Compost troubleshooting: smells, pests, heat and moisture
A table that goes from what you see or smell to the likely cause, one change and a check a week later, for piles, worm bins and bokashi buckets.
- Start here: the essentials
- Diagnose first, then change one thing
- The symptom table
- Animals and flies at the bin
- Fruit flies, worm bins and bokashi buckets
- What is normal
- When to change the method instead
- The three tables compared, and the oven test
- See it: Noor and the bananas, Hannah and David and the alley
- Practice: log a symptom, a change and a result
- Your system
- Check your work
- Knowledge check and scenario
Start here Essentials
Something is wrong, or you want to know that nothing is. This chapter asks for one symptom, one change and one check a week later, written on the problems sheet. If nothing is wrong, a week's check logged as "no problems" is the whole task.
- Name what you see or smell. Match it to a row in the symptom table (symptom table, indoor and bucket methods).
- Make the one change the row gives. Change one thing, so you know which change worked (diagnose first).
- Log the date, the symptom and the change. The problems sheet has a row for each (problems sheet).
- Check again in a week and write the result. If the same symptom is back after two changes, read the section on changing the method (when to change the method).
Minimum time: one sitting to log the symptom and the change, then one short check a week later. Done when: the problems sheet shows a symptom, a change and the result, or, if nothing is wrong, a week's check logged as "no problems".
Diagnose first, then change one thing Understand
Troubleshooting is a loop with four parts: what you see or smell, the likely cause, one change, and a check a week later. The order matters. Most compost problems are a shortage or an excess of the same four things from Choose your system: browns, greens, water and air. A smell is often the first report of which one is off, because microbes that run out of oxygen give off smelly gases. Cornell's composting science pages give the mechanism: when oxygen in the spaces between particles falls low, anaerobic pockets form (Cornell's Compost Physics page). Physical principle
Three habits keep the loop honest.
- Change one thing at a time. If you add browns, turn the pile and cover it with a tarp on the same day, a better smell next week tells you the pile improved and nothing about why. The problems sheet has one row per change for this reason.
- Treat the cause as likely, not certain. Two causes can give the same smell. A soggy pile and a packed pile both smell rotten, and the fix for one (dry browns) helps the other. The table lists the causes the sources give, in the order the sources give them; your log tells you which one fit.
- Write down the check. A week is long enough for a smell or a moisture reading to change after a correction, and short enough that you remember what you did. A week is this manual's convention for the re-check. Convention
Healthy compost has a baseline to compare against. Oregon State's composting specialists say healthy compost smells earthy, not sulfurous (Oregon State's compost troubleshooting article). The bin log's smell column (earthy, none, sour, ammonia, rotten) is that baseline written down.
The symptom table
The table covers the common symptoms of a pile, a bin or a tumbler. It is compressed from three extension troubleshooting guides: Kansas State's (K-State's Composting Troubleshooting Guide, revised 2017), the University of California Cooperative Extension in Napa County's (UCCE Napa's Compost Troubleshooting Guide, 2010) and Oregon State's (the article linked above, 2024). The symptom names match the options on the problems sheet, so a row here is a row you can log. The full troubleshooting table has more rows, a filter by method and a column for sources; this one keeps to what you will meet first.
