Compost tea - let’s talk about it! Just what exactly is this blackish colored, liquid substance people call compost tea and why do growers use it on their crops? How is it made and how is it applied? In the age of ever increasing food safety concerns, is it safe to use in the production of fresh produce?
First, let’s cover the basics. At its most elemental level, compost tea is a liquid brew made by steeping finished compost in water, usually aerated with pumps, to achieve an elixir containing the soluble nutrients and microbial biomass present in the parent compost – this is known as aerated compost tea (ACT). Some growers brew their tea without aeration by placing loose compost in a large, teabag-like netted screen, and letting it steep for multiple weeks without air pumps or constant circulation – this method is called Non-aerated Compost Tea (NCT). Many people believe the aeration helps shorten the brew time by aiding with the multiplication of beneficial microbes by the millions or even billions. These beneficial organisms tend to be aerobic in nature, so most commercial and home growers use the ACT method and this article will focus on this specific type of brewing.
Peer reviewed scientific literature on the efficacy of ACT is notoriously lacking, however some of the claimed benefits of compost tea to farming systems include the suppression of disease causing pests and organisms; improved nutritional quality of plants; improved soil structure via water infiltration and retention; increased biological activity in the root zone; and the chelation of nutrients making them more bioavailable for plants. While study results vary, one of the main goals of ACT production is to take the beneficial bacteria, fungi, and protozoa already existing within the parent compost and grow them exponentially to inoculate your soil or plant surfaces with beneficial organisms.
To aide in the growth of the beneficial organisms, some brewers use additives such as kelp meal, fish hydrolysate, humic acids, rock dust, molasses, etc. These additives provide a food source for the rapidly multiplying microbial population within the solution. Brewing the tea for 24 to 48 hours, growers then apply it as a soil drench or foliar spray direct to plant surfaces. Some brewers do not use additives to spur microbial growth, using the tea more as an extract for the soluble nutrient content within the finished compost.
Considering the safety of compost tea, a consensus seems to be emerging that the safety of the tea is all about the quality of the ingredients being used, as well as how those ingredients are used.
Let’s unpack this. Starting with completely finished, stabilized compost is absolutely key to reducing the risk of human pathogens such as E.coli, Listeria monocytogenes or Salmonella sneaking their way into the brewing process. The Food and Drug Administration’s Produce Safety Rule defines Stabilized Compost as “a stabilized (i.e. finished) biological soil amendment produced through a controlled composting process.” They also refer to composting as “a process to produced stabilized compost in which organic material is decomposed by actions of microorganisms under thermophilic conditions for a designated period of time (for example, 3 days) at a designated temperatures (for example, 131 degrees Fahrenheit) followed by a curing stage under cooler conditions.” So, it is critical to start with compost that has been validated to be free of pathogens.
Aerated compost tea is all about growing the beneficial microbes already existing within the finished compost, especially with the use of additives like humic acids, rock dust, fish hydrolysates, etc. If even one small portion of the batch of compost is unfinished and contaminated with a human pathogen like Salmonella or E.coli, the brewing process will ensure ALL microbes will grow in the brew, including those of the pathogenic variety! Some brewers argue that vigorous aeration ensures these pathogens will not grow, however, Salmonella and E.coli are facultative anaerobes and can therefore grow in both aerobic and anaerobic environments, and so neither ACT nor NCT are exempt from brewing without precaution.
Using water free from pathogenic organisms is also critical for the same reasons. Should some small amount of E.coli O157:H7 be present in the water used for brewing (e.g. surface water from the ditch), the greater the likelihood of the growth of that pathogen being amplified during the brewing process. Water validated as pathogen free reduces risk of the final brew being contaminated.
The FDA’s FSMA Produce Safety Rule (PSR) does not specifically define “compost tea”, however FDA does define and regulate what they call Agricultural teas, meaning “a water extract of biological materials (such as stabilized compost, manure, non-fecal animal byproducts, peat moss, pre-consumer vegetative waste, table waste or yard trimmings), excluding any form of human waste, produced to transfer microbial biomass, fine particulate organic matter, and soluble chemical components into an aqueous phase. Agricultural teas are held for longer than one hour before application. Agricultural teas are soil amendments for the purposes of this rule”. As you can see, the FDA’s definition includes more than just compost as a feed stock and they specifically call out an agricultural tea as a soil amendment for the purposes of the new FSMA Produce Safety Rule (PSR). The “soil amendment” language appears to exclude the use of agricultural teas for foliar applications, and as noted below, we will be working with FDA formally to find out if agricultural teas or compost teas can be applied via foliar methods.
