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Home > Protecting Our Lands & Waters > Sustainable Agriculture > The Greenbook > Greenbook 2017 > Using Compost Tea in Organic Farming

Using Compost Tea in Organic Farming

Principal Investigator: Becca Carlson, Seeds Farm
Keywords: biological activity, compost, compost tea, fruit, microorganisms, soil health, vegetables

Project Summary

Key Details

Project Description



Management Tips


Project Location

Other Resources

Project Summary

We are testing the effects of compost tea on vegetables, fruit bushes, pasture, cover crops, and hay ground. Compost tea inoculates the soil with microorganisms, including bacteria, fungi, protozoa, and nematodes, enhancing the soil food web. The six farms participating in this project grow vegetables, fruits, or pasture/hay, and are each comparing treated areas (compost tea applied) to similar control areas (no compost tea applied). Our overall goal is to determine whether applying compost tea to our crops can improve farm profitability. Reducing fertilizer needs, increasing yields, and/or increasing produce quality are all possible benefits of compost tea, but we are particularly interested in the potential for reducing purchased fertilizer.

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Key Details

Principal Investigator

Becca Carlson
Seeds Farm
201 Lincoln St S, Northfield MN 55057
Rice County

Project Duration

2014 to 2016

Award Amount


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Project Description

Seeds Farm shared the following story that illustrates why so many of us perceive a need to boost soil microbial activity: A college student buried dead squirrels in our vegetable fields, a nearby forest, and a nearby prairie. When she dug the squirrels up at the end of the season she found that the squirrel in the vegetable field had only barely decomposed, while the squirrels that had been buried in the forest and prairie were completely decomposed. These results showed us that agricultural practices can discourage soil microbes.

Compost tea is a liquid produced by extracting bacteria, fungi, protozoa, and nematodes from compost. The idea is to extract and replicate the beneficial biology and diversity of compost in a liquid form. Nutrients extracted from the compost (and/or added to the tea) grow beneficial organisms. Tea (extract) can be applied directly to the leaf surface of a plant as a foliar spray or used as a soil drench to improve root systems. Together, the beneficial bacteria and fungi result in a variety of many different species in the compost tea.

The value of compost tea is related to the importance of the soil food web. The soil food web is the community of micro-organisms living all or part of their lives in the soil, which includes bacteria, fungi, protozoa, nematodes, and earth worms. This community of organisms transfers nutrients through the soil, converts nutrients into forms plants can use, and helps protect crops from soil-borne pathogens. The very structure and health of our land is directly influenced by this complex set of biological and chemical interactions that decompose, retain, and recycle nutrients within the soil.

Compost tea works by putting beneficial microorganisms of the soil food web that your plant needs onto the leaf surface of the plant or the soil. To enhance this community of beneficials, the compost tea must remain aerobic.

The two key reasons to use compost tea are:

  1. Impart microbial life into the soil or onto the foliage of plants.
  2. Add soluble nutrients to the foliage or to the soil to feed the organisms and the plants present.

Note: The project leaders provided these descriptions of compost tea and the soil food web from information at Rodale Institute and Earthfort.

This project includes six farms in the Northfield/Nerstrand Minnesota area. Each participating farm in the project chose one crop (vegetables, fruit bushes, pasture, cover crops, or a hayfield) to spray with compost tea (Table 1). We are leaving unsprayed areas to serve as a control so that we can observe and measure any effects from the compost tea. We are evaluating yield, brix levels, plant health (through plant tissue analysis), and soil health (by analyzing the number of micro-organisms living in the soil).

Part of our project included figuring out how to brew and apply compost tea effectively and efficiently. Brewing consists of suspending a bag filled with biologically active compost in a container of water and using forced air to physically knock off the microorganisms and suspend them in the water. Bacteria, molasses, fish hydroloslate, kelp, steel cut oats, and/or humic acid, can be added to encourage these populations of microorganisms to grow. The tea must be kept aerobic and must be applied within 2 days of brewing. Foliar spraying, putting through drip irrigation lines, and gravity feeding behind a subsoiler can all be used to apply the compost tea, as we learned in year one of the project.

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Little Hill Berry Farm

Little Hill Berry Farm grows 4 acres of certified organic blueberries. We sell our berries primarily pick-your-own with 2016 being our fourth season open for picking.

In 2015 we applied compost extract to two varieties of blueberry plants, Patriot and Bluegold, (both 2 and 4-year old plants).

We made the compost tea by agitating 4 lb compost contained in a fine nylon mesh bag in 20 gal of water for 4 min. Our treatments included: control (no compost tea, no extra water), low (four applications between May-October), and high (11 applications between May-October). Each application consisted of one quart of compost tea poured at the base of each plant. Each variety/age pairing had five replicates of each treatment.

