How to compost: professionals’ tips for use and care

1. What is composting?

Composting is about creating favorable conditions for microbes like bacteria and fungi to break down organic waste. As these tiny organisms do their job, water vapor, carbon dioxide, nutrients, and heat energy are released. The end result is highly nutrient-rich humus, or compost.

2. Is composting just rotting?

No. Not all decomposition of organic matter is composting. If waste is packed too tightly in the composter, it starts to rot, releasing an unpleasant odor each time you open the lid. Composting is an aerobic (oxygen-driven) process, whereas rotting is anaerobic. Under controlled conditions at a biogas plant, rotting produces methane, which is then used as fuel for district heating and electricity generation.

3. How does composting work?

Composting takes place in three stages. During the first phase, the mesophilic phase, bacteria and actinomycetes break down sugars and proteins in the waste, causing the temperature to rise to about 40 °C.

In the thermophilic phase, the compost temperature can soar above 80 °C. The initial decomposers give way to microbes that thrive in high heat. As the easily broken-down nutrients start to run out, the temperature drops.

At this point, fungi, earthworms, and insects break down the remaining tougher materials, like wood. This cooling phase can last for several months. In a continuous composter, all three stages can occur simultaneously in different layers of the compost.

4. What makes a good composter?

A garden-waste compost can be just a pile of leaves and plants, a homemade wooden enclosure, or a store-bought composter. Typically, a garden composter isn’t insulated. According to regulations, in Finland, a kitchen waste composter must be rodent-proof and, in most municipalities, insulated.

A quick or hot composter is typically a plastic container with at least five centimeters of insulation on all sides. For a single household, the most common model of just over 200 liters is usually large enough. Well-known manufacturers include Biolan, Kekkilä, and UKM. Popular models cost between 300 and 500 euros.

A composter is a simple device, so no major technological breakthroughs are expected. The air vent is typically located on the lid, in the middle, or both. The lower section often has a hatch from which you can take out partially decomposed waste, which can be used for mulching.

Most models have a thermometer on the front, giving you an idea of the stage of the compost at the sensor’s level. The bottom of a sealed hot composter has a small opening for liquid from the waste and created during the process to drain out. You can let the liquid seep into the ground or collect it for use as a potent fertilizer.

5. Where should you place the composter?

You can place a quick composter outdoors or even indoors, provided there’s adequate ventilation. The most important thing is having it in a convenient spot, such as with your other waste bins.

Don’t put a garden composter in a pit, as it could get too wet, or in direct sunlight where it could overheat and dry out. Sunlight also kills beneficial microbes. A partially shaded, level spot is ideal for a garden composter.

6. Could I just drill holes in the ground with a soil auger, drop the waste in, and cover it with soil?

This “free composter” idea is occasionally offered on discussion forums. While the waste will decompose that way, it’s actually rotting rather than composting. It may also attract rodents and other pests, so it’s not a good idea.

7. Can you compost indoors?

The Japanese bokashi composting method has recently arrived in Finland. In an airtight bokashi container, or even in a tightly sealed plastic bucket, you add your bio-waste along with a bran-like material containing decomposing microbes. The result is a highly acidic compost mass. It is a great example of how to compost indoors effectively.

8. How do you fill a composter?

For the initial fill, create a drainage layer of about five centimeters of coarse bulking material (like bark chips) at the bottom. Then add your organic waste, followed by a peat-based bulking material. Ideally, the amount of bulking agent should be at least one-third of the amount of waste. Layering keeps the compost aerated so the process starts on its own without extra mixing.

9. How do I know the composter is working?

If you lift the lid and see condensation under it, see steam rising, and the smell is pleasant or neutral, that’s a good sign the compost is active. A compost thermometer makes it easy to verify. The most rapid decomposition occurs at around 50 °C, but some decomposition happens even before the temperature has risen that high.

In a quick composter, you can get compost suitable for mulching in just a couple of months.

In a garden composter, it might take about a year, and the decomposition can be less uniform than in an insulated unit. Occasionally turning the cooler, drier outer layers inward can help.

10. How do you control moisture, and what if the compost smells?

Dryness is the enemy of composting. Decomposer bacteria need moisture, but too much water leads to rot. A good rule of thumb is: if you squeeze a handful of compost, only a couple drops of water should come out. That means the moisture level is just right.

If the compost is too wet and smells rotten, add more bulking material and mix the compost. If it smells like ammonia, there’s too much nitrogen. In that case, add more bulking material to increase carbon levels, but avoid mixing.

In a garden composter, it’s best to add waste as moist as possible, and it’s also worthwhile to water the compost often. A store-bought garden composter typically retains heat and moisture better than a loosely built wooden structure. It also shields decomposers from light and keeps the compost mass together.

You can also build a compost enclosure yourself or make one from pallet collars.

11. What can you put in the compost?

In principle, you can compost anything that was once alive. All readily biodegradable waste, such as food scraps and spoiled food, fruit and vegetable peels, fish scraps, eggshells, coffee grounds, filter papers, tea bags, tissues, wood-pellet-based cat litter, and pet bedding waste are suitable. Be sure to cover fish scraps thoroughly and break bones into smaller pieces, as they decompose very slowly. If you’re uncertain about composting something, it’s safer to dispose of it with general waste.

