If you’ve been a gardener for a while, you may have come across articles referring to how important it is to keep your soil “loose” or “airy”.
But what does soil aeration mean? And why do we care?
In short, if a soil is well-aerated, there is enough exchange of oxygen between the soil and the atmosphere to support healthy roots. A lack of oxygen effects the root’s ability to absorb water, and reduces nutrient availability in the soil.
Overall, this leads to stunted growth, and in extreme cases, plant death.
In this article, we cover:
i) what is soil aeration, ii) causes of poor soil aeration, iii) how to test for soil aeration and iv) methods to aerate soil.
Let’s dive in!
What is soil aeration?
Soil aeration is a method to increase the amount of air within your plant’s soil.
Think of your plant at root level. Like all living things, root cells need oxygen to carry out its normal processes. At the same time, roots produce carbon dioxide as a waste product.
If soil is dense and heavy, there are insufficient air pockets in the soil. As a result, carbon dioxide levels rise. At the same time, oxygen from the atmosphere cannot easily enter the dense soil to support healthy root growth.
What causes poor soil aeration?
When you buy fresh potting mix, soil aeration is typically at its highest. Over time, soil tends to compact and harden. Anyone who has had a plant for more than a year would notice that the soil naturally becomes “stale”!
Besides old soil, poor soil aeration can also be caused by:
- soils that are naturally too dense or heavy, with no chunky amendments (see below!);
- waterlogged soils. When a plant becomes waterlogged, water molecules displace oxygen particles in the soil. This deprives the roots of oxygen.
What are the effects of soil compaction?
Soil compaction is very detrimental to your plant, affecting not just the roots but its overall health.
According to James W. Boodley from Cornell University, lack of oxygen due to soil compaction has been shown to reduce water absorption by the roots. It also reduces the ability for the plant to transport food from the leaves (where it is produced through photosynthesis) to other parts of the plant for nourishment.
Another study by the University of Warrick also shows that soil aeration also reduces the availability of some nutrient elements to plant roots, causing lacklustre foliage.
Symptoms of soil compaction are:
- Reduced growth rate, due to less nutrient absorption;
- A smaller overall size than normal;
- A less developed root system;
- Lots of yellowing leaves, or less healthy-looking foliage.
In extreme cases, soil compaction can lead to plant death.
How to test for soil compaction?
If soil has compacted significantly, you may be able to recognize that the soil is hard just by looking at it.
If it isn’t so obvious, gently poke a chopstick into the soil. You’ll be able to tell by the level of resistance whether the soil has compacted or not!
How to increase soil aeration? (3 ways)
There are 3 main ways we like to aerate our soil. These ways complement each other. We use all 3 in parallel, and recommend that you do too.
1. Use a chopstick
One way we’ve started to aerate soil is to use a chopstick. Any chopstick will do, even those you get from Chinese takeaways or sushi joints. 🙂
Starting at the pot’s edge, gently poke the chopstick into the soil, until loosened. It may take a few pokes for your chopstick to reach the bottom of the pot, depending on how compacted the soil is.
Then, twirl the chopstick to create air pockets within the soil.
Once the soil at the edge of the pot is loosened, work your way inwards, repeating the same steps.
If you find yourself hitting against the roots, don’t be too worried. Just move to a new spot.
We like to aerate the soil this way every few months, or when we notice hardened soil. If the soil is fresh, you won’t have to do this as often.
2. Use chunky amendments to lighten commercial soils
A easy way to aerate soil is by using chunky amendments.
Typically, we add a range of amendments to lighten commercial soil mixes. These include:
- Perlite. This is a type of volcanic rock that is heated and expanded. It can hold 3-4 times its weight in water. Perlite is good at delivering airflow to roots, and extra moisture.
- Vermiculite. Vermiculite is often compared to perlite. It has more water-retentive properties, but delivers less airflow to the roots.
- Orchid bark. Bark is usually made from fir or coniferous trees, and comes in different-sized chips. Its chunky nature naturally creates air pockets due to the way it falls against finer soil particles. It also retains some moisture.
- Pumice. Another kind of volcanic rock, pumice is heavier than perlite, yet is porous so it is good at delivering airflow to the roots. A benefit of it being heavier is that it doesn’t fly away like its light-weight sister, perlite!
- Charcoal. A type of processed carbon that, like bark, creates air pockets. Another benefit is that it reduces impurities in the soil. They are typically made from wood or coal, which is then heated in a process called pyrolysis.
The amount of amendments to add to your potting soil depends on the type of plant you have.
You’d want to aim for enough amendments for airflow and drainage, but not too much that the water and nutrients drain away before your plant can absorb its goodness!
Consult our plant guides to determine the recommended soil mix for your plant. 🙂
3. Make sure you replace your soil every few years
Soil is only meant to last for a finite amount of time, typically 2-3 years.
During this time, your plant draws nutrients from the soil. At the same time, key ingredients lose their structure and texture, as well as lose their ability to hold moisture effectively.
As a result, you should always try to refresh your plant’s soil every couple of years. That’s why we always recommend using fresh soil when repotting.
Frequently ask Questions
Does having well-aerated soil reduce the risk of root rot?
Yes it does.
Root rot is a condition where waterlogged roots are deprived of oxygen. This results in root decay and death. The key cause of root rot is heavy soils plus overwatering.
Adding amendments like bark and pumice increases both aeration and drainage properties of the soil… thereby reducing the risk of root rot!
However, there is a natural limit to how much well-aerated soil can help.
If excess water displaces the air pockets quicker than the chunky amendments can drain it away… then your plants will still suffer. 🙁
In this case, consult our guide on proper watering practices.
Deborah is a plant enthusiast and founder of Gardening Collective.