Compost is the single most important ingredient in a healthy, thriving garden. Not only does it improve the soil, but it provides a convenient and purposeful way to recycle kitchen scraps and old Amazon boxes.
So, what’s a gardener to do when the pile of grass clippings, shredded paper, and coffee grounds turns into a slimy, stinky mess? Or the pile of dead leaves and twigs hasn’t produced an ounce of usable compost in two years?
There are 4 possible causes for compost not breaking down:
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- Nitrogen/Carbon imbalance
- Surrounding temperature is too cold
- Pile is too wet/dry
- Poor airflow/lack of oxygen
If your compost pile isn’t decomposing, try turning or mixing the pile first. If it’s dry, add water. If this doesn’t work, use the troubleshooting guide below.
Signs Your Compost is Not Breaking Down
Compost is the end result of controlled decomposition. Although it may appear as though your compost pile isn’t breaking down, there will always be some form of decomposition in a pile of dead organic materials.
However, the speed at which they break down is dependent on many variables; primarily, how much effort you put into managing the pile. The goal is to turn waste into a resource, so if your compost pile is on pace to finish decomposing ten years from now, it may as well not be decomposing at all.
Active compost piles can produce finished compost in 2-6 months. So, if your pile hasn’t made observable progress in that amount of time, or, if your pile has stopped making observable progress, it’s time to troubleshoot.
There are 4 signs your compost pile isn’t actively decomposing:
- The center of the pile is wet and cool
- The center of the pile is dry
- The pile is slimy and smells bad
- Little or no insect activity
The size of your compost pile isn’t a good indicator of decomposition. After you build a compost pile, the green ingredients shrink quickly as they lose water, and the brown ingredients crumble as they’re turned. Although active compost piles do continue to shrink, shrinkage itself isn’t an indicator that the compost is decomposing.
The amount of finished compost also isn’t a good indicator of active decomposition. Obviously, the presence of finished compost means it was decomposing at one time, but it does not mean the pile is currently in a state of active decomposition.
Why Your Compost is not Breaking Down
If your compost isn’t breaking down, it’s because of a lack of microbial activity.
Active decomposition is a delicate balance between 4 different factors:
If these are all balanced, it will result in active decomposition. There is only one surefire way to know your compost pile is actively breaking down: heat.
If the center of your compost pile is warm, it means you have created ideal conditions for microbes to rapidly multiply and feast. A lack of warmth means a lack of microbial activity, which ultimately means a lack of decomposition.
Compost piles are composed of two basic ingredients: carbon and nitrogen.
Carbon is the brown, fibrous, bulky ingredients like dead leaves, branches, shredded paper, or cardboard.
Nitrogen is the green, fleshy, fresh ingredients like vegetable scraps, grass clippings, and animal manure.
Carbon provides a steady source of energy for microbes, while nitrogen provides an easily-digestible burst of energy that allows them to reproduce exponentially.
When the compost pile is turned or mixed, microbes begin to multiply as they feed on the new green material in the center of the pile. This frenzy of activity creates heat, which helps to break down the material even faster. Eventually, the microbes either create so much heat that they begin to kill themselves off, or they consume all of the available nitrogen and reproduction slows.
Either way, the pile begins to cool, and it won’t heat up again until it is mixed and new green material is introduced to the center.
If there is too much carbon, the microbes won’t have enough food to reproduce and heat up the center of the pile. Brown ingredients are also much bulkier, which results in increased airflow and difficulty retaining moisture.
If your compost pile is dry and cool, there is too much carbon.
If there is too much nitrogen, the microbes won’t have enough oxygen to maintain aerobic decomposition. The center of the pile may become quite warm, but it may also begin to smell and create a slimy film. This is due to a lack of bulk provided by brown ingredients like twigs and dead leaves, and an overproduction of ammonia.
If your compost pile is wet, warm, and/or slimy, there is too much nitrogen.
Microbes need both oxygen and moisture in order to break down carbon and nitrogen.
Oxygen supports the respiration of aerobic microbes, which are responsible for healthy decomposition within the compost pile.
Moisture supports the reproduction and metabolism of microbes, which allows them to multiply quickly and digest organic material.
When aerobic bacteria multiply and consume energy, they produce heat. This heat plays a vital role in the quality of the finished compost.
Weed seeds, pathogens, and many pest eggs are unable to survive the high temperatures of active aerobic decomposition.
If the pile is too moist, anaerobic bacteria will take over, resulting in a slimy, odorous compost pile. Anaerobic bacteria will break down organic material, but the process will be much slower, and the compost won’t be able to heat up enough to destroy weed seeds and pathogens.
Anaerobic bacteria is more likely to take over in compost piles that are too green. Moist, fleshy ingredients will form thick mats that obstruct air flow, which suffocates aerobic bacteria and creates the perfect conditions for an anaerobic takeover.
Aerobic bacteria are always present in a dry compost pile, but without moisture, they are unable to reproduce fast enough to create heat and break down tough ingredients.
The goal is to create a 50/50 ratio of airflow to moisture retention, which is easiest to maintain when you have the correct ratio of carbon to nitrogen ingredients (2:1).
If your compost pile is cool, wet, and has an ammonia (or, trash can) smell, it is too moist. This can happen when there is too much nitrogen, or if the carbon ingredients are too small (like shredded newspaper). It can also happen during a rainy season, or if the compost pile is in a container that restricts airflow.
If your compost pile is cool, dry, and has no smell, it is too dry. This can happen when there isn’t enough nitrogen, or if your compost pile is located in a dry, windy climate.
