Initially, soil that seems very fluffy and well aerated will tend to settle over time causing the soil structure to sink down. Breakdown of organic matter, rain, heavy mulch, and walking traffic can cause the soil to become even more compact and in some cases prevent proper drainage.
Sinking soil in the garden is the direct result of organic material breakdown within the soil. Organic materials are key to soil structure, providing pore space for air and water to infiltrate the soil. The breakdown of organic matter in a raised bed garden is accelerated by heat and moisture.
In this article, we are going to explain the possible causes of what causes soil in garden beds to sink and what you can do to prevent it.
What can Cause Garden Soil to Sink?
1. Break Down of Organic Material
Garden soil sinks because of the decomposition of organic material used in the soil. This shrinkage of soil usually occurs in a new garden or when the soil is amended with organics such as wood chips, compost, grass clippings and manure.
This is one of the main reasons why garden soil sinks down after the first crop is planted.
Bacteria and microorganisms are at constant work in the soil and organic materials provide a food source for them to consume and thrive and therefore their population grows tremendously.
This is beneficial for plants as they turn the organics within the soil into useful nutrients for the plant to grow.
Overwatering can also cause the soil structure to draw closer together as the water occupies the space between the soil particles. as the water draws away the particles also draw closer together resulting in the sinking of the entire soil mass.
Overwatering and compaction can cause one of the major problems when planning to grow in raised bed gardens, raising them too high can also cause water retention issues which means that over time the soil will not drain properly.
3. Compaction of Soil in Raised bed Garden
Garden soil can also sink because of the compaction caused by human activity. The constant walking, movement and footsteps in a confined space like raised garden beds will cause the soil to compact and settle over time.
The weight from the person’s feet tends to compact the soil into smaller particles which will increase the water capacity of most soils. This results in moisture retaining soil which will then become soggy.
This is one of the reasons most raised garden beds are very hard to maintain as over time they will sink and compact causing a very difficult drainage system.
4. Use of Heavy Mulch
Shrinkage of garden soil can be caused by heavy mulch, it can be prevented by leaving a small amount of space between mulch and soil. Sinks in garden soil because of the growth of some plants.
Some plants such as potatoes and other root vegetables grow long roots that can penetrate the soil structure causing the soil to sink.
5. Plants with Deep Root Systems
Plants with deep root systems are likely to pull away surrounding soil, leaving space for air to flow between the root zone and the surrounding soil.
Some plants like Nasturtiums and some underground perennial vegetables, do not require much space between the root zone and the surrounding soil. As a result, when they grow and pull away from the soil in some areas.
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How much will Garden bed Soil settle?
The depth at which garden soil will settle depends on the organic material within the soil. It will settle to a level where it will not impede the drainage system. The soil in a newly amended garden bed will sink between 2 to 4 inches over the course of 6 months which would indicate good microbial activity within the soil.
The average depth of soil in a garden bed is 12 to 18 inches.
The depth at which garden beds settle depends on: the type of soil, its texture, the variation in particle size and how it reacts to plant roots. Garden soils are very different from one another due to their composition and chemical makeup (pH, EC, organic matter content), so therefore they will settle at different depths.
New garden beds tend to have more readily available organic material for microbes to decompose and it is in these new beds that there is a more significant drop in soil level because of the activity of the microorganisms in the soil.
You will find that new garden beds are filled with
- Potting soil
- Organic material
- Soil amendments such as perlite and vermiculite.
These three when used together can create the perfect growing environment for plants as they provide sufficient nutrients, aeration and drainage (water holding capacity).
However, when this mixture begins to settle you will notice that the soil in the garden bed begins to sink and the amount it sinks usually depends on how much organic material you have used in the first place.
How to prevent Raised bed Garden soil from sinking
1. Add More soil
Adding extra soil and fill to the topsoil is an inexpensive solution to keep the soil higher than the surroundings.
The higher soil level will compensate for the sinking in garden soil when the organic material is broken down over the course of time. Another solution is to mulch over the surface with bark mulch, or similar material is to keep the soil from drying out.
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2. Mound Extra Soil to Compensate for Shrinkage
Mounding can also be employed to increase the soil mass above the bottom layer of soil.