| Symptom | Likely causes | First change | Check in a week |
|---|---|---|---|
| Rotten-egg or sour smell | Too much moisture, too little oxygen, or packed material (K-State, Oregon State) | Loosen or turn the pile and mix in dry, bulky browns such as straw, dry leaves or shredded paper; keep rain off with a tarp or a simple roof (Oregon State) | Smell the center and log the smell column; earthy or none means the change worked |
| Ammonia smell | Too much nitrogen compared with carbon (K-State); compacted leaves or too little air (UCCE Napa) | Mix in browns such as straw, wood chips or dry leaves (K-State, UCCE Napa); turn if the material is packed (UCCE Napa) | Smell again and log it; check that the new browns are mixed in, not sitting on top |
| Pile not heating | Pile too small, too dry, too little air, too little nitrogen, or cold weather (K-State); particles too large (UCCE Napa) | Pick the cause your log supports: a pile under about a cubic yard needs more material or insulation; a dry pile needs water while you turn it; a lack of greens needs grass clippings or manure (K-State) | Read the center with a thermometer; Oregon State says a well-built pile of 0.5 to 1 cubic yard should heat within a week |
| Hot only in the middle | Pile too small (UCCE Napa) | Gather more material and rebuild the pile larger (UCCE Napa), or insulate the sides (K-State) | Read the temperature at the center and at the edge |
| Too dry | Too much woody material (UCCE Napa); too little water; in a hot dry summer, water lost faster than it is added (University of Arizona) | Moisten the pile while you turn it, as damp as a wrung-out sponge, and add fresh materials (UCCE Napa, K-State) | Squeeze a handful (squeeze test) and log the moisture column |
| Too wet or soggy | Rain, wet fresh greens, or packed material (Oregon State, UF/IFAS Sarasota) | Mix in dry, bulky browns, turn, and cover the pile (Oregon State, K-State) | Squeeze a handful and smell it; log both |
| Matted grass or leaves | Wet, fresh clippings pack into clumps with no air inside (UF/IFAS Sarasota) | Mix clippings with bulky browns, or add them in thin layers; break up the mats when you turn (UF/IFAS Sarasota) | Pull a clump apart: it should be loose, not slimy |
| Flies at an outdoor bin | Food scraps left uncovered near the surface (UCCE Napa, NC State) | Bury new scraps in the center and cover each addition with browns (UCCE Napa, NC State) | Look for adults and larvae near the top; log yes or no |
| Rodents or other animals | Meat, dairy, fat or uncovered food; a bin with gaps or no floor (K-State, EPA) | Stop adding animal products, cover every addition, and check the bin for openings (animals and flies) | Look for droppings, digging or chewing at the bin and log what you find |
Three notes on reading it. First, not heating is a symptom only for a pile you mean to heat (The hot pile). A cold bin that you fill as you go is allowed to stay cool; that is the slow pile in The slow pile. Second, moisture is the cause behind more rows than any other, which is why the squeeze test is the first check. Third, quantities such as a pile's size or a temperature vary by source, and the table follows the source named in each cell; the Go deeper section sets the three guides side by side.
This table lists likely causes, not certain ones, and two causes can give the same smell. A fix that works tells you the cause was probably right; a fix that does not work tells you to try the next cause. The table cannot see your pile, and it does not replace the log. A rodent problem may be the neighborhood's rather than your bin's, and pest control beyond the bin goes to your landlord, a licensed pest professional or your city. This manual does not teach rodenticides or traps. For questions about any pesticide product, call the National Pesticide Information Center at 1-800-858-7378 or your extension office.
Animals and flies at the bin
Pests and the smells that bring them come from the same few causes, and the first change for nearly all of them is the same: take away what attracts them and cover what remains. Site and bin covers bin construction and sites (rodents, bears). This section covers what to do when something shows up at a bin you already have.
Rodents and other animals
EPA says a well-maintained pile should not attract pests or rodents, and lists the maintenance: no meat, dairy or greasy foods; food scraps covered with four to eight inches of dry leaves or other browns; and a bin with no gaps larger than 1/4 inch, a lid and possibly a floor (EPA's Composting at Home page). Convention New York State's guide gives a similar rule with different numbers: bury food under at least 3 inches of browns and put 1/2-inch hardware cloth under and around the pile (New York State DEC's Home Composting Guide). The two figures describe different parts of the problem, the burying depth and the mesh size, and Site and bin sets them side by side. Your first change when you see signs of animals at the bin is to check each of these in turn:
- What went in. Remove meat, fish, bones, dairy, fat and oil if any went in. K-State lists meat and fatty food as the usual cause and says to remove it and cover with soil or sawdust, animal-proof the bin, and turn the pile to raise its temperature (K-State's guide above).
- What is exposed. Cover scraps with browns at each addition and bury fresh scraps in the middle of the pile (Oregon State, UCCE Napa).
- What can get in. Look for gaps larger than 1/4 inch and for a missing floor (EPA). Cornell's urban farming guide asks for the same opening size and hardware cloth or wire mesh along the bottom (Cornell Small Farms' composting guide).