Forgoing tea additives is also seen as a way to ensure a safer agricultural tea, however as noted above, this type of tea will likely not pack the biological punch growers are brewing it for. Microbiologists David Ingram and Patricia Millner at the Agricultural Research Service (ARS) found that ingredients commonly added to compost tea can promote the growth of pathogenic and beneficial organisms, while teas made with finished (stabilized) compost and no additives did not show substantial growth of pathogenic organisms in the final tea product. FDA’s Produce Safety Rule (PSA) defines Agricultural tea additive as “a nutrient source (such as molasses, yeast extract, or algal powder) added to agricultural tea to increase microbial biomass”.
So, if a grower wants to make and use an agricultural tea and be compliant with FSMA’s Produce Safety Rule, how would they go about it? Well….as typical with new regulations, especially rules trying to capture the complexity of the agricultural industry, the answer is about as clear as a good batch of compost tea!
Some clues on how to comply with FSMA can guide us. It appears the key to using an agricultural tea in a manner compliant with the FSMA PSR, lies within how FDA defines and describes the use of “treated” or “untreated” biological soil amendments of animal origin, detailed in FSMA PSR’s Subpart F.
It appears the key question for FDA intrepretation is “are agricultural teas made with potable water (free of pathogens), stabilized compost, and no additives considered "treated biological soil amendments of animal origin" by FDA? And if so, additional clarifications on the application restrictions in FSMA PSR Section 112.56 will be helpful to the agricultural industry. For example, in what (if any) cases can a grower apply an agricultural tea as a foliar spray.
With regards to tea made with additives, it appears a key question for FDA intrepretation is "are agricultural teas made with potable water (free of pathogens), stabilized compost, with the addition of agricultural tea addivitives for the microbial benefits, considered "untreated biological soil amendments of animal origin" by FDA? If so, will agricultural teas made through this process need to be applied per FSMA PSR Section 112.56 and what specifically do those application restrictions look like?
The organic and food safety certification body I work for, CCOF, will be utilizing the FDA’s Technical Assistance Network (TAN) to submit specific questions about the brewing and use of agricultural teas. Through this system anyone can submit a technical question about the FSMA rules and FDA will formally respond publically so the entire agricultural community can benefit from the discussion. Please stay tuned to CCOF website and the FFSCN blog for the outcomes of these technical questions, or even ask FDA on your own!
But let’s get back to making a safe tea. Another important way to ensure your compost tea is safe is by sanitizing all the equipment used to brew the tea after each batch and flushing your drip lines with (an organically approved if your farm is certified or in transition) drip line cleaner. When brewing tea, biofilms can build up on the equipment and in the drip lines, serving as a protective habitat for pathogenic organisms, allowing them to survive on the equipment and potentially taint further batches of tea.
In 2006 the USDA National Organic Standards Board (NOSB) looked at the issue of compost tea safety and came up with strategies and recommendations that are similar to those found in this article. You can find these recommendations within a popular compost tea guide called Tea Time in the Tropics, which is a great reference for the would be tea brewer.
Bottom line, if you want to brew an agricultural tea and you are growing a product eaten fresh, it is a really good idea to use the very best stabilized compost, potable (pathogen free) water, standardized procedures and ingredients, sanitized equipment, and then take regular samples of the final tea for listeria, salmonella, and E.coli to reduce the risk that the final product being applied risk to the people consuming your produce. If your ingredients or processes change, update your brewing procedures and retest the final product.
As with everything food safety and organic compliance related, keep records on hand to show inspectors you are working to reduce the risk of your tea and to track your processes. Similarly, if your farm is certified organic or audited by third-party food safety certification body, be sure to check with your certifier to ensure you are complying with all the applicable standards.
Many successful growers believe compost or agricultural teas are a valuable asset in their crop health and soil nutrient programs. Knowing how to ensure a safe application of this biologically rich brew helps growers give back to the land while keeping consumers safe. Happy brewing!
by Jacob Guth