Composting Biology 101
Like a simple recipe, your compost pile needs the right mix of ingredients in order to produce the best results. The key materials are nitrogen-rich "greens," carbon-rich "browns," water, and air.
| Greens provide nitrogen, and act as a source of protein for the microbes. Examples of greens are green leaves, coffee grounds, tea bags, plant trimmings, raw fruit and vegetable scraps, and grass clippings. | Browns are a source of carbon and provide energy for the microbes. Examples of browns are straw, sawdust, twigs, dried grasses, weeds and leaves, and shredded newspaper. |
Like all living things, the microbes in your compost pile need water and air to live. Water allows the microbes in your compost pile to grow and travel around in the pile to decompost materials. Turning your pile each week with a spade or pitchfork will provide air to aid decomposition and control odors.
Common problems & solutions
| Symptom | Problem | Solution |
| Rotten egg smell | Not enough air due to compaction | Turn pile to fluff up and create air pockets. If particle size is small (under one inch), add a bulking agent such as wood chips about 2" in size. |
| Excessive moisture: During fist test, if water drips or runs our of your hand, the pile is too wet. |
Turn pile to add air and dry out pile. Wood chips or some other bulking agent could be added to increase air space. | |
| Ammonia Smell | Excess nitrogen (grass clippings, food waste, fertilizer) | Add more carbon materials (leave, non-recyclable paper, straw). |
| Pile doesn't heat up | Pile too small | In order to get the compost pile hot, it must be a minimum of 3' high by 3' in circumference. |
| Pile too dry—the most common problem. Using the fist test, if you do not see beads of water between your finger, the pile is too dry | Turn pile to mix materials. While turning the pile, add water with a hose or watering container. You should let the pile rest for several hours, then give it the fist test again. If beads of water do not form between your fingers, the pile is still too dry and more water is needed. | |
| Lack of nitrogen | Add materials containing nitrogen (grass clippings, food) or a plant fertilizer high in nitrogen. | |
| Poor aeration | Turn pile. Course materials, such as wood chips, may also be added to create air spaces in the pile. | |
| Cold weather | If the compost pile is small, it may not be able to heat up in areas that have very cold climates. | |
| Compost is finished | When appropriate, begin using finished compost in garden. | |
| Attracts rodents or other animals | Inappropriate materials | Materials such as meats, oils, fat, foods cooked in oils or fats, bones, and dairy should not be added to the compost pile. |
| Kitchen food scraps too close to surface of pile | Bury kitchen scraps beneath several inches of high carbon materials (leaves, straw, wood chips). | |
| Attracts insects, millipedes, slugs, etc. | This is normal | To minimize insect problem, turn the outside edges of pile into the center and make sure the pile heats up. This will kill the eggs laid by the insects and reduce the nuisance insects. |
The Fist Test
One of the most common reasons a backyard compost pile works slowly or even stops composting is lack of moisture.
The easiest way to check for proper moisture conditions in your compost bin is to randomly grab a handful of composting materials from the pile or bin. Make a fist with your hand and squeeze. One of three things will happen:
- You have water running between your finger. The material is too moist, and you need to turn the pile until it dries out.
- You have beads of moisture form between your fingers. The moisture level is just right. No additional care is needed.
- Youu have no moisture between your fingers. You need to add water.
Mix the pile well and repeat the fist test as necessary. This is no recommended if your feedstock is manures or contains food scraps.
Minnesota Pollution Control Agency
800-877-6300
www.pca.state.mn.us
