Pathway · about 20 minutes · 3 steps
The soil story — why the ground decides everything
Stand on bleached white sand and say what the rain has carried away, how long the emptying took, and what still manages to grow on it.
Rain soaks down through sand and takes something with it each time. Do that for long enough and the ground is sand and not much else. This short pathway follows that one slow force from the top down: what the rain lifts out of a grain of sand, where it puts what it takes, and how long the emptying runs. Three steps. The leaching itself, then the Cooloola dunes, where the same process is caught at every stage at once, then the heath still standing at the far end of it.
Keep the timescale in mind the whole way. Everything here happens by removal, one rainfall at a time, and nothing carries it back up. By the last step you should be able to stand on white sand and say what is missing from it.
- Podzolisation — how sand goes bankrupt — A force too slow to watch, and the reason a coastal dune grows poorer the longer it stands.
- Cooloola (Great Sandy National Park) — A field of dunes that get older the further inland you walk: a soil's whole life laid out in a row.
- The wallum — This heath runs on four scarcities at once, and cannot do without any of them.
The end-check — read both ends of it
You’ve walked the steps. Now put them together: answer these from what the pathway taught, not from the pages. Getting them here is what tells you it stuck.
Nitrogen can be topped up from the air by microbes and plants; phosphorus can only ever come from weathering rock. On an ancient soil, hundreds of thousands of years old, which nutrient runs short for good — and what does that mean for the country it can grow?
This is the hinge the whole Cooloola story swings on. Nitrogen-fixing organisms bank nitrogen out of thin air; nothing can do that for phosphorus. Once the original rock's phosphorus has weathered out and washed to sea, there is no resupply — so the oldest soils only ever get poorer, and the tall forest they once carried gives way to low heath. Along the Cooloola dunes, total phosphorus in the topsoil falls by roughly 90% from the youngest to the oldest sand. (Ch 4.)
You're standing on bright, bleached, bottomless white sand behind the beach. The scrub is low and hard-leaved and, in spring, loud with wildflowers and honeyeaters. What country are you reading — and why is the ground so poor?
Cues: Bright, bleached, bottomless white sand · Low, hard-leaved heath not much higher than your head · A spring riot of wildflowers and nectar-feeding honeyeaters · Still water the colour of cold, strong tea in the hollows
White sand is not clean — it is robbed. Rain and organic acids have stripped the iron and nutrients out of the surface grains over tens of thousands of years (podzolisation), leaving bleached quartz over a buried coffee-rock layer. That poverty is the signature of the wallum, and it is the very reason the heath is so species-rich: on ground this poor no single plant can dominate, so hundreds of specialists crowd in. (Ch 4; Ch 9.)
The wallum grows on some of the poorest soil in Australia, yet it is one of the most species-rich plant communities in the region. Why does such poor ground grow so many *kinds* of plant?
Poverty is a leveller. On rich ground a few fast, greedy growers monopolise the nutrients and shade everyone else out; on starved ground nobody can pull far enough ahead to dominate, so the field falls open to hundreds of specialists, each scratching a living a different way. The poverty is the cause of the richness. (Ch 4; Ch 9.) The strongest test of the strategy-diversity mechanism is Zemunik et al. 2015 (Jurien Bay, WA); the coast's classic chronosequence is Cooloola, and the wallum-diversity claim is kept at the regional level.