The soil microbiome: the living soil in plain words
Healthy soil is alive with bacteria, fungi, tiny animals and earthworms. What they do for your plants, what harms them and how to keep them fed.
5 min read 路 Patchmates
Good soil is not just dirt with fertiliser in it. It is a living system. A handful of healthy garden soil holds an enormous number of living things, most of them far too small to see. Together they break down dead material, release nutrients, build soil structure and help protect roots. Gardeners call this the soil food web, and looking after it is one of the best things you can do for your plants.
Who lives in the soil
Bacteria
Bacteria are the most numerous soil organisms. They feed on sugars, dead plant material and each other, and in doing so they unlock nutrients such as nitrogen. Some bacteria live in the root nodules of legumes like peas, beans and wattles and take nitrogen from the air, which is why legumes need little nitrogen fertiliser. Bacteria also make sticky substances that help glue soil particles into crumbs.
Fungi
Fungi grow as fine threads called hyphae that spread through the soil. Many break down tough material like woody mulch and fallen leaves. A special group, the mycorrhizal fungi, partner directly with plant roots. The plant feeds the fungus sugars, and the fungus acts like an extension of the root system, reaching further into the soil for water and nutrients, especially phosphorus. Most garden plants form these partnerships. The brassica family (cabbage, broccoli, kale) is a well known exception.
Protozoa and nematodes
Protozoa are single-celled animals, and nematodes are tiny worms. Most of them are not pests. Many graze on bacteria and fungi, and because they eat more nitrogen than they need, they release the surplus near the roots in a form plants can take up. A few nematodes, such as root-knot nematode, do damage plants, but in a balanced soil they are kept in check by everything else living there.
Earthworms and larger creatures
Earthworms, beetles, mites, springtails and slaters shred organic matter into smaller pieces that microbes can work on. Earthworms also tunnel, which lets in air and water, and their castings are rich in nutrients and help bind soil into stable crumbs. Seeing plenty of worms when you dig is a good sign.
What the soil life does for your plants
- Releases nutrients slowly, as organic matter breaks down, so plants get a steady supply.
- Builds structure. Crumbly soil holds water but still drains, and roots can push through it easily.
- Holds water. Organic matter and the crumbs it forms act like a sponge.
- Protects roots. A busy, diverse soil leaves less room for disease organisms to take over.
- Extends roots through mycorrhizal fungi, helping plants find water and phosphorus in dry soil.
What harms soil life
| Practice | Why it hurts |
|---|---|
| Bare soil | Bakes in summer sun, crusts in rain and leaves nothing for the soil life to eat |
| Compaction | Squeezes out air and water; walking on beds or working wet clay is the usual cause |
| Heavy digging | Breaks fungal threads, exposes soil life to drying and burns through organic matter faster |
| Too much soluble fertiliser | High salt levels can stress soil organisms, and plants with plenty of easy nutrients feed their fungal partners less |
| Regular fungicides | Can't tell good fungi from bad, so they can knock back helpful fungi as well |
| Long dry spells | Most soil life goes dormant or dies when the soil dries right out |
None of this means you can never dig, fertilise or spray. It means using these tools when you need them rather than by habit.
How to feed the soil life
The organisms in your soil eat what you give them. Feed them well and they will look after your plants.
Add organic matter
Compost, aged manure and leaf litter are food for the whole soil food web. Spread a few centimetres of compost on beds each year, more often in hard-working veggie beds. You don't need to dig it in deeply. Worms and other creatures will pull it down for you.
Keep it covered with mulch
Mulch shades the soil, slows evaporation and feeds soil life as it breaks down. In Adelaide's hot, dry summers this matters a lot. Straw-type mulches suit veggie beds and wood chips suit trees and shrubs.
Keep living roots in the ground
Roots leak sugars into the soil, and a lot of soil life gathers around them to feed. A bed that sits empty for months loses some of that life. Between crops, sow a green manure or cover crop, or plant the next crop soon after harvest.
Grow a mix of plants
Different plants feed different microbes. Mixing families, rotating crops and including flowers, herbs and legumes all help keep the soil community diverse.
Dig less
Where you can, add compost and mulch to the surface rather than turning the soil over. Use a garden fork to loosen compacted ground rather than a rotary hoe. Keep paths and beds separate so you never walk where plants grow.
Keep it moist, not soggy
Soil life needs both water and air. Deep, less frequent watering under mulch suits it better than frequent light sprinkles that dry out fast.
What about products sold to boost soil life?
You will see plenty of bottles and bags promising to add microbes or "activate" your soil. Some may help in particular situations, but in most home gardens the soil life is already there. It just needs food, cover and a bit of care. Compost, mulch and living roots are the reliable, low-cost path.
Signs your soil life is doing well
- Plenty of earthworms when you turn a spade.
- Dark, crumbly soil that smells earthy and sweet.
- Mulch and compost that disappear over a season.
- Water that soaks in rather than running off.
- Plants that grow steadily without needing lots of feeding.
In Patchmates, you can note when you add compost and mulch, so you can see how each bed changes over the seasons.