Potting soil is the most important ingredient to grow awesome plants in containers. But after a growing season, a lot of nutrients may get depleted as the plants absorb those. You need to improve the potting soil so you can use it again.
You can improve soil in a container by removing the top crusty layer and adding fresh potting soil to it. You can add compost to the top layer and mix it into the soil to give a boost of nutrients and beneficial microorganisms. You can add a slow-release fertilizer to increase the nutrients.
I’ve written the 8 steps that will help you rejuvenate your old potting soil so you save some money and recycle it for your next growing season.
1. Remove unwanted materials from the soil
The first thing you want to do is remove dead plant material from the old potting soil. You can pull out the old roots, stems, sticks, and other unwanted debris.
There may also be weeds growing in the potting soil that you need to pull out. Make sure you pull them out along with their roots or they may start growing again.
You can use your hands to pull these materials out. Using a trowel is easier and faster than digging for the old roots, stems, and weeds.
2. Aerate the potting soil
Now that you’ve removed the unwanted things from the soil let’s fix the next problem. Since the old potting soil has been used for a while, it may have compacted due to the pressure.
This means less space between the soil particles, which does not allow air and moisture to flow through with ease.
So you need to aerate the old potting soil and improve its texture. The easiest way to do this is to use a trowel or a fork to dig into the potting soil. If you have a large container, you could use a rake or shovel for this.
If you have a wheelbarrow, you can overturn or shift the potting soil to it and do this process. It makes this easier as you get a lot more space to work with the potting soil.
Poke into the soil and overturn it so it starts to regain its texture. Keep doing this till all of the potting soil has been mixed up to improve the texture.
I asked some of my fellow gardeners if they improve and reuse potting soil from the previous season. Most of them said they reused the soil after amendments as shown below.
3. Remove pests and diseases from the potting soil
It may happen that your plant in the container suffered from pests or diseases during the growing season. Or you just want to be safe before reusing the soil.
You should not use this infected potting soil without sterilizing it to kill the remaining pests and diseases in it.
The easiest way to do this is to put the potting soil in a black garbage bag and leave it under full sunlight for a couple of weeks. The heat will kill all the pests and diseases in the soil.
A faster method would be to place the potting soil on a baking tray and put it in the oven set at 120 degrees for an hour. This will kill the pests and diseases in the soil.
The drawback of doing this is that the heat will also kill the beneficial organisms and insects present in the soil.
Once the potting soil has cooled, you can add it back to the container. Make sure the container is also sterilized before doing this.
You can sterilize the container by placing it in a larger container or tub filled with 1 part bleach and 9 parts water. Let the container sit in this solution for an hour.
Then rinse the container with clean water and let it dry under the sun. Once it’s dry, you can add the sterilized potting soil to it.
4. Remove excess salts from the potting soil
You may keep adding organic fertilizer to the potting soil when growing plants. Over time, the salts keep building up in the soil.
If you use hard tap water, there will also be a buildup of such salts in the potting soil.
Before you reuse the potting soil, you want to remove such excess salt. The simplest way to do this is to give the potting soil several good waterings.
Use a watering can or hose to soak the potting soil with water until it drains out from the drainage holes at the bottom.
Repeat the watering process 3-4 times so you get all of the excess salts out of the potting soil.
5. Add 50% fresh potting soil
You can now remove 50% of the old potting soil and add fresh potting soil to the container.
This will help you save some money as you need to buy less potting soil. You can also reuse the old potting soil that you removed in some other containers if you want.
You can mix in the old and new potting soil with the help of a trowel. You may need a shovel if the container is large with a lot of potting soil.
You could even dump the old potting soil in a wheelbarrow and mix it with the fresh potting soil. This would be a bit easier to do without spilling the soil out of the container.
It’s best to add a little of the fresh potting soil, spray water and mix it up well. Keep repeating the process till all of the old and new potting soil has been mixed.
6. Add 25% compost to the potting soil
Adding compost is the best way to increase organic matter, nutrients, and beneficial organisms in your old potting soil.
The compost will also help improve the texture of the old potting soil. It will help restore some of the soil’s moisture retention and aeration properties due to the organic matter.
I suggest adding 10-25% compost to the old potting soil. You can mix it in the container itself.
Or you can place the old potting soil in a wheelbarrow, add in the compost, and mix it in well. You could use a trowel for a small batch of potting soil or a shovel if the quantity is large.
7. Increase the nutrients in the soil
The compost will provide a boost of nutrients to the potting soil. But I also recommend adding some slow-release organic fertilizer to it for plants that are heavy feeders.
The quantity you add will depend on the amount of potting soil and the manufacturer’s recommendations. So check their instructions before adding to the soil.
Once the seeds have germinated, they can get the required nutrients from the slow-release fertilizer. The slow-release fertilizer is released into the soil when you water it.
8. Improve the texture of the soil
You can take this optional step if you find that the potting soil still needs some improvement even after mixing fresh potting soil and compost.
If you find that the potting soil is not holding sufficient water and draining it out fast, the soil may have turned hydrophobic.
The best way to make it hydrophilic or get its moisture retention back is to soak it in water for an hour. Let the soil absorb the moisture slowly and get its texture back.
You can add moisture-absorbing materials such as vermiculite, peat moss, or coco coir to the potting soil to help it retain more moisture. I would suggest adding 10-20% of such material to the potting soil.
If you find that the potting soil is retaining more moisture than it needs to you can add materials that will help it drain water faster.
Some materials that can help you with this are coarse sand and perlite that have texture to drain the water out. I would suggest adding 10-20% of such material to the potting soil.
Make sure to use coarse sand and not the fine one because it will not drain the water well.