To quantify soil microbial activity we measured soil nitrogen mineralization. To quantify plant growth, we measured leaf carbon fractions. We found that N mineralization was higher in the “high” treatment compared to the control. We did not find a statistically significant difference in for the “low” treatment.

Neither variety nor plant age had any effect on N mineralization within treatments, and we found that compost extract alone (without any of the other ingredients frequently added, such as fish fertilizer, humic acid, kelp, molasses, etc.) produced a measurable increase in soil microbial activity.

The effects of compost extract on leaf carbon fractions were not as clear. We found the “low” treatment plants had less leaf carbon compared to the control, while the “high” treatment had no effect compared to control. We thought increased N mineralization by the soil microbes would lead to increased plant uptake of nitrogen and a corresponding increase in leaf carbon. The fact that we found no increase in the leaves may indicate that the microbes consumed the N before the plants could take it up. This nitrogen should be available to the plants in the future, so it is possible we will see an increase in leaf carbon next year.

In 2016, we experimented with two different methods of brewing compost tea. The first brewing method was to aerate the tea. We used Purple Cow Activated Compost in the tea and no other additions (i.e. fish, humic acid, etc.). The second method was to aerate the tea and add heat. We applied the tea weekly over the course of the growing season. All of the blueberry plants were the Patriot variety and 4 years old.

Preliminary results showed that heating and aerating the tea increased the protein content of leaf tissue by 10% over the control group. This increase suggests that plants in these treatments might have had greater access to nitrogen. Thus, the application of compost tea may increase plant-available nutrients and potentially serve as an alternative to other nutrient amendments in organic agriculture.

We measured microbial enzymes in the soils in the three different treatments but did not find any significant results. We were surprised that the change in leaf chemistry was not accompanied by any detectable change in the soil. Plant nutrient content reflects soil microbial activity and nutrient content. It may be that we were not testing the right things in the soil to capture the change in microbial activity.

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Open Hands Farm

We grow certified organic annual fruits and vegetables, certified on 15 acres of silt loam. In rainy or humid years we have struggled with fungal and bacterial pathogens. Since 2012, we have been using a probiotic brew to enhance plant health and resistance to these pathogens.

Our first line of defense against pathogens is resistant varieties, plus crop rotation, soil-building, adequate irrigation and other cultural practices to reduce plant stress. When pathogens become a problem many organic growers turn to copper or other organic-approved fungicides. We do not use pesticides--even organic-approved pesticides- choosing a probiotic approach instead of an antibiotic approach. Our goal is to manage pathogens by supporting diversity and abundance in the microbial communities in the soil and on crop leaf surfaces.

In previous years we applied compost tea to the soil, testing the soil for microbial activity before and after application. The lack of measurable results in the soil was most likely due to our tillage activity, both deep plowing and shallow cultivation, destroying microbial populations. Without scalable methods for organic no-till vegetables, we continue to till; so this year we re-focused our probiotic efforts on crop leaf surfaces, with a foliar application experiment in carrots.

Leaf health in carrots is important to yield and to the functioning of a mechanical carrot harvester, which grabs the carrot by the leaves. Weaker leaves break when grabbed and leave carrots in or on the ground, whereas strong tops enable proper functioning of the machine. Conventional carrots often receive multiple fungicide treatments, sometimes right up to harvest time, to keep carrot tops healthy for the harvester.

We have struggled a little with Alternaria blight, the primary pathogen responsible for weak tops in carrots. We typically apply the probiotic brew (supplier and brewing details below) 3-4 times per planting for most crops, including carrots. For the experiment, we left one patch (0.2 acres) of the carrot variety Bolero untreated, while treating the rest of the planting with three foliar applications. Soil type and all fertilizer, irrigation and other treatments were identical. We observed carrot top health, functioning with the mechanical harvester, and yield.

After we found it difficult to replicate quality compost tea with high microbial activity, this year we decided to experiment with a probiotic brew we’ve had anecdotal success with over several years. We use the recipe and supplier info in the book The Holistic Orchard by Michael Phillips. The supplier of the “mother culture” is SCD Probiotics in Kansas City, MO. We use the “Probio Balance Original” culture, a laboratory bacteria culture. One Gallon of this “mother culture”, fed with molasses and cultured 7-10 days, makes 21 gallons of activated brew.

We mixed this brew with water and fish hydrolysate, at a rate of 2 gal mature microbe brew and 2 gal fish per 100 gal water (we use a 100 gal sprayer, with a 50’ boomless nozzle). (For heavy feeding crops and/or high-stress conditions we will sometimes use 4 gal of fish hydrolysate per 100 gal tank, but the base rate is 2 gal.) Applied at a rate of 33 gal/A, each tankful covers 3 acres. Therefore, from 1 gal of mother culture we brew 21 gal of activated brew; enough brew for 10 sprayer tankfuls, to apply to 30 acres worth of crops.

