What is aerobic soil and how to create it?
Aerobic soil has air circulating through it.
Anaerobic soil has a lack of air circulating and tends to be water logged. Therefore, at a simple level, creating aerobic soil can be as simple as turning off the water and allowing the soil to dry out. It will, over time become somewhat aerobic.
Unless it’s a wetland, most soil is aerobic to some degree and another. However, any soil that is continuously wet or soaked can become anaerobic.
There are instances where aerobic loamy soil has become anaerobic because of flooding. An example is one of my current gardening clients whose rear garden has become partly anaerobic due to the constant inundation of water from a newly built complex next door.
Eight weeks of continual water run off has reduced the amount of oxygen in the soil. The soil is boggy. The lawn is dying off. The worms and soil life are building turrets and breathing vents.
There’s also a scum appearing on the surface of the water but no detectable and offensive smells yet. If that water run-off isn’t stopped, I’d expect to see the drought resistant shrubs in that garden to start dying. Why is that?
Why plants can die from waterlogged soil
Different plants need different things. Typically, a drought resistant plant won’t tolerate much water around its roots. There are some exceptions for example: Callistemon are drought tolerant and also flourish in and around fresh waterways.
Others, such as Heterophyllus (native Australian hibiscus) would suffocate in such wet conditions.
The soil conditions and the microbes that flourish in normal or drought resistant soils are very different from those that flourish in waterlogged soils. Without air many of the microbes helping to maintain a healthy soil ecology suffocate.
Microbes that enjoy a lack of air and oxygen can then colonise the soil. Plants that are susceptible to root rot and fungal infections will start suffering.
Reversing the damage, reversing the process is a bit more complex. That’s because soil isn’t some inert lump of dirt. When we dig into the ground we aren’t digging into dead matter.
We might think soil is a solid mass of earth but in fact it’s a dynamic and living interbeing of bacteria, fungi, nematodes, arthropods, minerals, organic matter, water, air, intelligences and so on.
Microbes and their habitats
Microbes can live pretty much anywhere. When we’re talking about our soil, we want more of the “good” microbes and much less of the baddies. We want more of the good bacteria not so much of the pathogenic bacteria. Yes, we also want more of the bacteria and fungi eating nematodes and not so many of the root eating nematodes.
Have you ever left the kitchen scrap compost for too long and smelt that acidic, acrid smell? When you smell it your face immediately recoils and pulls away? That’s the smell of anaerobic microbes. Generally, the good microbes need air, the baddies don’t.
Therefore, good microbes (bacteria, funghi, nematodes, protozoa etc) that are beneficial for most plant health tend to flourish in conditions favourable to them and that means having access to oxygen.
How to fix anaerobic soil
To reverse anaerobic soil conditions we need to reduce the water inundation. That’s the first key step. We need to get air back into the soil and we start that process by drying it out. Once the water’s turned off we need to encourage the good microbes and soil life back into the area.
Now, I wouldn’t want to dig up my clients back garden just to get the right microbes back in. What I’d do depends on the extent of the damage, what we want to see as a result and their budget.
Generally the result we want to see is aerobic soil. We want to provide a habitat conducive to a healthy soil community and that means organic matter. So depending on the situation, we might sprinkle bit of organic matter to the top of the soil or we might add quite a lot and then if its a garden rather than a lawn we might add a light mulch such as lucerne or sugarcane.
The key here is to add healthy, aerobic organic matter. We need to repopulate the soil with aerobic microbes and provide them with habitat and food. That’s what the organic matter does. If the organic matter is from a pathogenic compost heap, then it’s going to take a lot longer to get a positive result.
By turning off the water or diverting the water and adding organic matter we have started to re-create the conditions for aerobic soil life to flourish.
The earthworms will return with the addition of the organic matter and mulch. These magical creatures will aerate the soil and they will also contribute plant available nutrients in the form of worm castings.
In this way we help to create a habitat for the soil life that we want to see flourish.