Why Does Soil Sink In Potted Plants? Is it a Problem? [Hint: yes]

Did you see the soil level on your ceramic pot decreasing? What happened to the soil? Sunk, disappeared? I can hear you asking. The situation can be concerning but if we understand why this happens, we can apply appropriate measures.

Soil sink in a potted plant because of 1) soil bulk density and soil compaction, 2) decomposition of organic matter, and 3) soil leaching out of the pot’s bottom. The most common solutions for soil sinking in pots are: 1) limiting compaction, 2) repotting, and 3) adding organic media to the soil.

Is there a way to prevent the sinking of soil in a container? Does soil sinking happen to gardens as well? Let’s dive in to find out.

Soil Sinking In Container, 3 Reasons And Tips

Soil is composed of minerals, living organisms, dead organic matter, air, water. Once the balance of one of these components is altered the soil starts to shrink or sink.

1 – Soil Bulk Density and Soil Compaction

`The higher the bulk density, the more compacted the soil is. This is because soil particles are pressed closely together leading to a reduced volume of pore spaces in the soil. To the naked eye, heavy compaction of soil in the plant container can look like the soil is sinking.

Soil bulk density refers to the weight of dry soil per unit of volume. This volume includes the volume of soil particles and the volume of pores among soil particles.`

What Causes Compaction in Potting Soil?

Potting soil is light, aerated, and has less bulk density the first time you put your plant and soil in the container. As you water the plant, the soil gets compacted due to the water pressure. This pressure pushes the air out of the soil.

Compaction by water is not an immediate effect. It takes time (months) but will inevitably happen to reduce the airflow in the soil.

Does Soil Compaction Harm Potted Plants?

The effects of soil compaction (listed below) on the health of plants are interconnected:

1. Less soil infiltration
2. Poor water drainage
3. Aeration problem
4. Hardening of the soil.

Less Soil Infiltration: Soil infiltration is the ability of soil to absorb, and hold water. Poor infiltration results in decreased nutrient access by the roots and reduced nutrient cycling by soil organisms because they have less space to move around.

Poor Soil Drainage: Poor drainage occurs when water cannot easily infiltrate or drain downwards through the soil layers. Water will then displace the air in the soil. When this happens, the roots and soil organisms cannot get enough oxygen.

On the extreme end of poor soil drainage, water will stay standing on the soil for days eventually “drowning” the plant and causing root rot.

Aeration Problem: As stated, the air is displaced by water which leads to the roots and soil organisms having less oxygen to use.

Hardening of the soil: The more compacted the soil is, the more force the roots must use to penetrate deeper soil layers as the plant grows and matures.

When roots are less able to penetrate the soil, there is lesser access to nutrition leading to the plant exhibiting nutrient deficiencies. remember, not all plants have strong roots to penetrate compacted soil. Think about basil. Its roots are not as strong as mint for instance. So a compacted soil might severely harm basil.

2 – Decomposition Of Organic Matter

Potting soil contains organic matter. Organic matter decomposes over time due to the organisms in the soil that break them down. What’s left is only small amounts of mineral matter called humus.

Solution: Use new fresh potting soil or add organic matter into your soil. You can add peat moss, compost, agricultural manure, or fines from pine trees. The last one is great because it does not decompose that fast.

3 – Soil Leaching Out Of The Pot’s Bottom

Another cause of soil sinking in the pot is that you are actually losing soil because it is coming out of the drainage holes of your planter together with the water. It is said that you will lose roughly 20% volume of potting soil within 2-3 waterings.

Different from compaction such phenomena happen at the very beginning. After the first few waterings, the soil leak is very limited.

Prevention: Before planting, put a single layer of landscape fabric over the holes. The fabric will allow water to flow out while preventing the soil from leaving the pot.

Solution: For plants already in the pot, you can add more potting soil and firm it by tapping the pot gently on the potting table until you notice the soil has “settled”. You can also gently firm the soil with your hands.

How To Limit Soil Compaction In Pots – 6 Ways

First, assess the situation by analyzing the cause of the sinking soil. Is it compaction? The organic matter has decomposed? Is the soil leaching out of the pot?

To limit and avoid soil compaction here are 6 great tips:

1. Increase the organic content of the soil. This can be done by mixing any of these to potting soil: cattle dung, chicken manure, your own kitchen compost, grass clippings, fallen or dead leaves, and coir fiber.
2. Change watering can: If the compaction is due to water, use a watering can that has several small holes like this rather than one large hole as a water exit. You can also practice bottom watering if your pots are small enough to lift.
3. Add coarse material: Perlite for instance can be a great ally to limit soil compaction by “interrupting” the solid structure created by the soil. However, for this to work, it should be more than 30% of the soil volume otherwise it will not be able to “break” the compacted soil easily.
4. Moving the soil around: Your kitchen fork, skewer, chopstick, can be a great ally for soil compaction. What you need to do is just to gently move the soil so as to improve the air passage. Avoid moving the soil too close to the plant base to avoid harming the plant roots.
5. Shaking/Patting: With the planter on the ground (if of small dimension) just pat it with your hands on the side. The vibration will start breaking the soil. If your container is made of plastic it is even easier. Squish on the side.
6. Repot: changing the soil altogether is by far the most effective solution. However, it is not recommended all the time and mainly depends on the plant age and its size compared to the planter.