The control patch of Bolero carrots was clearly less healthy than the carrots that received compost tea; the control carrots had weaker tops and lower yields. In some places the tops were completely dead, and in the whole patch they were thinner and weaker, making mechanical harvest very difficult and requiring extensive hand gleaning behind the harvester. In the treated portions of the same planting, the tops were visibly strong and healthy, and mechanical harvest was effective, leaving behind very few if any carrots.

The treated areas, also had bigger carrots and higher yields. The yield comparison was approximately 21,000 lb/A in the treated sections, and less than 15,000 lb/A in the control. We suspected the weak and dying tops in the untreated section could not support the final stage of root growth – the final thickening and lengthening of the marketable portion of the plant.

The cost of 1 gal of mother culture (delivered) was about $70, plus we used about $20 worth of organic molasses and $220 worth of fish hydrolysate, for a total materials cost of about $310 for 30 acres of coverage. That’s under $11/A of materials cost for this probiotic brew, which can be made without special equipment (just a large stockpot, stovetop, and sleeping bag to keep it warm for a few days).

The labor and machinery cost of application is about $12/A: for every 9 acres, we use 1 hour of brewing labor ($15), 3 hours of application time ($90).

For valuable specialty crops susceptible to fungal and bacterial pathogens, we find applying compost tea to be a very cost-effective practice. More study would help confirm this conclusion. For example, an experiment that included treatments with and without fish hydrolysate, and on other crops would be helpful. While the rate of fish hydrolysate application is very low compared to typical foliar applications and is primarily meant as a starter food for the microbes, it is also probably having a beneficial effect on the crop. But is the fish the primary, or only, cause of benefit? If the fish is causing the benefit and the probiotic compost tea is irrelevant, skipping the brewing would mean saving labor.

The Bolero variety of carrot is one with strong resistance to Alternaria. It would be interesting to study the effects and interactions of the tea on varieties that do not have resistance to this pathogen. Pathogens of particular concern on our farm are black rot in brassicas, Septoria and Alternaria in tomatoes, and downy mildew and bacterial leaf spot in leaf lettuce. We need more non-toxic tools to keep crops healthy in rainy and humid years.

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Spring Wind Farm

We grow vegetables on 6 acres for our CSA members and several wholesale accounts. In 2015, we tested compost tea on six plots in a field that had been farmed conventionally the year before. We applied compost extract weekly to three plots in July and August, and applied water to three other plots as a control. We used the same “recipe” as Little Hill Berry Farm, put the compost in a mesh bag, and swirled it in the spray tank for 1 min. We applied the compost tea with a backpack sprayer in a single pass, trying to simulate the amount of spray that would have been applied with a tractor-mounted sprayer. We lightly incorporated the compost tea into the soil with a hoe.

After the last application, we took soil tests and had them tested for glyphosate. We were surprised to find the control plots had glyphosate levels of 67.75ppb while levels in the treated plots were higher (80.42ppb). Last year’s results were similar; soil samples from the control had 63.69ppb glyphosate and those that got the extract treatment had 84.17ppb. This is exactly opposite of what we expected; we thought that applying compost extract would boost soil microbial activity, and that this activity would reduce glyphosate levels. We’ve also begun to wonder if the higher rates of glyphosate on the experimental plots could suggest that there is glyphosate in our water. Our wells are 360’ deep, so this is concerning; next year we will test the water.

This year, we experimented with spraying a brewed compost tea on our cherry tomatoes. We used the same brewing process and compost as Little Hill Berry Farm. We applied the tea with a backpack sprayer on the foliage of a 200’ row of tomatoes once a week from June – August. We hoped that the application of the tea would reduce the amount of fungal disease in the tomatoes.

We did not see any visual difference in foliar disease between the rows of tomatoes in our trial. However, our cherry tomatoes did much better than our other tomatoes, which received no spray. We had 3 - 200’ rows of cherry tomatoes, spraying the middle one with tea and using the outer two as the controls. So it is possible that the beneficial microbes from the tea spread to the outer two rows. I am excited to keep experimenting with foliar applications as a disease management tool.

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Seeds Farm

We grow 6 acres of diverse vegetables at our farm. We want to know whether compost tea can be an effective tool to rejuvenate disturbed land. Our township is building a 10’ deep pond across the road from our farm. The fill generated from digging the pond will be placed on our field (after the topsoil has been stripped) to raise the field 7’ in some areas. The entire footprint of the disturbed land is 4 acres.

The pond project has been delayed and the soil had not yet been transported to our farm at the time we submitted the report for this article.

