When compost is finished and how to use it
The signs of finished compost, a germination test, curing and screening, and how to use compost, give it away or hold it, with the cautions that apply.
- Start here: the essentials
- The signs of finished compost
- The germination test
- Curing and screening
- Using finished compost
- Handling compost safely
- Compost tea, and why we do not teach it
- More tests, and rates side by side
- See it: Tomás checks the bin, Hannah and David and Ingrid read a test
- Practice: check, test, screen, log
- Your system
- Check your work
- Knowledge check and scenario
Start here Essentials
Finished compost is recognized by its signs and a short test, not by a date. This chapter asks you to check your oldest material against the signs, run one germination test, and write down where the compost goes. Most first seasons have nothing to harvest yet; in that case you write the date to check and read the test through.
- Check your oldest material against the signs. Look, smell and feel (signs of finished compost).
- If it passes, run the germination test. Seeds on wet paper towels, compost water against plain water (germination test).
- Screen what passes and return the rest to the bin. Unfinished material goes back to finish (curing and screening).
- Use it where your plan says, and log the amount. Garden bed, containers, mulch or a neighbor (using finished compost, harvest sheet).
Minimum time: one afternoon to check and set up the test, then a short look every couple of days for about a week. Done when: the harvest sheet shows one harvest, its test result and where it went. If you have nothing to harvest yet, done is the date to check written on your plan and the germination test read through.
The signs of finished compost Understand
Finished compost is compost in which the original materials have broken down and the material has stopped changing quickly. The word for that state is maturity. A pile can look like soil on the outside and still be active inside, so the sources give signs to check rather than a date.
EPA lists the signs: finished compost is dark, loose and crumbly, smells like fresh soil, shows no recognizable food scraps, and no longer heats up after you mix it (EPA's Composting at Home page). Convention Cornell Cooperative Extension of Tompkins County gives a three-sense version: it should look uniformly dark with no recognizable food, though pits and eggshells may persist; smell like rich soil and nothing more; and feel crumbly and moist, like a damp sponge (CCE Tompkins' Testing for Finished Compost).
| Check | Finished looks like | If it does not |
|---|---|---|
| Look | Uniformly dark; no recognizable food. Pits and eggshells may persist (CCE Tompkins) | Recognizable scraps, leaves or stems mean the batch needs more time |
| Smell | Like fresh soil or rich soil, nothing more (EPA, CCE Tompkins) | A sour, ammonia or rotten smell sends you to the symptom table |
| Feel | Loose, crumbly and moist, like a damp sponge (EPA, CCE Tompkins) | Soggy or packed material needs dry bulk and air; dusty material needs water |
| Heat | The pile no longer heats after you mix it (EPA) | If it heats, it is still working; wait |
| Time | Not a test. EPA says a well-maintained pile is usually ready in about three to five months, and that a pile left untended may take a year | A date tells you when to check, not what you will find |
Why wait for the signs? CCE Tompkins gives three reasons to treat unfinished compost with care: it may harbor pathogens, which heat eliminates; it may still be chemically active; and it may have unsettled pH. It adds that as carbon breaks down it uses nitrogen, which leaves soil temporarily short of it. The signs are a quick check that you are past those stages. CCE Tompkins says the senses check is fine for top dressing away from sensitive living plant tissue or for adding to a garden in late fall, where the compost can finish curing over the winter. For compost that will touch roots, leaves or seeds soon, it describes two further tests: a jar test, which we give here in a short paragraph, and a germination test, which we teach in full.
The first is a jar test: put a couple of inches of compost in a jar, add water until it is soggy, seal the jar, and leave it for a week. Open it carefully and smell immediately. CCE Tompkins says nice wet earth means the compost is done, and bad odors mean the materials were not sufficiently decomposed. Home test The jar test needs only a jar. The germination test below is the one that tells you how seeds respond.
The germination test
A germination test compares seeds sprouted in water that has soaked in your compost with seeds sprouted in plain water. Immature compost can harm seeds, so seeds are a sensitive check. This is the default test in this manual, and the method is CCE Tompkins'. Home test
- Make the compost solution. Mix equal amounts of compost and water, let it sit for about 5 minutes, stirring now and then, and pour off the brownish liquid into another container. Return the solids to your pile.