- What is in the bin type. New York City's guide says holding units should not take food scraps because they lack adequate protection against rodents (NYC Outdoor Composting Guide).
Then check whether the animal is there because of your bin or for some other reason. Rats seen in an alley, along a fence or near a neighbor's bins are not, by themselves, evidence about your bin. If a bin is enclosed, meat- and dairy-free and covered, and animals still gather, the problem is bigger than the bin. That is the point to ask your landlord, a licensed pest professional or your city's sanitation or health office. In bear country, your state wildlife agency's guidance comes before this manual's. Ask the authority
Flies at an outdoor bin
Flies at an outdoor bin usually mean food is near the surface. NC State's gardener handbook advises topping a pile with 4 to 5 inches of browns to keep out flies and other pests (NC State's Extension Gardener Handbook, composting chapter), and UCCE Napa advises putting fruit and vegetable scraps in the center of the pile and covering them. If you see small larvae on the surface, the scraps were exposed; bury them and cover. Convention
Bears
A bear at a bin is a wildlife-agency question, not a composting one. Follow your state wildlife agency's guidance, which comes before this manual's, and read Site and bin's bear section.
Fruit flies, worm bins and bokashi buckets
Indoor methods fail in different ways than piles do, and the sources for them are different. This table covers the indoor symptoms on the problems sheet.
| Symptom | Likely causes | First change | Check in a week |
|---|---|---|---|
| Fruit flies indoors | Ripened fruit and vegetables left out; a drain, trash container, recycling bin or other breeding site (University of Kentucky) | Eat, discard or refrigerate ripened produce, and clean or empty any other breeding site (University of Kentucky) | Set a jar trap for the adults (University of Kentucky) and count what it catches; log whether the number is falling |
| Rotten-egg or sour smell in a worm bin | Bin too wet; too little air; too much food (Oregon State) | Check that drain holes are open, add holes for air, cover food with dry bedding and add less food (Oregon State) | Smell the bin and feel the bedding; it should be damp, not soggy (NYC) |
| Worms trying to leave | Vibration from foot traffic or a washer and dryer; bin conditions that are not right (Oregon State) | Move the bin away from vibration; check the food supply, moisture and bedding (Oregon State); check the temperature, 55 to 80°F in NYC's brochure (NYC) | Look for worms on the lid or outside the bin and log yes or no |
| Worms not eating | Overfeeding, shown by untouched food two to three weeks after it went in (Oregon State); food pieces too large (NYC) | Stop feeding and wait up to two weeks (Oregon State); chop food smaller, or freeze and thaw it (NYC); if decomposition is still slow, feed less (NYC) | Dig gently where you last fed and log how much food is left |
| Bokashi bucket smells foul | Fermentation has failed and the contents are rotting rather than pickling (WSU Kitsap); too little bran (NC State Beaufort) | Add a generous layer of bran, press the contents down to remove air, drain the liquid, and check that the lid seals (WSU Kitsap) | Open the bucket briefly and smell; sour and pickled is right, rotten is not |
Fruit flies
The University of Kentucky's entomology fact sheet explains why fruit flies spread so fast: a female can lay about 500 eggs, and the cycle from egg to adult can take about a week (University of Kentucky's fruit fly fact sheet). Physical principle The fact sheet's central advice is to find and remove breeding sites; it says the problem continues while they remain, however often insecticides are applied to the adults. Ripened produce, drains, trash containers, a recycling bin that is rarely emptied and a forgotten potato or onion are all on its list. For the adults, the fact sheet describes a trap: a paper funnel rolled from notebook paper and set in a jar that holds a few ounces of cider vinegar. NYC's worm bin brochure gives a variation that adds a drop of detergent to the vinegar or beer (NYC's indoor worm bin brochure). Kitchen and collection covers fruit flies at the kitchen container, including the freezer; this row covers them anywhere else. We do not give advice on insecticide products; the fact sheet's mention of them is not a recommendation here.
