soil conductivity and leaf sap Brix was found, supporting previous evidence that good soil health results in good plant health. Although yields did not vary according to treatment, we found the weekly use of the soil conductivity measured and the Brix reader to be an easy and effective way to quantitatively gauge what was happening in the field.


One problem every farmer faces is supplying adequate nutrition to the plants and animals he or she is raising. Not only do we have to compensate for our soil’s natural deficiencies and mineral imbalances, but we also have to re-supply nutrients we remove through harvest. A second problem, which Shoving Leopard Farm shares with many other farms, is limited space; we have to maximize production on our acre-and-a-half. To address both these limitations, we hope to enlist the help of beneficial soil microbial communities that have been shown to promote for plant nutrition and health.

Soil biologists now have a greater understanding of the role that living soil organisms – from bacteria, yeasts and fungi to actinomycetes and algae – play in plant nutrition. Some bacteria are responsible for fixing nitrogen; some are associated directly to legume root hairs and provide plants with useful nitrogen; others are free-living; while still others decompose chitin and cellulose. Several fungi are known to have strong symbiotic relationships with plants and in the absence of any of these, plants are shown to have lower yields and poorer quality. The activities of soil microbes increase plants’ access to nutrition, resulting in more pest-resistance, higher nutritive value, and greater resilience in drought and frost. Diverse soil biological communities in agricultural systems improve both yield and quality of produce.

Shoving Leopard Farm has been growing mixed vegetables and fresh cut flowers since 2006 on 1.5 acres of heavy clay. Save for the work the rototiller does to prepare beds in spring and fall, all work is done by hand. We have improved the soil tilth through applications of finished compost, green manure, and hay mulch and have seen an improvement over the years in terms of production and plant health. We want to study the effects on crop yield (measured in quantity and weight) and quality (measured in Brix) by the promotion of soil biological communities by inoculating seeds and soil with beneficial species of bacteria, mycorrhizae, and other microbes. If this study shows positive results, other farms could increase their crop production without increasing land-use, and possibly decrease their use of some fertilizers over time.

The Real Food Campaign has been offering courses in the Northeast on how to increase the nutrient value of the crops we raise by, not only addressing the underlying deficiencies and relative imbalances of nutrients in the soil, but also by introducing and promoting soil biology. Scores of farmers have already begun using the biological inoculants the associated company, Nutrient Density Supply Company, sells. This study is designed to evaluate the effect of these inoculants on Red Norland potatoes grown on our CSA farm.

There are studies that demonstrate the benefits to plant health and yields of applying compost and mulch, using cover crops, and limiting soil-disturbing practices to plant health and yields. Microbial inoculants applied through compost teas have been used to reduce the need for pesticides in grapes (SARE F NC03-467, 2003) and in 2007, a study demonstrated that inoculation of arbuscular micorrhizal fungi produced an increase in yields in carrots, strawberries, and potatoes (LNE03-179). A 2010 SARE-funded project (LS10-227) is looking at the use of inoculants on legume rhizobia, and whether the native populations of soil bacteria, mycorrhyzal bacteria, and microbes that mineralize legume residues have an impact on the success of these rhizobial inoculants.

Shoving Leopard Farm has been in production since 2006. We have a 30 member CSA, we sell flowers at two farmer’s markets, and offer a PYO flower labyrinth. There is one full-time grower and two work-trade CSA members, as well as many occasional volunteers. Though we are not certified organic, our practices go beyond NY state organic standards, and every effort has been made to grow the best possible produce and flowers while improving soil tilth. Finding biological ways to increase our crops’ health and nutritive value, and integrating these solutions into our management practices is in line with our mission to promote healthy agriculture from the ground up. If we can healthily and sustainably produce more and better food without expanding our acreage and without increasing our need to fertilize, we can improve our economic bottom line.

Project Objectives:

The following measurements were taken for each plot:

• Days to emergence;
• Weekly measurements of: soil conductivity – 3 samples per plot, leaf sap Brix – 3 samples per plot;
• Biweekly measurements of growth: height, stem thickness;
• Size, weight, number, tuber Brix at harvest;
• Plant longevity;
• Cornell soil health test;
• CSA members blind taste test

Results were interpreted using SPSS.

A pre-trial and post-trial comparison of the Cornell Soil Health test was done, looking for any significant lasting effects in the control and trial plots.


Click linked name(s) to expand
  • Sarah Ashcroft
  • Wendy Sue Harper
  • Marina Michahelles
  • Stephanie Radin


Materials and methods:

The plants used for the study were Red Norland potatoes because potatoes have shown a positively response to inoculation in the past (LNE03-179), and because we grow several rows of them, so test plots could be distributed randomly. A saturated paste analysis and a Cornell Soil Health test were done in April, 2011 and again in April, 2012 for a complete assessment of soil health and fertility.

The study ran from April through September. In April, two 4’ x 60’ beds were prepared amended according to results of a soil test taken in fall 2010. Baseline measurements were taken in early May, and included a strong acid soil test, a Cornell Soil Health test, and soil conductivity of.