- Set up two trays. Label two shallow containers and set a folded paper towel in each. Place 5 seeds on one half of each towel and fold the towel over them.
- Wet one set with the compost liquid and one with plain water. Soak each towel fully. Cover each tray loosely with light plastic to hold in the moisture, and keep both in a warm spot.
- Keep the towels moist with the same liquid. Use compost liquid for one tray and plain water for the other until the end of the test.
- Look every couple of days for about a week. Compare how soon roots appear, how many seeds in each tray have sprouted 3 to 4 days after the first sprout, and how long the roots are at that point.
Which seeds? CCE Tompkins recommends seeds with thin coats, which tend to be the sensitive ones, and names radish and cress as easy to get and quick to sprout. It warns that tomato seeds are very resilient and are not good test seeds. If you plan to start particular seeds in the compost, testing those is a good strategy.
Reading the result
CCE Tompkins reads the result by comparison: if the seeds wet with plain water germinate better, the compost has not aged enough. Look for substantial delays, fewer seeds sprouting, or clearly shorter roots in the compost tray, and treat small differences as noise, because you are looking at only a few seeds. On the harvest sheet, passed means the compost tray kept up with the plain-water tray; failed means it fell clearly behind. That wording is this manual's, and the sheet's two labels are this manual's convention. Convention
A failed test has an answer. CCE Tompkins says the compost may have natural salts that need exposure to weather, and that if you test in the fall you can mix the compost into the soil and wait until spring to plant. We add one convention: cure it longer, then run the test again with fresh seeds.
The signs and the germination test say whether compost is mature enough to use around plants. They do not measure pathogens, nutrients or salts, and they do not find the herbicides that survive composting. Hay, straw, manure and grass clippings from outside your yard need the pea or bean pot test in Leaves, grass and yard, and Oregon State's extension notes that chemical lab testing might not indicate herbicide contamination (Oregon State's EM 9307). A pass on this test is not a statement about safety.
Curing and screening
Curing is the last stage: the finished pile stops receiving new material and rests while the slow breakdown of tough materials finishes. CCE Tompkins draws it as the third stage, running from about day 15 to 180 or more, and says worms, millipedes and sowbugs may show up in it. The sources give different lengths. EPA says to cure for at least four weeks; NC State describes covering the pile with a fabric weed barrier for six to twelve weeks (NC State's AG-791); the University of Arizona gives three to six months (University of Arizona's AZ1632). Convention Our default is to cure until the signs and the germination test agree, and to treat the shortest of those times as the earliest date to check.
Screening is passing compost through a mesh to take out what did not break down. EPA says to screen out twigs, pits, eggshells and stickers. Whatever stays on the mesh is not waste. Twigs and pits go back into the bin for a second round; stickers belong in the trash, because EPA lists produce stickers among the items to leave out of a pile. A bin that you add to as you go has a younger layer on top and an older one at the bottom; Tomás's case below shows why you take material from the bottom.
If your method was a hot pile (The hot pile), the log tells you what the pile did, and the signs and the test tell you whether it is done. If your method was a slow pile (The slow pile), no log of temperature exists, and the signs and the test carry the whole decision.
Using finished compost
Where finished compost goes depends on what you wanted from it. Your goals are in your setup: a garden, containers, leaves and trimmings, or only the trash. Not every goal needs the compost; a drop-off household may have none to use.
In a garden
How much to add is a soil question. Learn Vegetable Gardening owns it, with a soil test, rates and organic matter: see its compost and soil feeding chapter and, for what a soil test's organic matter figure means, its organic matter section. The rates in extension guides are conventions, and they differ. As one example, Oregon State's guide for existing vegetable beds is a layer a quarter-inch to 1 inch deep each year, worked into the top 8 to 12 inches of soil (Oregon State's EM 9308). Convention The other guides are in Go deeper. A soil test settles how much a particular garden needs, and your soil lab or extension office reads it.
Depth is not the only unit. The harvest sheet counts gallons, and a depth converts to a volume by arithmetic: a layer 1 inch deep over 12 square feet is 1 cubic foot, which is about 7.5 gallons. Compost settles after it is spread, so treat the figure as a starting point for the count, not a measurement.
