Long before there were trees, flowers or lawns, the first little plants crept out of the water and onto bare rock and mud. They didn't come alone. Fossils more than 400 million years old show fungal threads already living inside their roots, swapping water and minerals for a share of the plant's sugar. That deal has held ever since, and the grass in your backyard is still in on it. Right now, under every square metre of your lawn, roots and fungi are trading away, the same as they always have.
Then along comes the bag of green-up fertiliser. Most of us have done it: the lawn's looking pale, you chuck some out, water it in, and a week later it's lush. But a few weeks later it's pale again, you're mowing twice as often, and the same dry patches turn up every summer. That's because the bag doesn't just feed the grass. It quietly breaks a deal that's been running since plants first came onto land.
The deal going on under your lawn
Like most land plants, lawn grasses team up with mycorrhizal fungi: fine threads that grow into the roots and spread out through the soil, far beyond where the roots themselves can reach.
Think of the fungi as the grass's delivery drivers. They can squeeze into tiny pockets of soil the roots can't get into, pick up water and nutrients (phosphorus especially) and bring them back to the roots. And they're paid in sugar, the stuff the grass makes from sunlight. Along the way their threads help glue soil into crumbs, so water and air soak in instead of running off.
Around the roots lives a crowd of bacteria, fungi, worms and other critters, feeding partly on what the roots leak out. They break down dead roots and clippings and turn them back into plant food. A healthy lawn is really a partnership between the grass and the soil.
What happens when you hand the grass free takeaway
Plants aren't charities. That sugar costs the grass something. When the soil is suddenly loaded with easy, dissolved nutrients, especially phosphorus and big doses of nitrogen, it's like handing the grass free takeaway. Why keep paying the drivers when dinner's already on the doorstep? So it cuts back on the trade.
That isn't just a theory. Researchers who pooled results from dozens of field studies found mycorrhizal fungi generally decline when nitrogen and phosphorus fertilisers go on, with phosphorus having the most consistent effect. A large study of grasslands across several continents found the same pattern. How much it matters depends on the soil, the product and how heavily it's used, but the rule holds: feed the plant directly and it puts less into the partnership.
The knock-on effects
Shallow, lazy roots. Lots of soluble nitrogen pushes growth into leaves rather than roots. Add little-and-often watering and the roots have no reason to go deeper. Shallow roots are the first to cook in an Adelaide January.
Salt burn. Quick-release fertilisers are basically salts. Too much at once, on dry soil in hot weather, draws water out of the roots. That's the brown stripes and patches of "fertiliser burn".
Thatch, pests and disease. Push-growth makes stems and roots faster than the soil can break them down, so a spongy layer of thatch builds up. University turf guides list heavy nitrogen as a main cause. Thatch holds water at the surface, and soft, over-fed growth is more appealing to some lawn diseases and insects.
The fertiliser treadmill. A quick-release feed gives a flush of green that fades in a few weeks. The fungi and soil life that would normally keep feeding the grass have been sidelined, so it needs another hit, and another.
Runoff. Whatever the grass doesn't use has to go somewhere. In sandy soils it washes down past the roots. Off hard clay, paths and driveways it runs into the gutter, and Adelaide's stormwater isn't treated before it reaches Gulf St Vincent. The SA EPA has linked excess nutrients to seagrass loss along our coast.
Plants that warn the neighbours
Here's the bit we love: those fungal threads can carry a warning, too.
In one experiment, researchers grew tomato plants joined by a shared fungal network, then infected one of them with early blight. Its healthy neighbours, connected only through the fungi, switched on their defence genes and held off the disease better than tomatoes without the link. A few years later a UK team tried it with broad beans and aphids. When one bean plant got infested, its fungus-linked neighbours started giving off a scent that drives aphids away and calls in the tiny wasps that hunt them, before a single aphid had landed on them. Beans without the fungal link got no warning at all.
Picture it: an underground smoke alarm. One plant gets hit, the alarm runs along the fungal threads, and the plants next door are already arming up. These were careful experiments with garden plants, so we won't claim your lawn is chatting over the fence. But it's a good reminder of what a living soil can do, and what you lose when you break up the network.