Worm bins
Oregon State's worm bin guide has a troubleshooting table of its own (Oregon State's Composting with Worms, EM 9034). Its flies row separates two causes: food left exposed, and too many fruit or sugary scraps. For the second it says to reduce fruit peels or freeze them before adding them, cover the bedding with cardboard, set a small cup of vinegar or wine in the bin, and stop feeding for two to three weeks. NYC's brochure adds that freezing fruit or microwaving it for 60 seconds helps kill fruit fly eggs. Convention Oregon State's table also lists rodents in a worm bin and gives a bin with no gaps over 1/4 inch and no meat, dairy or grease as the cause and the fix. The same row names traps and baits, which this manual does not teach. Worm bins covers the feeding schedule.
Bokashi buckets
WSU Extension in Kitsap County gives bokashi a troubleshooting guide with one rule at the top: bokashi should smell sour, tangy and pickled, not like sulfur, rot or ammonia (WSU Kitsap's bokashi page). A rotten smell means the fermentation failed. Its fixes are the ones in the table. A Master Gardener article from North Carolina Cooperative Extension in Beaufort County gives the short version: if the waste smells foul instead of pickled, add more bran (NC State Beaufort's bokashi article). Convention Bokashi and burying covers the method; both pages are one extension office's or one gardener's practice, and no independent study of home bokashi results was found.
What is normal
Some signs look like problems and are not. Logging them as "normal" is a useful entry, because it is the record that stops you from fixing a pile that is working.
- Sowbugs, millipedes and worms in a cool pile. Cornell's invertebrate page says soil invertebrates are likely to contribute to a backyard pile, and lists millipedes, sowbugs and whiteworms among the primary consumers (Cornell's Invertebrates of the Compost Pile). Cornell Cooperative Extension of Tompkins County says worms, millipedes and sowbugs appear during the curing stage (CCE Tompkins' Testing for Finished Compost). Oregon State's worm table says sowbugs and beetles in a worm bin are not uncommon and coexist with worms.
- A few mites in a worm bin. NYC's brochure says a small mite population is good and a large collection is not; it advises removing food that has a crowd of mites.
- White mold in a bokashi bucket. The University of Saskatchewan's gardening site says white mold should start to grow within a few days and is not dangerous (University of Saskatchewan's bokashi page). WSU Kitsap agrees about white mold and treats green, black or blue mold as a sign of competing fungi or air exposure; its fix is to remove visibly affected material, add bran and press the contents down, and its advice for a bucket dominated by mold is to discard the contents and restart.
- Bokashi that still looks like food. WSU Kitsap says fermented food keeps its shape. Check the sour smell, the white mold and the softened texture instead.
- Eggshells and pits in nearly finished compost. CCE Tompkins says they may persist in compost that is otherwise ready. Finished compost covers screening.
- A pile that goes quiet in winter. Penn State says a pile goes dormant in winter and begins to decompose again when air temperatures rise above about 50°F (Penn State's Home Composting guide). That is a season, not a symptom. Your compost system's calendar covers it.
When to change the method instead
Most symptoms have a first change that works. Some do not, and a repeated symptom after two different changes is information about the method. Here are the patterns we mean, labelled as this manual's convention rather than a source's finding. Convention
- The fix needs more attention than you gave it. A hot pile that cools because nobody turned it is a mismatch with the "High" attention tier (Choose your system's three questions), not a pile problem. Moving to a slow pile, a tumbler or a drop-off is a legitimate answer.
- The method cannot take what you make. A bin that attracts animals every time cooked food or meat goes in needs a second route for those foods, such as a bokashi bucket or a drop-off, rather than more careful covering.
- The browns run out. A pile that turns wet and smelly every winter may be short of dry bulk. The browns calculator sizes the supply; a second storage bag is cheaper than a second bin.
- The space does not allow it. If a worm bin escapes into a room you cannot keep within the sources' temperature range, the bin may need another location, or another method.
The review in Your compost system is where a change of method gets written down, with the reason. You do not have to wait for it. If a problem has defeated two changes, write "change method" and the reason in the problems sheet's result column and carry it into the review.