Too much compost
More is not better. Washington State's extension says most organic amendments contain more phosphorus than plants need, and that repeated applications can create excessive phosphorus levels, which raises the risk of runoff (WSU's FS123E). A two-year Oregon State study sampled 27 urban gardens and found an average of 13 percent organic matter, against a recommended minimum of 3 to 5 percent; one gardener who used only compost reached 30 percent and their peppers burned (Oregon State's article on overdoing compost). It is a small study of Master Gardeners near one research site, and it does not set a number for your garden. The University of Minnesota's guidance for a soil that has too much says the first step is to test the soil (University of Minnesota Extension on too much compost). So: test, then add.
On lawns and landscape beds
One line each, from Oregon State. For an established lawn, spread compost one-quarter to one-half inch deep after core aeration. For a landscape bed before planting, spread 3 to 4 inches and mix it to a depth of 6 to 8 inches. WSU adds that established lawns seldom need an organic matter supplement.
In containers and houseplants
The extension sources we read set no percentage of compost for a houseplant or container mix; the nearest figures are under Go deeper. What they do say concerns fungus gnats. The University of California's pest notes say to avoid using incompletely composted organic matter in potting media unless it is pasteurized first, because it will often be infested with fungus gnats, and to purchase and use only pasteurized container mix or potting mix (UC IPM's Pest Note on fungus gnats). In practice, run the germination test first, and read the gnat caution before you mix any compost into a pot.
As mulch
EPA gives a three-inch layer of compost as mulch, kept a few inches away from plant stems and tree trunks.
Giving it away
A neighbor, a community garden or a friend should get compost with a short description of what went in. Say if it holds manure, grass clippings, hay or straw from outside, and say whether you ran the germination test. The recipient's own rules come first at a community garden, so ask before you bring a bag.
Buying compost
If you buy compost, the label may mention the Seal of Testing Assurance. BioCycle described it in 2015 as a testing and information disclosure program of the US Composting Council, one that standardizes the test parameters and methods for evaluating compost quality and can serve as a framework for others' numeric specifications (BioCycle's introduction to STA certified compost). In other words, the seal says a producer disclosed test results, not that a product passed a quality threshold. Ask for the results. Ask about herbicides, because lab results may not show them, and Oregon State's EM 9307 advises being wary of mushroom compost, animal manures and bedding as ingredients. We could not read the Council's current program rules, so the 2015 description is what we have.
Handling compost safely
Public health agencies give cautions for handling compost, soil and potting mix. The cautions below come from three kinds of source, and we label each by place and date because they are not all about the same thing.
Wear gloves when you handle compost, soil or potting mix, wet dry material before you turn or pour it, and wash your hands afterward and before you eat or drink. If you have a weakened immune system, a lung disease, or any other reason to worry about dust, ask your doctor before you handle compost, and ask whether you should wear a respirator. This manual gives no medical advice.
- Aspergillus spores (United States, CDC pages read October 26, 2026). Aspergillus is a common mold. CDC says its spores are found in indoor and outdoor environments, that some exposure may not be avoidable, that the illness is not spread between people, and that people with weakened immune systems or lung diseases are most at risk (CDC's About Aspergillosis page). Its prevention page advises avoiding dusty areas and close contact with soil or dust, and wearing an N95 respirator when you cannot (CDC's Preventing Aspergillosis page). That page does not mention compost or gardening by name, and it notes that these actions have not been proven to prevent the illness. A University of Georgia study (reported June 2024) found drug-resistant strains of one species in commercial soil, manure-based compost and flower bulbs; it looked at commercial products, not home compost, and the researchers advised people with compromised immune systems to take precautions when gardening and to discuss the risks with their physicians (UGA Today's report).