Many pesticides approved and marketed for home gardeners break down biologically, and plant material treated with them can be put in the compost. If you’re not sure, check with your local waste authority.

A garden composter can also take potting soil, plant waste, and mushroom or berry scraps. Be sure to cut branches and twigs into smaller pieces before adding them to the compost.

Weeds should be dried in the sun before adding them. They also need to go into the hottest part of the compost, the center, or their seeds may survive. Only a well-functioning quick composter gets hot enough to destroy weed seeds and roots. If you’re unsure whether your composter heats up sufficiently, avoid adding couch grass roots or other perennial weeds and seeds. In many municipalities, general waste is incinerated, and if that is the case, you can mix problematic plant parts and perennial weed waste with general waste.

It’s safest to compost diseased plant material in a hot composter, cutting it up and folding it into the compost mass. There are, however, a few diseases that can survive the composter.

You can add the following to a well-functioning hot composter:

12. What can’t go in the compost?

Avoid putting ash in the compost, as it raises the pH too much. Vacuum cleaner contents are also unsuitable.

Do not put the following in your compost:

13. Can you put pet waste in the compost?

If you plant to use the compost in a vegetable garden, it’s best not to add human or animal feces, as they may contain harmful bacteria such as salmonella. There’s no rule against composting feces if the compost is only used for soil improvement where there’s no contact with food production. Small amounts of cat litter can be added to the compost.

14. What about compostable plastics?

Compostable plastics aren’t recommended for home composters, as the temperature doesn’t rise high enough to break down green or gray biodegradable plastic bags. Municipal waste management systems have more heavy-duty processes that can handle them, although it’s still unclear whether they truly decompose or just break into microplastics.

15. What does aerating the compost mean?

Aerating, or turning, the compost keeps it from becoming compacted. Decomposer bacteria can’t function properly in a dense mass.

A quick composter usually has a built-in ventilation system, so the user’s main task is to add enough bulking material. Ideally, add bulking material whenever you add any waste. Every few weeks, you can stir the compost with a compost turner, pushing it straight down and pulling it back up.

You can also turn the contents of a garden composter.

16. How should you set the air vent?

Some household waste composters include a separate air valve that supplies the oxygen needed for decomposition. At first, it should be set to around 20 on a scale of 0–100. As the volume of waste and the outdoor temperature increase, the valve can be opened further in stages. In winter composting, keep the valve closed. When the process really gets going, gases generated by decomposition maintain a slight positive pressure in the composter, so fresh air mostly enters when you open the lid to add waste or when emptying the unit.

17. Which additives are needed?

It’s important to maintain the carbon-to-nitrogen balance in the compost. If there’s insufficient nitrogen, microorganisms won’t multiply as quickly and the compost won’t heat up enough. In that case, you’d need to add a nitrogen-rich addition, such as a compost accelerator. A good alternative to commercial accelerators is well-diluted urine. If there’s too much nitrogen, the compost will smell like ammonia, in which case you need more carbon-based bulking material.

You can use wood chips, coarse garden clippings, peat, or bark as bulking material, but avoid sawdust because it compacts too easily. You can also add products that help adjust acidity, nutrients, or amino acids to speed up decomposition.

18. How often should you turn the compost?

It’s good to turn the contents of a garden composter once or twice a year. The aim is to move any undecomposed material at the edges into the hotter center. An insulated quick composter doesn’t need turning.

19. Does the composter work in winter, too?

Yes, as long as you add waste regularly, three or four times a week, and use enough bulking material. A soggy compost freezes easily. You should also keep emptying it in the winter. In a continuous composter, the bottom layer is in the cooling phase, while the top layer can still be steaming hot. Removing the cooler material stimulates the process. If the compost does freeze, the process will stop, but neither the composter nor the compost is harmed. It starts up again when the weather warms.

A garden composter will freeze if the temperatures drop, as it’s not insulated. In the spring, you can revive it by adding a couple of buckets of hot water and mixing. Adding urea, chicken manure, or other nitrogen-rich materials also helps.

20. How do you know when the compost is ready?

Finished compost soil is uniform, and you can no longer identify the original materials. It’s dark brown or almost black and has a soil-like scent.

Kitchen waste can become compost that is suitable for mulching in just a couple of months. The compost is ready when it looks and smells like soil, and none of the original components (apart from eggshells) can be identified. Finished compost is dark brown or black in color. It’s best to use it right away, as it continues breaking down in the composter and the nutrient content gradually declines.

21. Is the compost ready to use as-is?

Compost is by itself an excellent slow-release fertilizer. If you’d like to use it more like garden soil, combine one part compost with one to three parts peat, sand, or a mix of the two. If you’ve used partially decomposed compost as mulching and want to make it into compost soil that can be used as a growing medium, it needs to mature for another six to twelve months. You can do this by shoveling the compost into a pile and covering it with a tarp.

22. How do you use compost soil?

Spread ‘half-finished’ compost as fertilizer around trees and shrubs in the spring. Remember that partially decomposed compost continues aerobic decomposition, so it shouldn’t be buried. Only fully matured compost soil is suitable as a growing medium.

23. What are the most common problems, and how do you solve them?

Experts: Business Director Kaj Paavola and horticultural advisor & communications specialist Riikka Kerttula, Biolan

Sources: kekkilä.fi, biolan.fi, kiertokapula.fi, and the Finnish Environment Guide