Can I Put Old Potting Soil in My Compost?

Instead of throwing old potting soil, you can re-use it in several ways. Adding potting soil to compost bin is one such way.

Reusing ensure your gardening budget is under check.

But, can you put old potting soil in compost without any issues?

Potting soil of infected annual flowering plants shouldn’t go to your compost bin. Else, pathogen and pests can spread to other plants through compost.

Old potting soil may consist of insect eggs, larvae, weed seeds and pathogen of diseased plants. Pasteurizing potting soil will help to get rid of these organisms to maximum extent.

While composting, most consider potting soil as an organic matter which eventually helps to add ample amount of nutrients to the soil.

Instead of adding it to compost bin, you can just spread old potting soil in the garden. In this write-up I’ll discuss other ways to reuse potting soil.

Can I Put Potting Soil in My Compost?

Yes, you can. But, as mentioned earlier you need to make-sure the soil is free from pathogen and harmful infected virus.

It might be surprising for few gardeners why should you add soil to compost, when you’re preparing compost to be added to the soil.

We all know that decomposition process in the compost is handled by helpful bacteria and microorganisms.

Soil has many such organisms that can contribute in decomposing green and brown material.

Benefits of adding soil to compost bin

  • Adding potting soil will help to get rid of flies and insects in the compost. Spread potting soil that is free from pathogens to remove the bad smell from the compost heap.
  • Soil has compost activators and inoculators that speed-up the composting process.
  • Rotten fruits or vegetables in the compost attract flies and ants, adding potting soil over the top layer of the compost will prevent flies from accessing kitchen scraps.

But, you need to pasteurize potting soil before dumping it in the compost.

How to Pasteurize Potting Soil?

Buying nutrient rich potting soil can be a considerable investment. But, should you throw this old potting soil when it loses its nutritional value?

Recycling and re-using is a skill that every gardener would like to posses.

But, before reusing potting soil in the garden you should get rid of possible soil gnats or pathogens in the growing medium. Else, the virus can spread to future plants.

Pasteurizing potting soil will remove most organisms that can harm your compost or garden.

Pasteurizing is done by heating the potting soil at 180F or 82C for 30 minutes. When the temperature is above 212F or 100C it is considered the soil is sterilized. (1)

You can use oven to sterilize the potting soil.

Preheat the oven to fasten the process by setting the oven to 200 F. Place the potting soil pan and cover it with aluminum foil. Set the temperature at 180F for 30 minutes.

Once done, allow the oven to cool down before using the potting soil.

You can check the video here.

Pasteurized or sterilized potting can be added to compost, mix it with new potting soil, spread around the plant or use it for bordering flowering plants.

How to add old potting soil to compost?

After ensuring that potting soil is free from harmful pathogen, use shovel to directly add the potting soil to the compost bin.

Old potting soil along with other compost material will make the compost nutrient rich.

Else, you can also mix old compost with old potting soil to spread around the plant. However, I recommend putting potting soil to compost.

How much soil should you put in compost?

Not more than 10 percent of the total volume.

Adding potting soil will add helpful organisms to the compost, but this doesn’t mean that you add huge amounts of soil to compost. This won’t benefit the compost.

Instead, add 4 inches of used potting soil to compost to get rid of the bad smell from rotten scraps.

Or, you can just add 2-3 shovels of potting soil for every 6 inches of other compost material.

Need not mention, mix the pile regularly to ensure complete decomposition.

How does adding potting soil to compost help your plants?

As mentioned, garden soil has activators and inoculators that fasten decomposition of kitchen leftovers and brown material that you put in the compost.

Each time you add old potting soil to compost, you’re adding these helpful organisms.

Just make sure you don’t end-up filling the compost bin with soil.

To avoid your compost going wrong, avoid adding wet potting soil. Use only dry and sterilized potting soil.

Other ways to reuse potting soil

You don’t have to change potting soil frequently, but if you’ve hanging annual flowering plants that probably you don’t want to throw the soil that took considerable amount from your gardening budget.

Apart from dumping it into the compost, you can reuse it to:

  1. Bulk large containers: Large pots gobble lots of potting soil. Instead of using fresh store bought soil you can just fill the bottom of the container with old potting soil that is free from soil gnats.
  2. Can be used as a base for raised garden beds: Similar to above principle, we can reuse old potting soil to fill the raised bed.
  3. Mix with compost to grow plants: Instead of composting, you can add old potting soil with old compost to revitalize it with nutrients.
  4. Use it to border flower plants: Instead of using fresh potting soil for borders, you can reuse old potting soil.
  5. Add it to your compost pile.

What are the things that you should not put in the compost?

Though composting is an excellent way to reuse wastage from kitchen to create nutrient-rich fertilizer, you should stay away from few things that aren’t suitable for composting.

As it can attract pests, or harmful germs that ruins your compost.

Below mentioned are few materials that shouldn’t go in your compost bin.

  • Meat, Fish, and Dairy Products can attract rodents and raccoons. As they take time to decompose, adding these things can cause foul smelling compost.
  • Grease, oil, and fatty foods should be avoided as they can create a compacted, anaerobic environment in the compost pile. This inhibits decomposition and can lead to foul smells.
  • Avoid adding plants that are infected with diseases, pests, or weeds to your compost. These can survive the composting process and spread to your garden when you use the finished compost.
  • Pet feces, such as from cats and dogs contain harmful bacteria or parasites that can contaminate the soil.
  • Non-biodegradable things like plastics, synthetic fibers, and treated wood should not be added to the compost.
  • Weeds that have gone to seed should be avoided, as the composting process may not kill the seeds. This can lead to weed problems in your garden when you use the compost.
  • While small amounts of citrus peels are generally fine, adding large quantities can make the compost too acidic, affecting the pH balance.


You can put old potting soil to compost, but remember few things:

  • Sterilize potting soil before using it in the garden.
  • Potting soil shouldn’t exceed 10 percent of compost volume.
  • Use 6 inches of other compost material with 2-3 shovels of potting soil.
  • Never use old potting soil to germinate seeds.
  • Avoid using infected plant’s potting soil directly in the compost.

Nature reuses everything it produces. If you visit forest, dropped leaves and twigs are reused naturally to add organic matter to the soil.

Reusing reduces pollution and gives your immense peace.

Apart from making yourself proud for preventing wastage, reusing potting soil for compost can keep your gardening budget at check.