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Compost Tea - Part II

by Diane Wells

Last month I wrote about compost tea and how it can aid in pest control and soil management. A number of folks use it, and a growing number of both organic and conventional growers are interested in it. This interest has led to some concern, though. Concern that the tea could coat your crop with hidden treasures, such as pathogens, and lead to a little more than bargained for.

The concern is this, from the USDA: “We have witnessed a number of large-scale food pathogen scares in recent years, from salmonella in tomatoes and peppers to E. coli in spinach and lettuce. Raw manure was often deemed the culprit. We know raw manure has a high probability of containing pathogens that may infect humans. That is why certified organic growers only apply it (to crops intended for human consumption) 90 to 120 days (depending on the crop) before harvest. And, that is why they can only use compost that has, among other things, maintained a temperature between 131 and 170 degrees Farenheit for three days (when using an in-vessel or static aerated pile system) or 15 days (when using a windrow system). These rules were written in an effort to reduce the risk of food-borne illnesses.”

In 2003, the pathogen spotlight was turned to compost tea. Even though derived from compost that should have been managed in such a way as to kill any pathogens present in the raw ingredients, concern that the tea could play a role in spreading pathogens persisted. It wasn’t the slow-steep method of tea production that raised eyebrows, but the aerated method that included adding molasses, yeast extracts or other nutrients to the tea solution. The tea-maker’s intent was to increase the size of the tea’s microbe community, which, in theory, would increase its beneficial properties. However, composting does not necessarily kill all pathogenic microbes; rather it drops their populations to what are considered acceptable levels. Could this practice of adding nutrients not only increase the number of beneficial microbes, but also have the unintended consequence of increasing the number of pathogenic microbes? Newly released study results indicate that it could.

Although links between compost tea and food-borne illnesses had not been made, the rule that states, “the absence of evidence is not evidence of absence” was oft repeated. It also didn’t help knowing that few replicated, peer-reviewed, scientific studies had been conducted on the stuff and its relationship to pathogens on the food we eat. The unknowns loomed darkly over the National Organic Program. Section 205.203 of the USDA Program’s final rule states, “The producer must manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil or water by plant nutrients, pathogenic organisms, heavy metals or residues of prohibited substances.” Confronted, the decision was made to take a proactive stance, and the National Organic Standards Board assembled a compost tea task force. The 13-member panel was charged with examining compost tea production and use practices and reviewing scientific data as it relates to human pathogen contamination of crops. Northeastern members of note include the panel’s chair, Dr. Eric Sideman, a botanist and organic crop specialist with the Maine Organic Farmers and Gardeners Association; Dr. Frederick Magdoff, a soil scientist with the University of Vermont; and Dr. Will Brinton, founder and president of Woods End Research Laboratory, an outfit in Mt. Vernon, Maine, that tests and measures organic matter and consults those who have a vested interest in it.

The panel reported its findings and recommendations in April 2004. The report is 21 pages long and easy enough to find on the Internet: go to www.ams.usda.gov and type “compost tea task force” under Search AMS. I do recommend reading it if you apply or are interested in applying compost tea to your crops. It’s a well-written summary of what we knew and didn’t know in 2004, including a review of the few scientific studies completed at the time.

What I’d like to touch on briefly are the panel’s final recommendations for certified organic growers (keep in mind this is a rundown; for details get your hands on that report). To start with, use potable water and sanitize your equipment between batches. Use compost produced according to NOP rules (compost piles, to start, should have certain carbon-nitrogen ratios that are derived from allowed feedstock materials, and then during the composting process the piles should be maintained at high enough temperatures to kill the pathogens—for details, see section 205.203 (c) (2) of the NOP rule). Compost tea, when made without additives, will not contaminate your crops and can be applied without restriction. If you are adding nutrients to your tea, and you want to apply it without restriction, it must be pretested and meet certain contamination guidelines recommended by the EPA. At least two batches derived from the same compost pile must be tested, and the average size of the bacteria population will determine whether your tea passes the test. A new batch of compost means another round of pretests. Once it passes, your tea can be used without restriction.

If you are adding nutrients to your tea but are not interested in going through the process of having it pretested, its application on human-consumed crops is restricted to the same 90 or 120-day preharvest period as raw manure. The only exceptions are crops for animal consumption, ornamental plants and grain crops for human consumption. For these, your tea does not have to be pretested, and it can be used without restriction throughout the growing season.

If you are responsible about what goes into your compost pile or windrow, and you keep track of how it is managed, you should not have to worry about feeding pathogens to your family or customers. Last point: compost is as diverse as the farms that produce it. Just think of all the variables at hand. However, your farm, with consistent management practices, physical characteristics and animal populations should produce a consistent tea profile. Send it out for testing a few times and wait for that report. You might find the results beneficial, never mind interesting.

The author, a brand-new contributor to Farming, is a biologist who lives and farms in Vermont’s Northeast Kingdom. Comment or question? Visit www.farmingforumsite.com and join in the discussions.