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Sinking Soil in Potted Plants: Causes and Solutions

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Soil is composed of different materials, such as sand, silt, clay, and organic matter. These materials work together to help retain water in the soil which helps with plant growth, but as time passes the soil level may drop because of certain factors.

Sinking soil in potted plants is the direct result from the breakdown of organic material by microorganisms within the soil. Organic materials provide pore spaces for air and water to infiltrate in and out of the soil. When these air spaces collapse the soil draws closer together resulting in a sinking effect.

In this article, we will discuss what causes the soil to sink in plant pots, the effect it will have on the plant as well as the ways to prevent this occurrence.

Normally, topping up soil will help, but repotting the plant will give the best results.

What Can Cause Potted Plant Soil To Sink?

Cause of sinking soil in garden and potted plants

If you notice that your potted plants seem to be sinking in, this can be a deep trouble sign for the health of the plant. Let’s take a look at some causes of sunken soil in potted plants.

Interested in getting a good potting mix? I use Miracle Grow Potting soil when repotting my plants. It’s cost-effective and specially formulated to give the best results after repotting. You can find it by clicking here.

Miracle Grow Potting Mix

1. Microbial Activity

Firstly, the natural soil is enriched with microorganisms including insects, bacteria, fungi, and nesting mammals. These microorganisms are primarily responsible for the natural nutrients balance and the protection of soil health.

Therefore, garden soil does not need much care. But when it comes to potting soil, the story is completely different.

Over time, the plants will absorb nutrients from the potting soil and use the organic material contained in it. Consequently, the potting soil becomes poor and hardened.

Depleted, hard soil will not hold as many nutrients and water as fresh soil. Closed pores in hard soil do not allow water to pass through, causing the soil to sink with water from the sides of pots. 

2. Decaying Organic Matter

Secondly, potted plants that have been growing for a while are more prone to sink the soil away with water. The organic component of the soil (peat moss, compost) decomposes over time, leaving only a small amount of mineral material behind. With organic matter gone, the hard soil is all left in the pot. 

3. Loss of Soil Through Drainage

Thirdly, in some rare cases, the soil may leak through the drainage holes in the bottom of the pot, but you can see it. When re-potting plants, use a landscape fabric or a layer of polyester fiber to fill in the holes to hold all the soil while allowing excess water to drain.

Lastly, the decrease in soil level in pots can be due to the rotting organic matter of the mixture. In garden soil, worms carry organic matter from the soil surface to the roots of the plants.

Since there are no worms in the potting soil, organic matter will rot away with the sinking soil.

How will the Shrinking Soil Affect The Plant? 

All potted plants demand one universal formula: 

  1. Drainage
  2. Aeration
  3. Moisture Retention

If a soil mix has these 3 qualities, the plant will grow beautifully.

Roots breathe air and, therefore, require good ventilation in order to receive enough oxygen.

Excessive watering and waterlogging will kill the plant and, therefore, the soil must have good drainage to avoid this.

Lastly, the soil must retain moisture so the plant can absorb nutrients and water until the next watering.

However, what happens in plant pots is that after a while the soil breaks down and it compacts the soil. 

When air gaps in the soil get hardened (closed), ventilation and water drainage to the roots get prevented. When the water cannot reach the roots, it starts to flow out of the pots, taking the soil with it.

Furthermore, the soil is made up of particles and spaces. Particles can be organic or inorganic. Organic particles are organic substances that have been separated into many different-sized parts. 

Inorganic particles are rock fragments reduced in size by the digestive actions of organisms as well as natural actions such as erosion and weather conditions.

Soil (loose soil) with normal particles and spacing does a few things to help the plant grow. Air (oxygen) not only helps many living things in the soil but also reaches the roots easily. The loose soil allows water to reach the root system as well.

Therefore, the shrinking Soil will adversely affect the plant as both the air and water will not pass through it. Consequently, oxygen will not reach the roots. Water will also encounter difficulties as it passes through the shrunk soil.

