Compost tea is a liquid extract of compost made with water, as LSU AgCenter describes it. Extension and USDA-hosted sources explain how it is made, what trials have found, and the precautions that apply on edible crops.
This page covers one topic from the wider guide to organic gardening for beginners.
What compost tea is and how it is made
WSU Extension defines compost tea as a liquid extract produced by diluting compost with water. The LSU AgCenter fact sheet separates aerated tea, which uses agitation or forced air to maintain an oxygen-rich atmosphere during brewing, from non-aerated tea, which steeps with no effort to maintain an oxygen-rich environment; LSU notes that this tea is produced in an anaerobic environment. The same fact sheet says that compost tea is not a plant tea, meaning a water extract of fresh, uncomposted plant material, and not a manure tea, meaning a water extract made using uncomposted animal manures.
UConn Extension describes the aerobically activated version: mature compost is added to water in a container through which air is passed, and after 24 hours the formulation would be sprayed on plants or on the soil, according to its food safety fact sheet. That document adds that kelp, fish extracts or sugars such as molasses are sometimes added as a source of nutrients for the beneficial microbes. LSU AgCenter lists molasses, soluble kelp, yeast, humic acid and fish hydrolysate as commonly used additives.
What the research says about benefits
WSU Extension states that few controlled, replicable scientific studies exist to support claims of the tea’s effectiveness. It also says that a lack of consistency in compost age and ingredients, and in tea preparation and application methods, makes evaluating claims of efficacy difficult and often leads to inconsistent and conflicting results.
The same WSU fact sheet reports one trial on cherry blossom brown rot. Thirty-two trees of eight different cultivars were studied, and two treatments were compared: spraying with compost tea and spraying with water. According to the WSU report, the study found little difference in rates of brown rot infection between the two treatments. The compost tea did not reduce disease rates, and in some cultivars tea applications worsened the infection rates, but WSU reports that treatment with compost tea was not significantly different than treatment with water. The trial used two trees per cultivar per treatment. WSU adds that more research needs to be done before any definitive recommendations can be made.
The conclusions in the LSU AgCenter fact sheet are along the same lines:
- A few studies have shown slight disease suppression under laboratory conditions.
- Science and the compost tea industry disagree on the effects and benefits of compost tea.
- There is no scientific evidence for most beneficial claims regarding compost tea.
- There is no scientific evidence for the preferred use of compost tea over compost.
WSU’s bottom line is that compost teas have not been suitably characterized, nor have their purported benefits been validated scientifically. It also notes that properly composted organic material makes a wonderful mulch.
Food safety and edible crops
UConn Extension notes that backyard compost piles often do not heat up to 130 to 150 degrees F, so pathogens are typically not controlled in home piles. In its fact sheet on compost, compost tea and manure, Salmonella, E. coli O157:H7 and other bacteria, viruses and parasites are described as able to live in a pile of unmanaged compost. The fact sheet notes that compost teas can be made from leaf- or manure-based composts. Even though commercial composters use horse and dairy manures in their recipes, it says home gardeners should not use any animal waste, including pet waste, because animal waste may contain pathogens that might not be destroyed during the compost process; even birds, guinea pigs and rabbits can carry pathogens such as Salmonella, and it is also best not to put meat scraps or dairy product waste into a compost bin. From a possible pathogen contamination standpoint, it adds, it may be safer to use a plant-based compost in the vegetable garden.
The same fact sheet reports that USDA Agricultural Research Service microbiologists David Ingram and Patricia Millner have found that ingredients commonly added to compost tea may promote growth of bacteria that can cause illness in humans, including Salmonella and E. coli. It advises that, unless a gardener is positive the methods used will minimize the presence of human pathogens, it would be wise to save the teas for nonedible plants. It also says little is known for sure about the uptake by plants of human pathogens from the soil, and says it makes sense, at the very least, to minimize the contact of compost teas that have not been managed to kill or prevent the growth of human pathogens with edible plants, particularly close to harvest.
The LSU AgCenter fact sheet makes several points about edible crops and additives:
- Manure teas should never be used for crops grown for human consumption, because human pathogens are a major concern.
- Most compost teas are not approved by the Environmental Protection Agency (EPA) for use on edible crops. Minimally, approval would require EPA-approved compost, approved agricultural water, and no post-brewing additives.
- Additives can increase the number and variety of both good and bad microorganisms in the compost tea.
- Use of compost teas on commercial fruit and vegetable production is prohibited in most instances because of additives.