The three tables compared, and the oven test Go deeper
The table in the chapter is one reading of three guides that differ in age, scope and detail. Here is where they differ, so you can tell which row to trust when your pile does not match.
| Topic | K-State MF3371 (2017) | UCCE Napa (2010) | Oregon State (2024) |
|---|---|---|---|
| Rotten smell | Excess moisture, lack of oxygen or compaction; turn or add dry porous material such as sawdust, wood chips or straw, or make the pile smaller | Too much moisture; turn or add straw | Low oxygen and excess moisture; turn, add dry bulky material, keep rain out |
| Ammonia smell | Too much nitrogen; add sawdust, wood chips or straw | Too much nitrogen, compacted leaves or inadequate air; add straw, sawdust, wood chips or dry leaves, or turn | No ammonia guidance in the article |
| Low temperature | Pile too small, too dry, poor aeration, lack of nitrogen, cold weather | The same list plus particles too large | Volume too small; aim for 0.5 to 1 cubic yard during peak garden season, otherwise cool-compost or use a worm bin |
| High temperature | Above 140°F: pile too large or poor ventilation | Not covered | Targets 130 to 135°F to kill weed seeds and many pathogens |
| Dry or warm only in the middle | Not a separate row | Dry throughout: too much wood. Warm only in the middle: pile too small | Not covered |
| Pests | Meat or fatty food; remove, cover, animal-proof the bin, turn | Animal products or uncovered scraps; put scraps in the center and cover | Skip meat, fish, fat, oil, dairy, bread, grains and bones; bury scraps; line the bin with hardware cloth |
| Flies | Grouped under pests | Not covered | Not covered |
Where two or three of them cover the same symptom, they point the same way. They differ in what they leave out. K-State is the only one with a high-temperature row, UCCE Napa the only one with particle size and the only one with the dry-throughout and warm-only-in-the-middle rows, and Oregon State the only one with a heating target and a volume. None of the three covers worm bins or bokashi, which is why those rows come from other sources. The oldest, UCCE Napa's, is dated December 2010, and no source we read says its rows have been revised. Where a table here uses a quantity (a cubic yard, a temperature), it follows the source named in the cell.
Moisture by percentage: Cornell's oven test
The squeeze test is the default because it needs no equipment. For a number, Cornell's moisture page gives an oven method. Weigh a 10 gram sample, dry it for 24 hours in an oven at 105 to 110°C (221 to 230°F), weigh it again, and compute the moisture as the weight lost divided by the wet weight, times 100 (Cornell's Moisture Content page). Our example, with made-up weights: a 10.0 gram wet sample that weighs 4.0 grams dry lost 6.0 grams, and 6.0 divided by 10.0 times 100 is 60 percent. A household oven may not hold a temperature that low, so check yours before you start, and use the squeeze test if it does not.
Sources do not agree on the target. Cornell says most of the literature recommends 50 to 60 percent by weight. NC State gives approximately 40 to 60 percent, the University of Arizona 40 to 60 percent, Utah State 40 to 65 percent (NC State's AG-791, University of Arizona's AZ1632, Utah State's backyard composting guide), and Oregon State quotes about 60 to 65 percent for a hot pile. The ranges overlap near 60 percent. All of them describe the same thing as the wrung-out sponge.
See it: Noor and the bananas, Hannah and David and the alley See it
Noor Haddad is a fictional person, and every fact about her here is invented for this manual. She is 28 and lives alone in a fourth-floor apartment with no balcony, kept between 64 and 72°F. Her scraps go into a lidded 1-gallon container in the freezer, which she empties each Saturday at a drop-off. Fruit flies appear over a bowl of bananas on the counter.
Known: flies are over the bowl; the scrap container is in the freezer. Her assumptions: the ripe bananas are the source, because the flies are over the bowl and not over a drain or a bin. Open: whether the bowl is the only breeding site. The University of Kentucky fact sheet lists drains, trash containers and a rarely emptied recycling bin as other sites, and Noor settles it by checking them and watching the trap over a week.
What the table does. The row for fruit flies indoors makes removing the source the change and the trap the check. The fact sheet says ripened produce should be eaten, discarded or refrigerated, so she puts the ripe bananas in the refrigerator. She sets a jar trap with cider vinegar and a notebook-paper funnel by the bowl, and counts what it catches.
