There are a few reasons you might want to put new soil on top of old soil. For example, you may have decided to create a new garden, but then you realized its soil needed a little sprucing up. Is putting new soil on top of old soil a good idea then (or in general)?
You can put new soil on top of old soil. However, you should loosen the soil and make it less compact. Otherwise, the roots will not grow deep enough in soil that is too compact. You also need to add a slow-release fertilizer to the old soil to make it more nutrient-rich.
This article will discuss what you need to know about putting new soil on top of old soil. I’ll talk about its advantages, the best time to replace old potting oil, and how to rejuvenate old soil.
Why You Shouldn’t Plant in Old Soil
There are a couple of reasons why you shouldn’t plant in old soil without rejuvenating it in some way.
- The old soil will likely have fewer nutrients. Over time, the nutrient levels in your garden soil decrease because your plants are using them up. Additionally, being constantly subjected to the elements erodes away these nutrients, leaving you with soil that cannot support healthy plant growth. Fertile soil is especially important for germinating seeds and newly transplanted plants.
- When the soil dries out, it becomes hydrophobic. In other words, your plants will be unable to absorb water through the roots if they’re growing in old soil.
One way to rejuvenate old soil is to add new soil to it. But how does new soil do that, exactly?
How New Soil Improves Old Soil
I’ve established that it’s all right to put new soil on top of old soil. Here are the reasons doing so is a good idea.
- The new soil makes the old soil less compact. During the process of tilling and mixing the old and new soil, you can break up large chunks of dirt. This makes it easier for plant roots to penetrate the soil.
- The new soil adds more minerals and microorganisms to the old soil. As I’ve mentioned, soil degrades over time, partly because plants use up the nutrients without them being replenished. Other things that contribute to soil degradation include soil erosion, pollution, compaction, organic matter decline, and biodiversity loss. New soil helps to mitigate the effects of these.
- The new soil helps to improve the quality of the old soil. Topsoil is critical for plants because they use this layer to absorb nutrients. Adding fresh topsoil to old soil will allow you to start your garden in areas that otherwise have poor soil quality.
- New soil makes old soil less hydrophobic. When soil is exposed to high temperatures, it dries out and forms a waxy layer that makes it hydrophobic. When you try to water hydrophobic soil, the water will either sit on it or run off. If you add in new soil, it’ll absorb the water and make your garden less hydrophobic.
When To Replace Old Potting Soil
You don’t need to change your soil every growing season. However, if you’ve been using the same soil for 12 to 18 months, it’s probably time to replace your old potting soil.
Apart from your soil’s age, here are the signs your old soil may need to be replaced:
- If you’re spending too much time, money, and pesticides to deal with one pest after another, your soil may be the problem.
- Despite the care you give your plants, they aren’t growing as well as they should — which is likely a sign that the soil doesn’t have enough nutrients for them.
- You’re growing plant species that tend to use up a lot of nutrients.
- You’re switching between different types of plants that have significantly different nutritional requirements.
That said, you don’t need to throw away your old soil unless pests and diseases have contaminated it. Instead, pour the affected soil into a plastic bag and dispose of it. Alternatively, you can sterilize the soil and prepare it for the next growing season.
How To Sterilize Old Soil
If you still want to salvage old soil even when it’s riddled with pests and diseases, you need to thoroughly sterilize it first. Otherwise, if you add it to new soil, those pests and diseases will affect the new soil as well, forcing you to replace your soil all over again.
One good way to sterilize old soil is to use a pressure cooker or sterilizing container. Either way, the ideal temperature for sterilizing soil should be 180°F (82°C).
Here’s how to sterilize your soil with a pressure cooker:
- Place the soil in small pans that fit in the pressure cooker.
- Add water to the pressure cooker.
- Place a rack in the pressure cooker to hold the pans containing the soil.
- Use foil to cover the pans of soil and place them on the rack in the pressure cooker.
- Cover the pressure cooker. Remember to leave the steam valve open to prevent steam buildup.
- Steam the soil for 30 minutes.
When using the pressure cooker method, you should ensure the soil wasn’t recently fertilized. Otherwise, nitrate-rich soil or soil that has manure will explode when exposed to high temperatures in a sealed chamber.
The same steps above apply when you’re using a sterilizing container.
After sterilizing the old soil, fertilize it. Then, split and mix it with the new soil.
These VIVOSUN Heavy Duty Plant Grow Bags (available on Amazon) are great for separating soil. The fabric is breathable and sturdy. Plus, they have durable reinforced handles.
How To Rejuvenate Old Soil
When rejuvenating old potting soil, you don’t need to get rid of all the soil. If the soil is only old, compacted, or out of nutrients, it’ll likely need only a boost — unless it’s filled to the brim with pests and disease-causing microorganisms, in which case you’ll have to sterilize it as I described above.
Here’s how to rejuvenate old soil:
- Make the soil less compact. Dig out as much of it as you can, and break up large chunks into smaller ones. If some chunks of the soil are too hard, remove them from the pot. Don’t forget to relocate any plants that are already growing in the soil.
- Nourish the old soil with slow-release fertilizer. Since these fertilizers take time to work, make sure the soil absorbs them properly before you decide to mix in fresh soil (if that’s necessary).
- Use perlite. This Miracle-Gro Perlite (available on Amazon.com) is rich in plant food. It prevents soil compaction and encourages strong root development. It also improves soil drainage and is best used in container gardening.
This YouTube video gives tips on how to reuse old potting soil.
How To Maintain Soil Quality
Adding fresh soil to old soil is one way to ensure that your plants grow under optimal conditions. However, if you want your soil to stay as healthy as possible, here are a few more tips.
- Apply fertilizer properly. Fertilizer is a beneficial substance on its own. However, if you want to make the most of it, you need to know what amount to apply and when depending on what you’re growing.
- Use organic ways to keep away pests. Instead of spraying harmful pesticides on your soil and plants, you can introduce the pests’ biological predators. Make sure these predators won’t feed on your plants in turn, though.
- Avoid disturbing the soil as much as possible. As long as you’ve created the optimal conditions for plants to grow, you can pretty much leave your soil to its own devices for most of the year.
Old soil doesn’t have much going for it. It’s often compact, dry, and barren, meaning it has no nutrients left to support healthy plant growth. At some point, you’ll have to rejuvenate old soil — and one of the ways to do so is to add new soil into it.
The new soil will breathe new life into the old soil by adding nutrients and improving the water absorption rate.
To learn more about improving soil quality, you could check out my other article here: How to Improve Soil Quality (The Ultimate Guide)