Usually, it is between the first 4 to 6 inches of soil that see the most microbial activity and it is here that most of the organic material is broken down first.
This accounts for the richness in top soil and also the reason why a great deal of the root structure of plants with fibrous root systems are found in the top few inches of soil.
Back to the sinking, mounding will account for the drop which is expected when using a higher ratio of organics in the soil and is something to take into account if you intend to add more organic material to your soil.
3. Wait After Tilling
If tilling a raised bed or garden bed, wait till the soil settles before planting (1 – 2 weeks)
Tilling is a term that usually refers to the breaking-up or turning of the soil. In agriculture, it is used for loosening the soil and incorporating organic matter to aerate and enrich it.
When this is done the soil should be left to settle for 1 t 2 weeks before planting. This allows time for the soil particles and organic matter to draw closer together so that there are no surprises when the plants are sown into the soil.
However, over the course of time, the soil will still drop and you may have to simply top up the existing soil to the desired level.
4. Add Soil in the Dormant Period
The dormant planting period typically begins in November and lasts until May. It’s a wise idea to start preparing your gardens for the colder months by adding soil and finalizing harvest arrangements.
For this reason, you should add organic material during this period so that it has enough time to be broken down and settle before the next planting season.
Additionally, you should avoid fertilizing your growing plants and be extra cautious when preventing excessive watering or adding mulch to conserve water.
5. Pull the Root Ball and Add Soil at the Bottom
If you have plants with a shallow root system you can opt to remove the plant, adding extra soil with amendments and replanting.
Consider it somewhat like transplanting and your plants will thank you for it as the extra amendments would increase aeration and drainage around the root system.
6. Add Mulch
Add mulch to prevent rain from compacting the soil causing soil erosion.
Mulch is a gardening product that is meant to add organic material to the soil surface. Mulch, as it decomposes, improves the quality of the soil and can also provide a home for microbes. Mulch also prevents rain from compacting the soil and causing erosion.
Rain can also and everyday watering activity can also add to the sinking of soil in the garden.
When water is applied to raised beds, soil within the higher part of the bed will be wetter than the surface. Additionally, when rain comes for an hour or two, it can wash away exposed sand on the surface and also compact the soil below.
While this may not cause serious problems, if a garden is subjected to regular watering, a raised bed can sink over time.
Mulch will also prevent the top layer of the soil from being eroded away from watering and rain. This is a sure way to prevent the valuable top layer of soil from washing off and the over height of the soil from dropping.
7. Lessen foot Traffic in Clay Soil for Garden Beds
How will the sinking soil affect the plant?
Sinking soil will not affect the plant as it is just an indication of good microbial activity within the soil. The soil will lose its initial mass as the organic material is broken down into nutrients for the plants. Additionally, drainage and aeration will also not be affected leaving the plant with optimal growing conditions.
Can you Top-Up Soil and Adjust the Soil Level?
Yes, you can top up the soil level but you must be careful not to bury your plants with the additional soil. This can be done with only certain types of plants. Soil can be added around vegetable crops with longer stems and trunks and they would welcome the additional nutrients from the soil.
However, you should avoid adding too much soil that would bury the crowns and rhizomes of some plants such as canna lilies, irises, rhubarb, kikuyu, ginger and bamboo.
The crown is the part of the plant below the soil surface from which new shoots grow and plants that grow from underground stems called rhizomes which include potatoes and ginger.
How High Should Soil be in a Raised Garden Bed?
Soil placed in raised garden beds should be 2 inches below the top of the raised bed barrier. The soil depth can range between 12 and 18 inches which gives the roots of the plants enough room to grow in search of nutrients and support.
How to fix soil shrinkage in raised bed gardens?
Soil shrinking in a raised bed garden is a result of under-watered soil normally caused by drought conditions. It is more predominant in soil containing a high percentage of clay. When the water molecules are removed, this causes the soil particles to draw closer together resulting in soil shrinkage.
To prevent the soil from shrinking you should consider adding soil amendments such as perlite and vermiculite along with organic material including wood chips, grass clippings and compost.
You can also reduce the percentage of clay in the soil by mixing in loamy soil and sand to help increase drainage and the water holding capacity of the soil.