Sinking soil can expose roots to light which may not have such an appealing look. You can have a look at our detailed article on how light affects plant roots.

Can you Add Soil To The Mix (Top-up):   

Long story short: Yes, you can add. Replacing soil entirely isn’t the only option. 

Whether the soil should be changed or not depends on the condition of the plant. If your plant looks good and the potting soil looks good as well, there is no need to entirely replace the soil. 

In such a scenario, some fresh, good-quality soil is needed to boost the soil.

You can add a bit of healthy soil (weed-free) to your old potting soil without any worry. Don’t forget to add some special boosters (ingredients) to the new soil that will improve plant growth and health.

Using a fork or other garden tools (if available), mix the fresh soil into the old potting soil. Remember, blending well into the old soil is the key to success. 

This fresh soil will enrich the old soil and make it more fertile for what you plan to grow.

How To Prevent Potted Plant Soil From Sinking:

One can prevent the soil from sinking by following these rules:

  • Always buy a good quality potting mix. It is worth the money over time. 
  • Another trick is to add peat moss. To prevent potting soil from sinking, simply add peat moss to it. 
  • Give your pots a good drink every week with seaweed or fish emulsion. But, don’t forget to sprinkle some water on potting soil after adding the peat moss for the very first time. It will not only help them mix well but will also moisten the soil. 
  • Adding soil amendments into the soil can help to solve these soil issues because it helps to keep space between the soil particles. Soil amendments will improve the drainage of the soil.

I use this cost-effective soil amendment when repotting, which enhances the soil and gives my plants the best growing conditions.

Soil Enhancer

Potting soil, or soil in houseplants, is much more different than outdoor soil in the wild It’s a different kind of ecology and a different set of physics.

As mentioned earlier, the biggest problem with potting soil is that it slowly compacts and sinks over months and years, closing off spaces that need to be filled with water and air. 

Even with the right amounts of fertilizer, compacted soil will slowly bring the plant to an end.

That’s why the plants need to be repotted every year or two. They can be substituted in the same pot if you don’t want them to get bigger. 

Whereas, in forested areas, you will find that the soil is constantly being replenished by the breakdown of plant matter (dropped leaves, broken stems, cut pieces, etc.) and by animal life (insects, fungi, bacteria, etc.). Therefore the soil in these areas neither sinks nor shrinks.

Soil Pulling Away from Plant Pot

There are a few different reasons that could cause the soil to pull away from the sides of the pot.

Soil will pull away from a plant pot as a result of drought conditions. In soils with a higher ratio of clay, drought conditions can cause the moisture content within the soil to drop very low. This causes the soil to contract as the moisture is removed which results in it pulling away from the sides of the plant pot.

Clay silt and sand

Also the pot may be too large for the type of plant. This will cause an excessive amount of water to accumulate on one side, which will create an imbalance with gravity and make your pot lean in one direction.

Another possibility is that there is a drainage issue. The issue could be due to the wrong type of potting soil or bad drainage at ground level near the bottom edge of your pot’s outside wall.

Take a look at the container you are using before watering it and ensure there aren’t any cracks or holes along its surface; this can also lead to water retention in certain spots within your container.

The Takeaway:

To conclude, the prime factor causing sinking soil is compact, hardened soil. Pores in the soil from where air and water pass, get closed. 

When water cannot get to the roots, it starts running out from the side, taking a portion of soil with it. Why do the pores in the soil get closed?

Well, after a certain time, nutrients in the soil are used up. Even if chemical fertilizers are added, microbes in the soil that allow the fertilizers to break down and reach the plant are reduced in number with time. 

Microbes are reduced because the organic matter in the soil is also depleted for a certain time.

For this reason, the soil in the potted plants is changed periodically to ensure that fresh organic nutrients and microorganisms are present and the plants regain their health.

Water intake into the soil should be improved to prevent soil from sinking. Add more organic matter to the potting soil for improvement. 

So far, this has been considered the most effective way to prevent soil from sinking, as coarse organic matter will separate clay particles and create pores for water to pass through easily.

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