- Production and use of compost teas at home often carries the risk of human pathogens.
Organic rules and the NOSB guidelines
The USDA National Organic Program (NOP) guidance on processed animal manures, effective July 22, 2011 and with links updated August 31, 2018, says the regulations require uncomposted animal manures to be applied at least 90 days before harvest for crops whose edible portions do not come in contact with the soil, and at least 120 days before harvest for crops whose edible portions do. The same NOP 5006 document states that it does not apply to compost teas or other products that may contain raw or composted manure.
The compost tea section of a compost tipsheet from the National Center for Appropriate Technology, published July 2015 and hosted on the USDA Agricultural Marketing Service site, summarizes the NOP’s draft guidance on crop materials for compost tea. In the tipsheet’s words, “The NOP draft guidance on crop materials states that compost tea made from compost not meeting the requirements of § 205.203(c) or NOP 5021 is subject to restrictions of § 205.203(c)(1) for raw animal manure.” This is NCAT’s 2015 summary of draft guidance; as noted above, the NOP’s own guidance on processed animal manures states that it does not apply to compost teas. The tipsheet then lists guidelines that were set forth by the National Organic Standards Board and are not regulatory requirements:
- Use only potable water to make compost tea or to dilute it.
- Sanitize all of the equipment used to prepare compost tea.
- Make compost tea only from compost that has maintained a temperature of at least 131°F for three days and has been mixed so all of the pile or windrow has heated up, as listed in the tipsheet.
- Avoid additives when fermenting compost tea, as these can promote the growth of harmful organisms. In particular, simple sugar sources, like molasses, should be avoided.
- Additives can be used if sample batches of compost tea are tested before use to make sure the tea meets the EPA’s Recreational Water Quality Criteria with regard to coliform bacteria.
- If compost tea is made with additives but is not tested, or if it does not meet water quality guidelines, food crops may not be harvested until 90 to 120 days after the tea has been applied, as with raw manure use on organic farms, according to the same tipsheet.
Organic rules and guidance can be revised, and the tipsheet describes draft guidance, so growers working under certification should confirm the current requirements for compost tea with their certifier or the National Organic Program.
Nutrient load and other ways to use compost
WSU Extension compares tea with mulch: unlike compost used for mulch, which provides a slow release of nutrients, compost teas most certainly add increased levels of nitrogen, potassium and other minerals all at once. The WSU fact sheet says it is unlikely that these are completely absorbed by the plants, and that they may instead contribute to the eutrophication of watersheds. Its bottom line also states that compost teas can be overused and potentially contribute to ground water pollution.
Gardeners comparing ways to feed soil can also read about organic fertilizer types and how they release nutrients, or about vermicomposting.
How current the sources are
The sources differ in age. The LSU AgCenter fact sheet is marked as published in April 2022, the NOP 5006 guidance took effect in July 2011, and the NCAT tipsheet was published in July 2015. The WSU and UConn fact sheets carry no date on their pages. Among its NOSB references, NOP 5006 lists an April 2002 Crops Committee Compost Tea Task Force Final Report and a September 13, 2006 Crops Committee recommendation for guidance on the use of compost, vermicompost, processed manure and compost teas; the 2015 tipsheet presents its compost tea rule as a summary of NOP draft guidance.
Frequently asked questions
What is compost tea?
It is a liquid extract that results from steeping compost in water, using a variety of preparation methods, as LSU AgCenter puts it.
Is compost tea safe to use on vegetables?
UConn Extension advises saving teas for nonedible plants unless the grower is positive that the methods used minimize human pathogens, and LSU AgCenter states that most compost teas are not approved by the EPA for edible crops, as detailed in the LSU AgCenter publication. The same publication adds that manure teas are never to be used for crops grown for human consumption.
Does compost tea control plant disease?
LSU AgCenter reports slight disease suppression in a few laboratory studies, while the WSU trial on cherry blossom brown rot, described in its fact sheet, found that the tea did not reduce disease rates.
Should molasses be added to compost tea?
The National Organic Standards Board guidelines quoted in the NCAT tipsheet say to avoid additives when fermenting compost tea and that simple sugar sources like molasses should be avoided, as the tipsheet explains. Those guidelines, which the tipsheet describes as “not regulatory requirements,” still allow additives where sample batches are tested first and meet the EPA’s coliform criteria for recreational water. For additive tea that goes untested, or that fails those criteria, they say food crops may not be harvested until 90 to 120 days after application, as with raw manure on organic farms.