The two beds were divided into twelve plots, each with a blue field flag. Each plot was randomly assigned one of the four possible treatments. There were three plots of each treatment, five plants per plot for a total of fifteen plants per treatment. Only Marina knew which plot corresponded to which treatment.

May 3rd, potatoes were planted, and each week thereafter soil conductivity measurements were taken from the same three spots in each plot, (total of 12 weekly measurements per treatments per week) and when the plants emerged leaf sap Brix measurements were taken weekly from three plants per plot. Monthly measurements of stem girth and length were taken, and observations about predation and pests were made. Plants were given applications of soil drench and foliar feedings on alternating weeks.

The control group received only water,
T1 received water and liquid fertilizer,
T2 received water, fertilizer and inoculant, and
T3 received water and inoculant.

The fertilizers used in the foliar sprays and drenches were fish/kepl emulsion and Sea Agri salts. The inoculants used included “Mycotonic” and “Biogensis III”, which include mycorrhizae and bacteria, “Biogenesis”, which includes beneficial microbes, and “Pepzyme”, a microbial stimulant.

Research results and discussion:

The hypothesis was that there would be a measurable difference in potato production – number and weight, in soil conductivity and Brix measurements between groups.

First, biological inoculant and liquid fertilizer did not have an effect on emergence of plants in the spring.

There is a moderate positive correlation between soil conductivity and plant sap Brix (Pearson = 0.78). As soil conductivity increased, so did leaf sap Brix. Analysis of variance (ANOVA) showed that the treatment had an effect on both soil conductivity (F=4.416, p=0.01) and leaf sap Brix (F=3.474, p=0.027). The plants that were given liquid fertilizer had higher soil conductivity, and the plants that received inoculant had higher leaf sap Brix. Table 2 shows the seasonal measurements taken, reported as averages for each treatment.

Leaf sap Brix was not correlated with potato Brix, nor did treatment appear to have an effect on the potato Brix. This was confirmed with the taste test, as scores did not significantly vary according to treatment.

Treatment did not have a significant effect on the number of potatoes per plant, pounds of potatoes produced per plant, or mean tuber Brix. Table 3 shows the mean scores of post harvest data.

The 2011 season started off very wet and potatoes were slow to emerge. There was also much higher deer pressure than expected, and the problem was not solved before a number of the plants were eaten back a couple of times. As the deer did not eat from each of the treatment sections, the results may have been skewed by the predation.

In August, Hurricane Irene caused a flood in the back part of the fields, but the potatoes were hilled and in a slightly higher part. Nonetheless, conditions were very wet for the last month of growth, and production was low overall compared to previous growing seasons. Had conditions been better, perhaps greater differences would have been found. A combination of small sample size and less-than-ideal conditions may have had an effect on results.

The final component of the trial was a soil health test comparison between post-trial soil samples and samples taken in the spring of 2011, conducted by the Cornell Soil Science department. Samples were taken at random throughout the plots that had been inoculated, and were compared to samples taken from plots that were not inoculated, as well as to the baseline.

Overall soil health improved in both the control and the inoculated samples (scores of 86.1 and 88 out of 100, respectively, compared to 75.6 baseline). Overall soil health did not differ between the control and treatment samples.

Although the study does not support the hypothesis that potato yields can be improved by applying beneficial inoculant to the soil and plants, the weekly use of the soil conductivity measurer and the Brix reader proved to be a good way to keep track of soil and plant health. We will continue to use these tools to monitor field conditions in the future.

Research conclusions:

This study fails to show that the use of biological inoculants can increase yield and profitability.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary

Education/outreach description:

A demonstration of the methods was conducted on a farm day in July, when 19 Hudson Valley farmers and apprentices came to the farm. We demonstrated how to conduct the Brix readings, and soil conductivity readings, then applied the foliar spray and drench. The final results were subsequently shared with the group on the email listserve.

Two reports were given to the “Nutrient Density” course offered at Hawethorne Valley during the 2011 season. Between thirty and forty attendees were learning how to use the inoculants on their own fields, and regularly shared anecdotal reports.

Reports have been sent to Cornell cooperative extension to be published on their website, and to NOFA’s quarterly publication &”The Natural Farmer” for possible publication in an upcoming issue.

The results will be posted on the “Real Food Campaign” website, where other research on nutrient dense food production is collected.

A report has also been published on the author’s website,, and on the farm’s newsletter.

Project Outcomes

Assessment of Project Approach and Areas of Further Study:

Future Recommendations

This study should be repeated with better weather conditions (to avoid the Irene effect), and with different soil types. It would be interesting to see if inoculants are more or less effective in different soil environments, like some that are higher or lower in organic matter, and soils that are sand, silty, or clayey.

Further studies could look at the effect of inoculant on other nightshades, such as tomatoes or peppers, and measure the size, weight, and seed production of the fruit.

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.