The nitrogen–phosphorus asymmetry

Of all the numbers you could measure in a soil, two decide almost everything, and they could not behave more differently. One is nitrogen. The other is phosphorus. Learn how these two part ways and you have the asymmetry this coast’s ecology turns on, the point where the Cooloola story starts.

The rule is short. Nitrogen can be replaced from the air. Phosphorus can only come from rock.

Nitrogen makes up most of the atmosphere, and a whole guild of living things has learned to reach up and take it: the bacteria in the root nodules of wattles and native peas, the cyanobacteria in the crusts on bare ground, all quietly pulling nitrogen gas out of thin air and banking it into the soil for free. So a young, nitrogen-poor soil does not stay poor. Give it time and biology, and it grows richer, drawing down the one major nutrient that is there for the taking.

Phosphorus has no such trick. The air holds no phosphorus worth the name, and nothing fixes it the way rhizobia fix nitrogen. Every phosphorus atom in a soil was once locked in the parent rock, in apatite and a handful of other minerals, and released grain by grain as that rock weathered. Once it is out, it moves one way only, and slowly. Roots take it up, leaf and litter cycle it round, drainage water carries it off. Worst of all, some of it is occluded, sealed molecule by molecule into iron and aluminium compounds that no root can prise back open. Once the original rock’s phosphorus has weathered out and washed to sea, that is the end of it. There is no second delivery.

Hold on to that one fact and you can predict the long-run fate of a soil anywhere, given enough time, which, fortunately, is the one thing soil has in abundance. In 1976 two soil scientists, Walker and Syers, turned it into a model that has been bedrock ever since. A young soil arrives with a decent stock of phosphorus still locked in its minerals but almost no nitrogen; nitrogen is what limits it, and nitrogen is what biology steadily supplies, so its fertility climbs. A middle-aged soil has nitrogen in plenty and phosphorus still within reach, old enough to be rich and young enough to be solvent. An old soil has been so thoroughly stripped of reachable phosphorus that it runs short for good, and because phosphorus cannot be replaced, the shortage only deepens. The system tips into what ecologists call retrogression: it winds down. This is the part people find hardest to swallow. Soil does not improve forever like a good wine. It matures, then ages, then runs down to almost nothing.

To watch that happen you would need a row of soils identical in every respect, same rock, same rain, same air, differing in age alone across hundreds of thousands of years. Such a thing has no business existing. It exists at Cooloola, where the asymmetry has been measured directly. Along that sequence the total phosphorus in the topsoil falls by close to ninety per cent, from a couple of hundred kilograms a hectare on the young dunes to a bare handful on the oldest. The ancient sand is not merely low on phosphorus. In nutrient terms it sits about as near to nothing as a soil can while still holding up living things at all.

And much of what follows downstream begins with this one lopsided rule. The dazzling white of a leached podzol, the tough-as-boots leaves of the wallum: each is a different answer to the same non-negotiable scarcity. Nitrogen the country can earn back. Phosphorus, once gone, is gone.