Do You Need To Remove the Old Soil When Repotting?


Transferring a potted plant to a new container can be tricky, especially when choosing the best soil for the new container. Many gardeners wonder if they need to remove the old soil when repotting and transferring plants to a larger container.

It’s best to retain the uppermost layer of old soil when repotting, as this layer is likely still nutrient-rich. Gardeners can test the soil’s nutrient level in the original container before and during the repotting process to ensure the plant roots have access to the necessary nutrients.

This article will address the factors you’ll want to consider when repotting with old soil and review the methods you can use to determine the health of a potted plant’s current soil. You can use this information to ensure that the repotting process goes smoothly.

Factors To Consider When Repotting With Old Soil

The growing media you choose for your plants significantly impacts the plant’s health and growth rate. But when you change a plant’s soil when repotting poses particular challenges.

After all, torn root systems (especially delicate ones) are a relatively common risk during the transplant process. 

When shaking off or removing old soil from a plant’s roots, you may damage the root system, leading to stunted growth or plant death. For that reason, you may want to repot your plants with some of their current soil. 

Still, there are a couple of factors to consider when repotting old soil, including fertilization regularity (and type) and the size of the new container.

Fertilization Regularity and Type

Fertilizing potted plants is a great way to keep the soil nutrient-rich and help plants grow. But overfertilizing potting soil is easy to do, and chemical fertilizers can harm plant health. 

If you’ve been fertilizing your potted plant’s soil too often (or with chemicals), it can lead to root burn. This condition kills plant roots, significantly harming the overall health and longevity of the plant. In this case, it’s often best to discard the old soil when repotting.

The Size of the New Container

The most common reason gardeners transfer plants to new containers is to allow the plant to grow larger. Unfortunately, when a plant has become too large for its current container, its growth is often stunted

Naturally, you’ll need additional soil when transferring a plant into a larger container. So, you can either mix old and new soil in the new container or only use fresh soil. But there are some instances when old soil may be unusable, necessitating gardeners to use entirely new soil.

Reasons Old Soil May Be Unusable

In some cases, a plant’s current soil may be unusable, necessitating a full or partial soil change during the repotting process. For example, old soil may need to be discarded or composted when it:

  • It contains excessive amounts of fertilizer
  • The soil lacks the nutrients required to keep plants healthy
  • It harbors fungi that can harm plants.

As we mentioned earlier, overfertilized potting soil can damage plant roots and inhibit plant growth. As such, it may be unusable when repotting. 

But in addition to excess nutrient levels, a lack of nutrients can make old dirt a poor choice of media. Soil sapped of its nutrients can slow plant growth and contribute to premature plant death.

Additionally, you should not add soil that contains fungal spores and mold to a new container. Mold and fungus in the soil can consume nutrients and spread to the plant. Avoiding this issue may be as simple as using fresh potting soil.

To determine whether old soil is usable, you’ll want to consider its nutrient levels and check for signs of mold growth.

Gauging Soil Health When Repotting

If you’re thinking of mixing a plant’s oil soil with new soil when repotting, you’ll want to ensure that the old soil is healthy and nutrient-rich. 

Fortunately, you can use visual indicators (soil color and texture) to determine soil health. You can also use testing kits to reveal the soil’s nutrient levels. 

Visual Indicators of Soil Health

Healthy potting soil is typically dark brown or black (often depending on its moisture levels). It should also be loose, with a crumbly texture. Light brown or chunky soil may be nutrient-poor, making it a lackluster growing media.

It’s also essential to look for mold or mildew growth when examining old potting soil. Dirt with a web-like covering of white mold should be spread out as an even layer and exposed to sunlight for several days.

This exposure to air and sunlight helps kill fungal spores growing in the soil. However, this process isn’t immediate. As a result, gardeners may want to use fresh dirt to repot their plants. 

Testing the Soil’s Nutrient Levels

Sometimes, visual indicators aren’t enough to determine whether old soil is still usable. For example, nutrient levels in old soil are virtually impossible to gauge without the help of a soil test kit.

If you’re considering reusing a plant’s current soil when repotting, you may want to invest in a soil test kit. 

The Luster Leaf Soil Test Kit (available on Amazon.com) is an excellent option, as it allows gardeners to discover a soil sample’s potassium, nitrogen, and phosphorus levels. Results typically only take a few seconds, making it easy for you to determine a soil sample’s reusability. 

These nutrients will fall onto a broad scale ranging from depleted to surplus. Soil that registers as depleted should be discarded or composted, and samples with surplus nutrients may be overfertilized. In either of these instances, it’s wise to use fresh soil when repotting. 

Benefits of Using New Soil When Repotting

If your plant’s current soil is sapped of nutrients, contains excessive fertilizer, or shows signs of fungal growth, it might be best to compost or discard the old soil. Besides, using only new soil when repotting can be beneficial.

For example, gardeners using new soil when transferring plants to new containers will find that:

  • They don’t need to test soil nutrient levels
  • New soil expedites the repotting process
  • The old dirt is recyclable.

There’s No Need To Test Soil Nutrient Levels

Testing soil nutrient level only takes a few minutes, but it can make the repotting process more complex. When testing old soil, you need to remove the plant from its container to access the bottom ⅓ of the soil.

To ensure the plant remains viable, you have to keep its roots wet. You can do this by propping a plant upright over a water container, being careful that you submerge its roots in clean water. But this extra step can be frustrating for gardeners looking to complete a quick transfer.

By using new soil, you can skip the testing process, thus allowing you to immediately repot the plant without setting up a temporary root-watering rig.

Using New Soil Expedites the Repotting Process

If you choose to discard or recycle your plant’s old soil when transferring it to a new container, you may find that the repotting process is far faster. 

When using new soil, the transfer process is as simple as filling the new container with new soil (between ⅓ or ½ of the container), placing the plant into this fresh soil, then adding new soil around the plant to keep the roots protected and help make the plant stable.

You’ll then water the new soil, kickstarting root growth and helping the plant anchor itself while absorbing life-sustaining nutrients. Besides, when you only use new soil, you can recycle the old soil by adding it to a compost bin.

Gardeners Can Recycle Old Soil by Composting It

Did you know that you can recycle old soil? Even overfertilized soil can become safe and fertile again, making it a valuable compost for future gardening projects. 

One of the best ways to recycle old soil is to add it to a compost bin. Tossing this dirt into a compost bin and mixing it with other types of compost (dead leaves, decaying twigs, rotten food scraps) enriches it with naturally-derived nutrients.

Compost bins also allow small amounts of water to pass through the compost material. As water seeps through this compost mix, it helps slowly flush out fertilizer (especially chemical fertilizer), resulting in a more fertile mix. 

The small holes in a compost bin that allows water to pass through also increase ventilation, reducing the risk of fungal growth in the compost. Still, the composting process can take several months or more than a year. 

Gardeners must be patient when recycling old soil into usable compost. But this patience will be rewarded with nutrient-rich soil, and it’s an eco-conscious alternative to chucking old dirt into the garbage or spreading overfertilized soil around your property.

Final Thoughts

Removing the bottommost layer of old soil when repotting it is an excellent way to ensure a plant has access to plenty of nutrients. Plant roots descend toward the lowest layers of the soil when grown in a container, and this bottom layer is often far less nutrient-rich than the upper layers.

Potted plants that you may have overfertilized may benefit from a complete soil change. Alternatively, if you are transferring plants into a larger container, you may not need to remove any soil. You will only need to add a fresh layer of additional soil to fill the new container.

Alexander Picot

Alexander Picot is the principal creator of TheGrowingLeaf.com, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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