Onions in compost often bring strong reactions among many gardeners, as many say that even placing an onion in compost bins can ruin the entire compost. Some gardeners disagree by proving their successful methods to placing onions in compost.

Backyard Compost Pile

Actually, it can be quite easy to compost onions. While you can toss onions into your compost pile, it really depends on a lot of factors, keep reading to learn how to compost onions without harming your composting pile.

Can You Compost Onions?

💥 Yes, you can compost onions, the key point is to be careful when placing composted onions since the amount is crucial.


Onions are acidic in nature with their low pH levels. Having in mind that compost is naturally alkaline, when you add onions to your compost pile, the acidity of the onions can lower the alkalinity of your compost.

When you place too many onions into your compost pile, only do so if your sole purpose is to lower the pH level of your composted pile. Onions inhibit bacteria and other living things in the compost such as earthworms and compost grubs. 

When these organisms are inhibited, composting time gets delayed, however, as a precaution, add onions to your compost pile sparingly.

Balancing the Acidity in Your Compost

Alkaline levels are important to compost piles, as high alkalinity helps break down the organic compost matter more quickly.

When the onions in your compost seem to be too much, you can balance out the acidity of the onions by adding chalk or crushed eggshells.

How To Compost Onions

Before anything, you should be most aware that adding large chunks of onions can harm your compost pile. The larger the pieces of onions are, the longer they tend to decompose. Note that onions are not recommended for vermicomposts as the worms tend to steer clear of the onions and do not break down as easily.

Now that we’ve gotten that out of the way, let’s begin the steps on how to compost onions.

– Create a Composting Bin or Identify Your Composting Area

Everything begins with the composting bin, this is when you have to first identify the spot or the area where you would like to place your composting bin. The location could be anywhere in your kitchen, your terrace, or even your yard.

The composting bin can be made of a pot or a bucket, where holes should be drilled at differing levels of the bin to allow air to circulate and initiate the composting cycle. If you already have a composting bin or a composting pile in your yard, you can skip this part and move on to the next.

– Prepare the Onions for Composting

If your onions are still whole or in large chunks, cut them into smaller pieces. Doing so allows them to decompose more easily and reduces the chances of the onions sprouting new shoots.

Moldy onions can be composted safely, as long as you cut them up into smaller pieces. Having in mind that mold is a fungus, which is needed to help break down food particles in compost bins.

You may need to add newspaper or cardboard if the odor of onions does not appeal to you. Aside from reducing the odor of onions, newspapers, and cardboard decompose easily as well.

You can also place onion skins in compost bins if you do not intend to compost the onion itself. It is still best to have them in small pieces as well to hasten the composting process. While the odor of onions is greatly reduced, you may still opt to include newspapers and cardboard if the smell does not agree with you.

– What Types of Onions to Avoid

While onions, in general, are fine to be added into compost bins, there are some exceptions. Certain conditions can make onions harmful to your compost bins, such as:

– Diseased Onions

Avoid placing diseased rotten onion bits into your compost bins. The diseased parts can pose a risk to your whole compost and become an agent of infection when compost is applied to the soil. Pathogens such as leaf blight, white rot, or onion mildew can wreak havoc if you allow diseased onions into your compost bins.

While some composting enthusiasts believe it is perfectly safe to compost rotten onions, you may want to double-check. The safety of your soil affects many plants, after all.

You can, however, burn the suspected diseased onions into ash and add the ash to your compost bin. Nonetheless, ash is alkaline in nature and will greatly help the alkalinity of the compost pile.

– Cooked Onions

Cooked onions should not be added to your compost bins. Compost piles in general take longer to break down greases and fats, if at all. The grease and fat may also spread over other compost materials and inhibit them from composting properly.

Decomposing fats also have strong odors. These odors tend to attract pests and scavengers, which can make your compost bin problematic. Decomposing fats also make the whole compost pile slimy, which interferes with the correct aeration process of the composting cycle.

– Place Your Prepared Onions Into Your Compost

You can easily add the onions on top of the compost bin; however, if the odor of onions affects your olfactory senses, you may want to bury them. By burying them at least 10 inches deep into the composting bin, the odor of onions is less detectable.

