The Truth on: ‘Can you compost onions’

Many food lovers who love to garden often ask ‘can you compost onions?’

The reason for this is simple- Onions are a flavour powerhouse. They add an appetizing taste to your food and this makes them a favourite for those who cook.

But they leave behind a lot of waste. Skin peels, leftover chopped onions and moulded onions are all thrown away. As a plant lover and environmentalist, this is a concern.

To counter this concern, kitchen scraps can have a purpose. Think about it… this kitchen waste is all organic matter. We can can add this to a compost pile.

So answering the question ‘can you compost onions?’- yes you can. 

Let’s dive in and learn how onions can help to give nutrient-rich compost to help plant growth. 

Benefits of Composting Onions

Onions can be broken down in to organic matter that can benefit your compost. The benefits include:

Improve soil quality

The importance of compost can never be underestimated. But the addition of onions can help the compost soil in many ways. 

The skin has antioxidant and antimicrobial properties that help keep your soil away from pests and diseases.

The onions are also rich in useful substances such as potassium, zinc, sulphur, pectins and vitamins. This fills your compost with essential nutrients plants need.

Onions also release natural chemicals that protect plant roots from soil erosion. The acidic properties also help to kill off pests that can cause plant disease.

Many gardeners soak onions in water and leave them for a couple of days. This produces a liquid mixture that we can use as a fertiliser for indoor or outdoor plants. This can increase disease resistance, growth and productivity of the plant.

Reduces costs

Buying compost bags or fertilisers can be expensive. If you grow plants , it is best to make compost yourself. Onions are full of chemicals and nutrients that plants need to grow. 

It’s a win-win as you are saving money, recycling kitchen waste and making quality compost. 

There are other ways onions can help with growing your plants. This includes:

• They are able to hold onto moisture. This allows the plant to take up water when it requires. 
• Reduce excess water. Over-watering your plant can also kill them. It’s important to make sure your plants do not have excess water around them. For example, succulents dislike moist soil as they prefer soil that has good drainage.
• Increase stem growth and productivity.

Limitations of adding onions to compost

Despite have beneficial properties, there are a few points to be careful of:

• Be cautious not to use many onions. They are acidic and a lot of them can lower the soil pH. This can affect soil properties and prevent plant growth.
• Also, the composting process relies on microbes and protozoa to break down the organic matter. An acidic environment inhibits the good microbes, and this will prolong the composting process.
• Using whole bulbs can be difficult to break down. The onions won’t decompose and the compost won’t have the rich properties you desire.
• Also, onions grow easily and this is not good. Adding a full onion to a compost pile can sprout new onions.
• Cannot add to vermicomposting. Thus, not suitable for certain composting methods.

How to compost onions

1. Have a composting spot. Make sure it is big enough to make the compost.
2. If using a bin-make sure it has holes to allow air circulation. Good aeration is important for compost. 
3. Cut the onions into small pieces- This will make it easier for microbes to break down the organic matter.
4. If you add hard onion skins, it is best to soak them in water for approx. 24 hours before adding in. 
5. Cover your onions with cardboard and newspapers to minimise unpleasant odours. If you turn your pile during the composting process, add odourless cardboard or paper again so the smell does not spread.
6. Bury onions around 20cm down into composting bin. This reduces the awful smell when onions decompose. 
7. Allow it to degrade

Tips for Composting Onions

Composting onions can be difficult. For those who want to try it, below are some useful tips to make it easier:

Use small pieces of onions

If you have full onions to add to your compost pile, it is best to cut them into small pieces. This makes it easier for bacteria and microbes to break down the organic matter.

You can also add moulded onions. This is great, especially if you have onions that have gone a bit off and you cannot add to cooking.

Avoid onions cooked in fats

Cooked onions can be added to your compost pile. However, avoid onions you would cook in oil or butter. This is because they are difficult to break down. The fats can spread to other materials in the compost and this prevents them from composting.

The cooked fat gives an unpleasant smell that attracts rodents and rats. They can interfere with the natural decomposition and will not allow good aeration in the compost pile.

Soak the skin

The food scraps from onions are the harder, outer skin that is not used when cooking. We can add this to compost, but in this state, it is difficult to break down.

Cut into smaller pieces and then soak in water for 24 hours. This will leave a moist, soft skin that will easily break down when added to a compost pile.

Reduce the smell

Onions have a strong odour that can be unpleasant. When the onions rot, the foul smell gets worse. Avoid this by burying your onions deep into the compost. 

Also, reduce the unpleasant smell by adding odourless material around the onions such as newspapers or cardboard.

As we use onions daily when we cook, there is a lot of waste. Hence, try to keep a compost bin in your kitchen to collect this waste. It will help keep a tidy look to your kitchen.

Another reason to control the smell is because you may be growing plants to improve the landscape. You don’t want to be growing large succulents or the beautiful Korean Succulents covered in a lovely top dressing of rocks, and then have a foul smell ruining all this.

Avoid onions when vermicomposting

In vermicompost, we add material to allow all those tiny worms to break it down. But when odorous materials like onions are added, this decomposition process does not happen.

The reason for this is that worms dislike the smell of onions. They will not feed and will leave those onion peels to rot.

Sometimes, worms may break down the onions, but this will let out a foul smell. This is something no one will want!

Also, adding acidic materials can kill worms. The compost will not decompose in time and the vermicomposting method will be ineffective.

I would strongly advise against using vermicomposting when you add onions. 

Can you compost whole onions

It is best to avoid adding whole onions to your compost pile. There are two main reasons for this:

Using whole bulbs can be difficult to break down. The natural decomposers find it difficult to break down the onions as they do not thrive around the acidic environment. The onions will not decompose and this can be a problem for the composting pile.

Second, if the whole bulb breaks down quickly, there is a chance of regrowth. This is because onions can grow very easily. Therefore, adding a whole bulb to a compost pile can cause the sprouting of new onions. This is not something you would want.

It is better to use skin waste or cut a whole onion into smaller pieces.

Can you compost moulded onions

Yes, you can! The whole point of composting is to allow organic matter to rot and breakdown in a natural process. When you use moulded onions, you have already started that process.

So if you have any moulded onions, add them to your compost pile. It is a great way to get effectively rid of your unused onions.

Can you compost diseased onions

No. Avoid this at all costs. The disease will pass on to the compost pile and kill off plants when we add the compost. 

Onions with neck rot, mildew, purple blotch and leaf blight, for example, all have diseases that can spread. Thus, it is better to dispose of them differently. 

Using onions peels to make a fertiliser

Now we have established onions can be used to make compost, I would like to touch on the benefits of using onions peels as a fertiliser.

The fertiliser is easily made by soaking onion peels for a period of time. The resulting mixture consists of calcium, potassium, magnesium and iron, and they are all vital minerals your plant will need to grow.

The organic fertiliser eliminates the need to use chemical fertilisers or pesticides as it helps with both.

Below is a video that shows step by step how to make an organic fertiliser using onion peels.

Final words

As weird as it may sound, composting onions can be beneficial for plant growth. Not only do they let out essential nutrients, but they also help the soil, promote water retention and prevent diseases. 

Also, we all are guilty of throwing away our onion waste daily. Composting onions is a great way to recycle our waste. If we all play a part, there is less waste sent to landfills and this will have massive beneficial effects on our climate. 

Overall, onions are a great way to get nutrient-rich compost, reduce costs and save the planet.