Other Causes of Compost not Breaking Down
If you’re new to composting, it’s possible your compost pile isn’t breaking down because you’ve used ingredients that were never meant for the compost pile. While these ingredients may decompose, it’s not going to turn into the crumbly, spongey, dark compost you were expecting.
Compost piles are not trash heaps. You should only use raw, plant-based ingredients.
Acceptable green/nitrogen ingredients include:
- Grass clippings
- Fruit/veggie scraps
- Weeds (non-invasive and without seed heads)
- Dead garden plants (pest and disease free)
- Coffee grounds
- Tea leaves
- Horse/chicken/sheep/goat/cow manure (no carnivorous animal manure)
Acceptable brown/carbon ingredients include:
- Dead leaves
- Shredded newspaper
- Shredded cardboard
- Wood chips
- Egg cartons (cardboard)
- Egg shells
- Straw (not hay)
Although a compost pile may smell if it’s too wet/green, most compost piles will have an earthy, soil-esque smell if they are built with the right ingredients. Healthy compost will also have earthworms, isopods, beetles, and other soil-dwelling insects breaking down the materials.
However, if your compost smells like a trash can, is swarming with flies, shows evidence of rodent activity, and has an overall gross odor/appearance, you probably added something you weren’t supposed to.
Unacceptable ingredients include:
- Bakery items
- Human waste
- Dairy products
- Tape/glossy items on cardboard/paper products
- Any chemically-treated item (pressure-treated wood, sprayed grass clippings, etc.)
While these items may eventually break down, they attract pests and take much longer to decompose. Furthermore, these ingredients can lead to harmful pathogens in your finished compost, like E. coli, salmonella, and other diseases carried by rodents or found in spoiled food.
These ingredients also lead to the physical problem of keeping your compost pile in one space. Enticing food items can lead to animals digging through your pile and taking bones, meat, etc., and leaving a scattered mess. This makes it even more difficult to maintain a warm, moist environment at the center of the pile, and the remaining plant materials will be slow to break down.
How to Get Compost to Break Down
If a compost pile is built correctly and maintained, it will break down. If it’s not breaking down, it either wasn’t built correctly, or it wasn’t maintained.
How to Balance Carbon:Nitrogen Ratio for Compost
Whether you’re building a new compost pile or troubleshooting an old one, the root cause of compost failures is an incorrect Carbon:Nitrogen (C:N) ratio.
As a general rule, compost piles should be 2 parts carbon to 1 part nitrogen.
This works out to roughly 4” of brown ingredients to 2” of green ingredients.
Always start with a 4” layer of the bulkiest brown (carbon) ingredients. If you’re having a problem with your compost pile retaining too much moisture, lay down a layer of bulky twigs or wood chips, and turn the pile onto this new bottom layer. It will help with airflow and help balance the carbon:nitrogen ratio.
Grass clippings count as green ingredients. Many beginning composters use brown grass clippings as a brown ingredient, but grass clippings are high in nitrogen and will form a thick mat that prevents water and air movement. Whether grass clippings are fresh or aged, they are a green ingredient and must be balanced out by adding bulky, carbon-rich materials.
Shred or tear paper and cardboard. Large sheets of paper or cardboard will create a mat that prevents air and water movement, and it will create large sections of nitrogen-deficient material within the pile. Shred or tear cardboard and paper before you add them into the compost pile to increase the surface area available for microbes and make it easier to mix into the center.
How to Maintain Active Decomposition
Even if you build a perfectly-layered compost pile, decomposition will eventually stall after the microbes have devoured the ingredients at the center.
If you want to maintain constant decomposition, invest in a compost thermometer. These thermometers measure the temperature at the center of the compost pile. It will allow you to watch as the microbial activity increases and temperatures rise, and then track when the microbes begin to die off from lack of food as the pile cools.
Thermometers also help with tracking whether or not your compost is hot enough to kill off pathogens and weed seeds. Maintain a temperature above 131 degrees for 15 days to kill harmful pathogens and most weed seeds (source).
Turn the compost every 4-7 days. Or, if you have a thermometer, turn it when the pile cools down and reaches a steady temperature. This introduces new material into the center of the pile and restarts decomposition.
Maintain consistent moisture at the center of the compost pile. Microbes use water to move, reproduce, and eat. If the center of the pile is too dry, they won’t be able to decompose things as quickly. Green ingredients help add moisture, but if you live in a dry, windy area, lightly water the compost pile each time you turn it.
Add new ingredients to the center of the pile. If you’re building your compost pile over time, avoid the temptation to just throw new materials on top. Not only will this attract more pests, it also dries out the new material and doesn’t introduce anything into the center where decomposition happens. Dig a small hole into the compost and throw in your veggie scraps, coffee grounds, egg shells, etc., and cover it up.
How to Tell When Compost is Done
If your compost pile has stopped making any observable progress, you may have done everything right.
After a few months of actively managing your compost pile, you should have a dark pile of soil-like material that is roughly ¼ the volume of your original pile (source). You shouldn’t have any recognizable ingredients, like leaves, twigs, paper, etc. (although small, recognizable pieces is fine).
At this point, the bacteria have run out of enough material to rapidly reproduce, so the temperature will slowly drop off. This indicates your compost is done.
Although bacteria will continue breaking down the materials for years, it has reached a stage where it is usable and beneficial for your soil.
For information on how compost affects your soil, read Does Compost Turn Into Soil?