- Legionella in potting mix and compost (a CDC-published review, 2011; NSW Health, Australia, 2022 and 2023). Legionella is a group of bacteria that can cause Legionnaires' disease, a lung infection. A review in CDC's journal Emerging Infectious Diseases says the major source of human infection with Legionella longbeachae is considered to be commercial potting mixes, and that all cases with a confirmed source were associated with potting mixes and composts. It lists precautions: a face mask, avoiding inhalation of dust and aerosols, and washing hands after use. It says bagged potting mixes and composts carry a health warning in Australia, and that no strategies are available to control or eliminate the bacteria in potting mixes (Whiley and Bentham, Emerging Infectious Diseases, April 2011). NSW Health's fact sheet (current as of June 2022) says the bacterium is common in soil and potting mix and advises wetting the mix to reduce dust, wearing gloves and a mask (a P2/N95 if possible), and washing hands afterward and before eating, drinking or smoking (NSW Health's Legionnaires' disease fact sheet). Its 2023 release extends that advice to mulch and compost, adds that symptoms can develop up to 10 days after exposure, and says most people who breathe in the bacteria do not become ill (NSW Health's September 2023 release). The higher-risk groups it names are older and middle-aged people, smokers, people with chronic lung disease and people with weakened immune systems.
- A patient group's guidance (UK, reviewed August 2026). Aspergillosis Patients and Carers Support advises people who already have aspergillosis to avoid turning compost heaps and opening dusty compost bags, and not to open or tip compost in a greenhouse, shed or other enclosed space (its gardening guidance). It is written for patients, and it is the reason we say to ask your care team.
These sources are about different organisms and different products. The Legionella evidence is mostly about commercial potting mixes and composts, in Australia, and CDC's current general Legionnaires' disease page (read October 26, 2026) does not mention soil, compost or potting mix. Nothing here measures the risk from your own pile. We pass on the cautions; we do not say how likely a problem is for you.
Compost tea, and why we do not teach it
Compost tea is a liquid made by soaking or brewing compost in water, sold and shared as a plant tonic and disease control. We do not teach it. The reasons are two. The evidence for the benefits is thin and mixed, and brewing it with additives can grow the very microbes you hope to avoid.
On the first, Washington State University's Linda Chalker-Scott found only a handful of peer-reviewed papers on compost teas, said that anecdotal evidence abounds but controlled, replicable experiments do not, and reported that the one field test she found showed no difference in disease control between compost tea and water (WSU's "The Myth of Compost Tea", an undated paper whose file information says 2008). On the second, USDA's Agricultural Research Service reported in 2006 that when compost with very low levels of Salmonella and E. coli was used to make tea, the pathogens multiplied when additives were included in the water, and stayed undetectable in teas made without additives (ARS's "Recommendations for a Safer Compost Tea"). LSU AgCenter's food-safety guidance for farms says no compost teas should be applied to produce, because no safe pre-harvest intervals have been determined (LSU AgCenter's on-farm composting guidance, 2015). That guidance is written for farms, and we pass it on as written.
This section is a reason, not a method. If you read a recipe that calls for molasses, kelp or another additive, the University of Vermont's extension says to avoid additives when fermenting compost tea and singles out simple sugars such as molasses. If you want what compost offers a plant, use the finished compost itself, as a mulch or worked into the soil. WSU calls compost a wonderful mulch.
The compost tea evidence in full Go deeper
- A field test (WSU fact sheet, file dated 2004). Researchers at the Washington Park Arboretum sprayed 32 cherry trees of eight cultivars with compost tea or water at popcorn, full bloom and petal drop. There was little difference in brown rot between the two treatments, and in some cultivars tea made the infection worse (WSU's "Compost Tea: Miracle Cure or Marketing Gimmick?").
- A two-year study (University of Vermont summary, 2005). The Vermont fact sheet says research sometimes shows that compost tea suppresses disease and sometimes it does not, and blames part of the variability on how different composts and brewing methods are (UVM Extension's compost tea fact sheet). It describes a Rodale Institute and Penn State study on grapes, potatoes and pumpkins in which tea made with kelp, humic acid and fish hydrolysate gave about 50 percent suppression of powdery mildew on grapes in year one, failed to suppress it on pumpkins in year one, and did not reduce late blight on potatoes in year two. The study found more measurable benefit for crop growth, yield and quality than for disease suppression.
- Additives (ARS, 2006). The additives ARS names include soluble kelp, fish hydrolysates, humic acid, rock dust and proprietary nutrient solutions. ARS says testing the final tea before application may be necessary to ensure the absence of human pathogens. Vermont adds that if a tea is made with additives and not tested, food crops may not be harvested until 90 to 120 days after it is applied.
