Podzolisation — how sand goes bankrupt

Some of the forces that shape this coast announce themselves. Fire arrives in a day; a flood in an afternoon. Podzolisation works on a scale that hides it from a human life. It belongs with deep time and moving water among the slow forces, and of those it is the quietest: the one-way draining of a sandy soil’s wealth, over spans that make a whole forest look like passing weather. You will not see it happen. But on the Sunshine Coast there is one place where the process has been laid out in the open, in order.

The place is Cooloola, the great sand mass north of Noosa, where the dunes lie ranked by age: the youngest built fresh along the beach, the oldest stranded far inland, each row a little further along in the same slow leaching that carries iron and phosphorus down out of the sand and leaves a bleached column standing over buried coffee rock. Walk inland from the shore and you walk forward through time. The young grey foredunes hold no bleaching at all. Further back the pale band appears near the surface and the staining forms below it, and the two pull apart and drive downward, dune by older dune.

How do we know how much time each step buys? Every buried grain of quartz keeps a clock of its own. From the moment the wind last covered it, a grain soaks up a faint drizzle of natural radiation, and that stored dose, teased back out as a glow in a darkened laboratory, dates the day the dune stopped moving. The oldest sand at Cooloola reads at seven or eight hundred thousand years. On those most ancient dunes the leaching has run to an outlandish extreme: the bleached column plunges past twenty metres, a six-storey fall of white sand standing over its hidden floor, among the deepest soils of their kind anywhere on Earth. They were startling enough to be written up in the journal Nature in 1981.

What all that time removes matters more than the colour it bleaches. Measured dune by dune along the sequence, the phosphorus held in the topsoil falls by something close to ninety per cent from the youngest sand to the oldest, and phosphorus, alone among the major nutrients, cannot be refilled from the air once it is gone. The point here is only the direction of travel. Podzolisation never reverses. Each pulse of rain takes a little more and hands nothing back, and no dune along the sequence grows richer for growing older.

This is the force, then, behind the poverty every wallum plant is built to live with. It works below the surface and below notice, on a clock measured in ice ages, draining the same clean sand a beach delivers into the most exhausted ground on the coast. What the heath makes of that is the wonder, and it belongs to the wallum’s own telling. The force itself is still running, under every white dune on the coast.