Fertiliser isn't the villain
Let's be straight: nutrients are nutrients, and sometimes a soil really is short of something. When a test shows that, a targeted feed is the right call. The problem is feeding on the calendar, heavy and fast, without knowing what the soil actually needs.
Stop asking what the lawn needs. Ask what the soil is missing.
What TwigYard does instead
Look before we feed. We lift a small soil core to check root depth, thatch, compaction, soil type and worms. Where it's warranted we test pH and nutrients, so any feeding is based on facts, not guesswork.
Organic matter, every time. Sifted compost topdressing, often straight after coring so it works down into the holes, plus charged biochar (more on that below). In sandy soils organic matter holds water and nutrients; in heavy clay it helps open things up.
Feed the soil life. Worm-casting tonic, and no synthetic chemicals hammering the soil.
Slow-release organic inputs, only when needed. Soil life releases these gradually, so they're gentler on roots and less likely to wash away.
Water deep, and less often. A good soak that reaches the root zone, then let the top dry out a bit. Roots follow the water down, and every litre goes further.
Mow higher, and leave the clippings. Taller leaves catch more sun and make more sugar, and more sugar means more pay for the fungal delivery drivers and deeper roots. We never take more than about a third of the leaf at once, and set the height to suit your grass. Fine clippings left on the lawn break down into food for the worms and microbes, so the nutrients go back where they came from instead of into the green bin.
Biochar: a home for soil life
Biochar is plant material, usually wood, heated with very little oxygen. That heat does two things. It locks the carbon into a stable form that soil microbes can barely break down, so instead of rotting away in a season like compost, it stays put for generations. Research puts most of it at hundreds of years or more in the soil. And it leaves behind a honeycomb of tiny pores: a ready-made block of flats where bacteria and fungi can set up house, sheltered from drying out, with water and nutrients held close by. More life in the soil means more food for the worms.
The catch? Plain charcoal is just carbon. Dig raw char into your lawn and it's an empty building, and it can even soak up nutrients for a while before it does any good. So we charge ours first, soaking it with aged sheep manure and compost so the pores are full of food and microbes before it goes in the ground. It arrives with the tenants already moved in.
Whether you're on sandy coastal soil that drains like a sieve or heavy clay that sets like concrete in February, the fix is the same: more organic matter, more soil life, deeper roots. That's what gets a lawn through a hot, dry Adelaide summer.
Book a free soil peek
Not sure what's going on under your lawn? Book a free on-site soil peek. We'll lift a small core and show you the roots and soil on the spot.
And we'll count the worms. They're the scorecard. Worms only hang around where there's food, air and moisture, so a core full of them tells you the soil's alive and working. Find none and we know where to start. They earn their keep, too: a big research review pooling 58 studies found that, on average, earthworms lifted plant growth by around a quarter.
Then we'll tell you honestly what we see and what (if anything) we'd do about it. No obligation.
Quick questions
Do I have to stop fertilising altogether?
No. If your soil genuinely needs something, we'll feed it, usually with slow-release organic products. What we steer away from is heavy, frequent doses of quick-release fertiliser applied by habit rather than need.
Isn't coring disturbing the soil?
Not the way digging does. Coring pulls out small plugs spaced across the lawn and leaves the soil in between alone. That relieves compaction and opens channels so air, water and compost can get down to the roots. Tilling or rotary-hoeing churns the whole top layer and shreds the fungal network. Coring leaves most of it intact, and roots and fungi grow straight into the fresh, compost-filled holes.
How long until I notice a difference?
It's gradual. Some things, like water soaking in better after coring and compost, can show up fairly quickly. Rebuilding soil life and deeper roots usually takes a season or more. We'd rather be honest about that than promise overnight miracles.
Can't I just buy a mycorrhizal product?
They can help in some situations, such as new turf or badly degraded soil. Most established lawns already have native fungi in the soil, though. They just need the right conditions: less soluble phosphorus, more organic matter and sensible watering. Fix the conditions first.
What actually happens at a soil peek?
We take a small plug of turf and soil, look at it with you on the spot (roots, thatch, soil type, moisture, worms), then pop it back in. It takes a few minutes and leaves barely a mark.