- Nutrients. WSU says teas add nitrogen, potassium and other minerals all at once, which could contribute to eutrophication.
- Dates. Every source here is from 2004 to 2015. We found no newer review that changes these findings; if you find one, we would like to see it.
More tests, and rates side by side Go deeper
The radish and extract tests
The University of Florida's Sarasota County extension gives three maturity tests (UF/IFAS Sarasota's Compost Maturity Test). The simplest is a pot test: plant radish seeds in pots of your compost, and if three-quarters or more of the seeds sprout and grow into radishes, it says the compost is ready to use in any application. The extract test is a more careful version of the germination test above: soak 1/4 cup of compost in 1/2 cup of water for two hours, filter the liquid, wet six paper towels in six glasses (three with plain water and three with the filtered liquid, five seeds each), keep them in the dark at about 80°F, and count germination at 24, 48 and 72 hours. The page says 80 percent germination, compared with the control, is considered satisfactory. The third test sets six pots of potting soil beside six pots of compost with six seeds each and counts at seven days; markedly lower germination in the compost means it needs more curing. These thresholds are one extension office's conventions, and they differ from CCE Tompkins, which compares trays and sets no number.
Rates side by side
| Use | Colorado State (GardenNotes 243) | Washington State (FS123E) | Oregon State (EM 9308) | EPA |
|---|---|---|---|---|
| New vegetable bed | 3 to 4 inches into the top 8 to 12 inches | 1 to 3 inches into at least 6 to 8 inches | 3 to 4 inches into the top 8 to 12 inches | 2 to 4 inches into the top 6 to 9 inches |
| Existing vegetable bed, each year | 0.25 inch | Typically about 1/2 inch | A quarter-inch to 1 inch | Not separated from new beds |
| Established lawn | Not given (a one-time 1 to 2 inches for a new lawn) | Seldom needed; no more than 1/2 inch, then aerate | One-quarter to one-half inch after core aeration | Not given |
The sources are Colorado State's GardenNotes (CSU's Using Compost in Colorado Gardens) and the three linked earlier. Colorado State adds that rates should be cut substantially for manure-based, biosolids-based or high-salt compost. EPA's range matches the guides' figures for a new bed, not for a yearly top-up. None of these replaces a soil test.
The nearest figures for containers
CCE Tompkins says starting seeds with up to 25 percent compost is an excellent idea, but that seeds are sensitive, which is why it gives the germination test. Oregon State's garden guide says to add up to 25 percent compost by volume when filling a raised bed with excavated soil. Oregon State's worm guide says worm compost typically makes up 25 percent of the total soil volume for container plants (Oregon State's EM 9034); that is worm compost, a different product. WSU gives 15 to 20 percent by volume for a new lawn. None of these is a rate for an ordinary compost in a houseplant mix.
See it: Tomás checks the bin, Hannah and David and Ingrid read a test See it
Tomás Reyes is a fictional person, and every fact about him here is invented for this manual. He is 34 and rents the downstairs of a duplex in a northern city. His enclosed plastic bin holds about 11 cubic feet, sits on a square of 1/2-inch hardware cloth, and runs add-and-cover with browns from his stored leaf bags. The following October, he checks the bottom of the bin.
Known: a bin filled as he went, with browns over every addition. His assumption: the oldest material is at the bottom and has had the longest to break down. Open: whether it passes the signs, and whether it passes the germination test. Nothing is settled in advance; the bin and the seeds settle it.
What he does. He lifts the lid, digs down past the newer layers and looks, smells and feels a handful from the bottom. Then there are three outcomes, and his plan has a line for each.
| What he finds | What he writes on the harvest sheet | What he does next |
|---|---|---|
| The signs are not met: recognizable food, a sour or rotten smell, or soggy material | Date, "bin", signs not checked, test not run | Leaves it, fixes any smell by the symptom table, and writes the next date to look |
| The signs are met | Date, "bin", signs checked; sets up the germination test and writes the result a week later | If compost matches plain water, he marks it passed, screens it and writes where it goes. If it falls behind, he marks it failed and cures it longer |
| The bottom passes and the top is not finished | The amount he took, in gallons | Takes only the bottom, covers the rest with browns and keeps adding |
The case stops here on purpose. His plan decides where it goes, and the tests decide whether it can.