What she logs. Method: other. Symptom: fruit flies indoors. Likely cause: ripe bananas left out. Change made: ripe bananas to the refrigerator. Checked on: seven days later, by the trap count. Result: written in a week.
Under these assumptions the count could fall as the adults die and no new ones hatch. If it does not, the result column says so, and the next cause on the list is the drain or the recycling bin.
Hannah Bauer and David Lin are fictional people, and every fact about them here is invented for this manual. They own a house with a backyard about 40 by 60 feet. Rats have been seen in the alley. The town has no food scrap collection. They run two enclosed bins with hardware-cloth floors in rotation, with no meat or dairy and every addition covered.
Known: rats in the alley; the bins have floors, covers and no animal products. Their assumptions: the bins are as they built them in Site and bin and nobody has added meat or dairy. Open: whether the rats are drawn to the bins or only pass through the alley. A licensed pest professional or the town's health office settles that, and this manual does not.
What they do. No symptom has appeared at the bins, so they check the causes the table lists for a rodent problem: nothing animal-based in the bins, every addition covered, no gaps over 1/4 inch, the hardware cloth intact. They log a week's check as "no problems at the bins; rats seen in the alley" and write the same check into the routine they already keep.
This manual does not say the rats will stay away. If animals start to appear at the bins, they stop adding food, log the symptom and ask. The alley is the neighborhood's.
Free references that work through the same symptoms, fetched on 2026-10-26:
- Oregon State's compost troubleshooting article: three symptoms, each with a problem and a solution.
- K-State's Composting Troubleshooting Guide: a one-page table from symptom to cause to fix.
- WSU Kitsap's bokashi page: a bokashi troubleshooting guide with a normal-or-not column.
Practice: log a symptom, a change and a result Practice
Minimum: look at your method, name a symptom from the problems sheet or "no problems", make the one change the table gives, and log it with a check date a week out. Time: about 10 minutes to log, plus a short re-check a week later, once. Materials: your bin, bucket or kitchen container; the problems sheet; your hands for the squeeze test; a thermometer if you run a hot pile.
- Look, smell and squeeze. Open the bin, bucket or container. Note the smell, the moisture and any animals or flies. Take the temperature if you run a hot pile.
- Find the row. Match what you saw to a symptom in the symptom table or the indoor table. If nothing is wrong, skip to the last step.
- Make one change and log it. Make the first change in the row. In the problems sheet, write the date, the method, the symptom, the likely cause and the change.
- Write the check date. Pick a date a week out, in the "Checked on" column.
- Re-check and write the result. On that date, look, smell and squeeze again, and write what changed. If nothing is wrong, write "no problems" as the symptom and the date as the check.
Log the normal things too. If you saw sowbugs, white mold or a pile gone quiet for winter, add a row with "normal" as the result. When a new symptom appears later, the older rows show what a quiet week looked like.
Your system Your system
Check your work Check your work
Criteria
- Does the problems sheet row name a symptom from the sheet's list, or say "no problems"?
- Does it record one change, not several?
- Does it say when you will check again, and later what the result was?
- Does the likely cause say "likely" (or "probably"), or does it state a cause as settled?
Worked example
Noor's row names the symptom (fruit flies indoors), a likely cause (ripe bananas left out), one change (the bananas to the refrigerator), a check date seven days later and a blank result. Hannah and David's row names no symptom, records a check ("no problems at the bins; rats seen in the alley") and carries one change to the routine. Both pass. A row that read "fixed everything: added browns, turned it, moved the bin and covered it" would not, because the next reader cannot tell which change worked.
Common mistakes
- Several changes at once. You cannot tell which one worked.
- Treating a cold bin as broken. Not heating is a symptom only for a pile you meant to heat.
- Chasing the animals and ignoring the bin. The causes the sources list for animals are what is in the bin and what the bin leaves open.
- Reaching for a product first. The fruit fly fact sheet says the problem continues while breeding sites remain.
- Skipping the re-check. The row is not finished until the result is written.
What is still unresolved
Which cause fit your pile is settled by your log, a week after the change. Whether rats near your home come to your bin is a question for a pest professional or your city. Whether the method is wrong for you goes to the review in Your compost system.