Hence burying the onions deeper into your compost also eliminates the risk of pests or wildlife poking into your compost bin looking for some snacks.

– Take Care of the Compost Pile

a Compost Pile

You can add soil once a week to your compost pile. Soil helps break down the components of the compost pile faster. Microbes in the soil improve the speed of the composting process while lessening the risk of insect infestation.

Ensure that your compost pile gets turned every two to four weeks, this allows your compost to aerate even more. Nevertheless, a compost pile with good aeration improves the entire composting process.

– Check Your Compost Pile

Aside from adding soil and turning over your compost pile, you should learn to check if your compost pile is ready to be used. Compost ready for use that includes the composted onions should ideally be a rich dark brown color. Keep in mind that the compost should also smell like earth and easily crumble in your hands.

You will know if your compost is not ready for use when recognizable food materials are still present and visible. When the pile is still warm to the touch, your compost is also not ready to be used.

The warmth indicates that the whole composting process is still underway. Large lumps in the pile are also indications that your compost is not ready for use.

– Harvest and Use Your Compost

When you have indications that your compost is ready for harvest, place containers near the compost bins. These containers can serve to hold your compost if your plants are not near or if you plan to use the compost at a later date.

Compost from onion peels can make a wonderful soil supplement for plants. You can use compost as an alternative to inorganic chemical fertilizers, by simply spreading the compost over your garden soil to add nutrients.

Compost can be used as mulch to protect the roots of your plants from soil erosion. Mulches in general greatly help prevent wind damage as well as reduce the risks of soil and plant diseases and infections.

The onion skins in compost increase the potassium content of your soil, which increases their size, growth, and resistance to many diseases. Furthermore, the onion peels in compost also provide trace micronutrients and minerals such as iron, calcium, copper, and magnesium.


Bonus: How To Make Your Own Liquid Fertilizer from Onion Peels

Onion peels can make a great fertilizer feed for your plants. If you have a handful of onion peels and you don’t know what to do with them, you can make liquid fertilizers from onion peels. Here is a quick instruction on how to make onion peel fertilizer:

Simply place two to three handfuls of onion peels in a liter of water. Some gardeners recommend boiling the water with the onion peels while others do not.

If you are going to boil the onion peels in water, do so for about 15 minutes up to an hour.

However, if you are not going to boil the onion peels in water, allow them to soak for 24 hours.

Whichever process you prefer, strain out the liquid after 24 hours. Use the liquid fertilizer as is, or you can dilute the solution with some water if you frequently fertilize your garden plants. The liquid fertilizer will keep for 10 to 15 days.

Compost Heap


Do compost worms like onions?

Compost worms can eat onions, but they may not prefer them due to their pungent smell.

Can you put moldy onions in compost?

It is generally safe to put moldy onions in compost, but it’s best to avoid them to prevent spreading mold.

Will onions break compost down faster?

Onions contain sulfur which can inhibit compost breakdown, so they may not speed up the process.


With many believing that onions do not belong in composts, the savory bulb has become somewhat polarizing in the eyes of many, but now that you know that onions can in fact work well as composting components, you can now breathe a sigh of relief.

Let’s just go over some things that we need to keep in mind when we add onions to composting piles.

  • Add onions to your compost bins and piles sparingly. Make sure the onions are cut into small pieces before you add them to your compost bins.
  • To lessen the strong pungent odors of onions, add some newspapers and cardboard along with the onion bits. Bury the onion scraps and bits at least 10 inches deep into the compost pile to further lessen the chances of the strong odors coming out.
  • There are some types of onions you should avoid adding to your compost pile: diseased onions and cooked onions. Take care of your compost pile with onion bits by adding soil every week to hasten the composting process.
  • Turn your compost pile every 2 to 4 weeks to aerate the pile for a better composting cycle. Check your compost pile texture every few weeks to see if it is ready to be used.
  • Use the compost when it is a rich dark brown color that smells like earth and crumbles easily. You can even make your own liquid fertilizer from onion peels.

Now that we’ve gone through how onions can be safely added to compost, you won’t look at onions and onion peels the same way again. Initially thrown into garbage bins, onion scraps can now be turned into solid and liquid gold for your garden and for your plants!

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