Hannah Bauer and David Lin are fictional people, and every fact about them here is invented for this manual. They own a house with four raised vegetable beds of 4 by 8 feet and run two enclosed bins in rotation. Their lawn service applies a weed-and-feed product in spring, and they leave their grass clippings on the lawn. Their germination test passes. Ingrid Sorensen, another fictional person, who has 5 acres in bear country, has a passing germination test as well.
Known: a passing germination test; four beds of 32 square feet each. Their assumptions: the compost holds kitchen scraps and their own leaves, because the clippings stay on the lawn. Open: whether it can go on the beds, and how much. A soil test settles the second, their soil lab or extension office reads it, and Learn Vegetable Gardening's chapter covers amounts. This manual does not.
What a pass tells them, and what it does not. It tells them that seeds sprouted about as well in water that had soaked in their compost as in plain water. It does not tell them what the compost holds in phosphorus, in salts or in herbicide. The conversion is arithmetic: one inch over one 32-square-foot bed is 32 times 1/12, about 2.7 cubic feet, or about 20 gallons. Whether that depth is a quarter inch, an inch or none is the soil test's question.
For Ingrid, the test passes and is not the whole story. A germination test does not find persistent herbicides, which is why the horse manure from a neighbor, flagged by her pea pot test in Leaves, grass and yard, stays off the vegetable garden, whatever a test on a compost containing it showed.
Free references that show the tests being done, fetched on 2026-10-26:
- CCE Tompkins' Testing for Finished Compost: a two-page sheet with the senses check, the jar test and the germination test.
- UF/IFAS Sarasota's Compost Maturity Test: three further tests, with counts and thresholds.
- Oregon State's EM 9308: uses by garden, landscape and lawn.
Practice: check, test, screen, log Practice
Minimum: check your oldest material against the signs and write the result. If you have nothing old enough, write the date on your plan when you will check, and read through the germination test. Time: about 20 minutes to set up the test, then a few minutes every couple of days for a week, once per batch. Materials: a handful of compost; two shallow containers or plates; two paper towels; ten radish or cress seeds; a jar; water; a mesh for screening if you have it.
- Look, smell and feel. Take a handful from the oldest part of the pile. Compare it with the signs table.
- Run the germination test. If the signs are met, follow the steps in the germination test and label the trays.
- Read it and screen. After about a week, compare the trays. If the compost passes, screen it and return what stays on the mesh to the bin.
- Log it. Add a row on the harvest sheet: the date, where it came from, whether you checked the signs, the test result, whether you screened it, the amount in gallons and where it went.
Run a second test. For compost bound for seed starting, add the radish pot test or the extract test from the Go deeper section, and note in the harvest sheet's notes column which test you used.
Your system Your system
Check your work Check your work
Criteria
- Does the harvest sheet row say whether the signs were checked and what the germination test showed?
- Does it record where the compost went and the amount?
- If you have nothing to harvest yet, is the date to check on your plan?
- For any outside material in the compost, is there a herbicide answer from Leaves, grass and yard?
Worked example
Tomás has nothing to record in his first season, and his plan says to check the bottom of the bin the following October; that line passes. Hannah and David's row records the test as passed, a screened batch, the gallons that went to the beds and a note that the amount followed their soil test. Ingrid's plan keeps the neighbor's manure off the vegetable garden whatever a germination test shows. Noor Haddad, another fictional person, drops her scraps off and has no pile; her plan has a line asking the program whether it offers finished compost to residents, and she reads the germination test through. Each passes.
Common mistakes
- Going by the calendar. A date tells you when to check. The signs and the test decide.
- Treating a pass as a safety test. The germination test does not look for pathogens, salts or herbicides.
- Adding more because you have it. Test the soil before adding, and add what the test supports.
- Putting uncured compost into houseplant pots. The extension sources tie incompletely composted material to fungus gnats.
- Brewing compost tea. The evidence for it is thin, and additives can grow pathogens.
What is still unresolved
How much compost your garden needs is settled by a soil test. Whether your compost holds persistent herbicide is settled by the pea or bean pot test or by the source of the material. Whether you should handle compost, if you have a weakened immune system or a lung disease, is your doctor's question.