In the meantime, we brewed and applied compost tea on our farm four times this year: May 8, June 10, July 30, and November 26. We brew in a 275 gal tote with a ½ horsepower regenerative pump blowing air through a 1.5” tube through the bottom of the tote at full force, right underneath a suspended bag of high quality compost. We used 3 gal of activated compost, 0.5 gal fish emulsion, 16 oz kelp, 16 oz molasses, and 0.5 cup sea salt (with minerals). We brewed the mixture for 24 hr and applied at 20 gal/A with a 100 gal boom sprayer.

We sent a sample of our compost tea to Crop Services International in Michigan. They told us our compost tea was good quality, with bacteria, fungi, and protozoa all in the desired levels. The recipe and ingredients we used consistently produced a quality tea, but indicated a lack of nematodes. In the future we will use vermicompost in addition to the compost. Starting in spring 2017 we will apply compost tea to portions of our disturbed field (leaving portions as a control) to see if the tea we brew can be used to aid in rejuvenating the soil faster.

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Woodskeep Orchard

Woodskeep Orchard joined the compost project as a new partner in 2015. We have a high density apple orchard with nearly 30 varieties that we grow primarily for cider. We are interested in becoming certified organic and want to see whether we can meet many of our fertility and disease management needs through foliar compost tea applications.

We brewed compost tea in a homemade brewer and sprayed two rows four times in 2015, leaving two rows of varieties unsprayed as a control. We compared soil tests, disease/general appearance, and fruiting in the treated and control rows.

We saw no differences between the treated and control rows for the same variety and location this year. However, apples are a perennial crop, and many of the benefits of compost tea may not be seen for a couple of years. We think the health of the orchard in general is very good, but it is difficult for us to quantify.

In 2016, we saw some differences between rows in the Sweet 16 block. There were no visual differences, and only insignificant soil differences. However, leaf analysis in August showed differences between the two rows of Sweet 16 rows in almost every measurable factor.

The sprayed rows had increased fertility in all but two measured elements, though this was just one test, and we would have more accurate results if compared to multiple tests over time.

The visual health was comparable. Due to a late frost, most of the fruit was killed and therefore there were none to compare. Apples are a perennial crop and many of the benefits of compost tea may not be seen for a couple of years. The health of the orchard in general is very good, but it is difficult to make definitive conclusions without long-term measurements.

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A second project replicating Little Hill Berry Farm’s method of spraying compost tea on the soil in perennial fields would help solidify our results. Testing across many different crops, including other perennials, would be the best way to determine whether or not applying compost tea is helpful.

Little Hill Berry Farm, Woodskeep Orchard, and Seeds Farm plan to continue their use of compost tea, since the results from this project indicate that it improves the health of crops. Little Hill Berry Farm expects that the benefits of spraying compost tea will increase each year over a period of 5 to 10 years. Though it may not entirely replace commercial organic fertilizer, it will likely allow them to reduce the amount of commercial fertilizer they use.

This project has given our group of farmers working on it more confidence in compost tea. We recommend the use of compost tea due to the minimal costs of applying and the potential gains. Applying compost directly at large rates on our fields would be ideal, but expensive. By brewing compost tea, we are instead growing the microbial populations that live in the compost and applying them to the field. This method is second best, and much more economical. Once a farm has set up a system to brew and apply compost tea, it is relatively easy to implement an application plan.

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Management Tips

  1. Compost tea’s effectiveness depends on how it is brewed. Heating and aerating the tea during the brewing, seemed to produce a better result.
  2. Don’t take short cuts when making your brewer! Though it isn’t a difficult process, it is important that it is done correctly. Pentair makes a great air pump for brewing the tea.
  3. Plan ahead! This probiotic brew takes 7-10 days to mature. Adjust the amount of each brewing batch for the amount of acres you expect to cover in a week.
  4. Watch the weather. Timing is crucial for foliar feeding, and spraying in inclement weather affects coverage.
  5. Start your spray schedule as early as possible in the year in order to give your crops maximum coverage and exposure.

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  • Dane Terill, Crop Services International, Portage, MI
  • Andrew Ehrmann, Spring Wind Farm, Northfield, MN
  • Molly Haviland, Living Soil Lab, Fairfield, IA
  • Dan Hernandez, Carleton College, Northfield, MN
  • Erin Johnson and Ben Doherty, Open Hands Farm, Northfield, MN
  • Tracy Jonkman and Nate Watters, Woodskeep Orchard, Dundas, MN
  • John Porterfield, Cherry Leaf Farm, Northfield, MN
  • Aaron Wills, Little Hill Berry Farm, Northfield, MN
  • Kathy Zeman, Simple Harvest Farm, Nerstrand, MN

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Project Location

This project took place on several Northfield/Nerstrand, MN area farms. To reach any of the participants, contact project leader Becca Carlson, whose information is provided at the top of this article.

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Phillips, Michael. 2012. The Holistic Orchard: Tree fruits and berries the natural way. Chelsea Green Publishing. White River Junction, VT.

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MDA Contact

Meg Moynihan

Ag Marketing & Development Division