Repotting as a Solution of Compaction

I would like to add that with each situation discussed earlier repotting can be a solution as well however, the decision to transfer a plant depends on whether:

a) The plant is still young and thriving or

b) The plant has overgrown the pot or

c) The soil is drastically depleted.

Note that if you decide to repot, it is recommended to do it at a time when the plant is dormant. This way stress to the plant is reduced because they are “sleeping.”

The Plant Is Still Young And Thriving

If the plant is young and healthy there is no need to repot. You can instead just do the following:

Step 1 – Add new potting soil up to the brim of the container.

Step 2 – Spread the soil around and level it.

Step 3 – Tap the container once or twice on the potting table.

The Plant Has Outgrown The Pot

If the plant has not yet matured but has overgrown the pot, you need to repot. The container should match the size of the plant at maturity as the roots need enough room to grow and spread.

The signs that the plant no longer fits the pot are: a) the roots are coiling inside around the container and b) the roots are growing out of the pot’s drainage hole.

The Loss Of Soil Is Drastic

If you notice a significant loss of soil in the pot, you should transfer the plant because a) the plant gets its nutrition from the soil and b) for larger plants, the roots need to reach enough depth that will enable it to keep the plant upright.

5 Causes Of Garden Soil Sinking

The reasons for garden and farmland soil sinking are similar up to a certain extent to potting soil. Due to the larger scale, other environmental factors contribute to this phenomenon such as:

1. Compaction
2. Decomposed organic matter
3. Erosion
4. Removal of original soi
5. There is a void under the garden

What follows is a discussion of each reason and what possible actions can be taken to solve the situation.

1 – Compaction

In garden soils and farmlands, this is caused by human traffic, limited crop rotation, rainfall, livestock roaming around. In addition, farmlands are further impacted by the use of heavy machinery.

Solutions:

• Minimize human traffic.
• Do not allow livestock to roam on cultivated soil.
• Practicing multi-crop planting composed of plants with varying soil depths.
• Mixing sawdust in with garden soil: ideally 50% to the volume of garden soil or higher (up yp 70%) in case of very heavy clay soil.

2 – Decomposed Organic Matter

After organic matter decays, it turns to humus which has very little volume.

Solution: This can be solved by adding new soil and organic matter amendments. Before adding organic matter to your garden soil, it is recommended to have a soil analysis done first so that you’ll know exactly the amount to add and to avoid over-fertilization of the soil.

3 – Erosion

More than “sinking” erosion is actually soil disappearing (that can be confused with sinking).

It is caused by the action of water and wind on the soil. Water carries the soil away and the wind blows soil particles. Erosion happens to topsoil. This causes poor soil quality because most soil organic matter rests on topsoil.

Erosion is also caused by tilling soil because it makes it easier for water to carry away the soil during rainfall

Solution: Erosion can be prevented by planting cover crops, reducing tilling practices, tilling in months where less rainfall is expected, and the addition of soil organic matter.

4 – The Original Soil Was Removed

If the original soil was removed and organic matter was added, after several years, that soil will appear to be lower than the soil around it again due to the decomposition of organic matter.

Solution: Adding new soil. Mulching with organic matter is also recommended. Mulching on clay soil will improve its drainage and water holding capacity.

5 – There Is A Void Under The Garden

The void or “sinkhole” as laymen call it, causes topsoil to slowly sink over time. The sinkhole is the result of water going through loose layers of soil underground. It can also be caused by tree stumps that were left to rot underground. Another cause is burying rotting material such as grass clippings and tree branches.

Solution: The first thing to do is to determine what caused the void to appear. If due to underground water, professionals should be called. If the cause is buried rotting materials, filling the hole with new soil is the answer.

Sinking Soil: A Problem For Foundations

A little subsidence of soil can be expected. In the construction industry, the soil is intentionally leveled and compacted by machinery to make it strong and stable to be able to support the foundation of structures that will be built on it.

Improper filling and compacting of the soil before construction will result in it sinking at a faster rate and unevenly. This can trigger structural problems that will manifest as cracks on the basement flooring, gaps between the soil and the steps, leaning of walls, and gaps between the floor and the wall.

Solution: To know for sure if the problem is structural, it is recommended to get a professional who has relevant experience. If the house is newly constructed, the homeowner should approach their contractor.

If the problem is not structural, the homeowner can ask for tips from professionals for appropriate solutions.

Takeaways

1. Soil amendment is done to improve the quality of growing media.
2. Soil settling is normal but losing a significant amount of soil is not good for plants.
3. Sinking soil not only affects hobby gardening and horticulture but also causes great concern for people in the building construction industry.

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Sources

“Soil Quality Indicators” by n.a. in US Department of Agriculture

“Soil compaction” by Jodi DeJong in University of Minnesota

“The connection between soil organic matter and soil water” by Anna Cates in University of Minnesota

“Soil Infiltration” by n.a. in US Department of Agriculture

“Soil Basics” by n.a. in Soil Science Society of America

“What Does Organic Matter Do In Soil?” by Eddie Funderburg in Noble Research Institute

“Sinkhole – Solving Drainage and Erosion Problems” by n.a. in Northern Virginia Soil and Water Conservation District