Concept · keystone idea · Ch 4
The nitrogen–phosphorus asymmetry
Nitrogen can be replaced from the air, but phosphorus comes only from rock — so once an old soil has leached its phosphorus away, it is gone for good.
Two things about any soil matter more than all the rest, and the second is the one that decides everything downstream: how much nitrogen it holds, and how much phosphorus.
Here is the asymmetry. Nitrogen can be replaced from the air — certain microbes and plants haul it straight out of the atmosphere and bank it in the soil. Phosphorus can only come from rock. Nothing whatsoever can pull phosphorus out of thin air. So once the original rock’s phosphorus has weathered out and washed away to sea, that is the end of it: no resupply, no second delivery, not ever.
Hold on to that one fact and you can predict the fate of any soil on Earth, given enough time — which, fortunately, is the one thing soil has in abundance. A young soil is short of nitrogen and slowly builds it. An old soil has plenty of nitrogen and has run out of phosphorus for good. The whole life story of the ground, and of the forest or heath riding on it, is written in that single lopsided rule.
In depth
Two nutrients set the fate of a soil, and they behave in opposite ways. Nitrogen is effectively renewable: free-living and symbiotic microbes (the rhizobia in legume and wattle root nodules, cyanobacteria in crusts) fix N₂ straight from the atmosphere and bank it into the system, so a young, N-poor soil steadily grows richer as biology accumulates. Phosphorus has no atmospheric source at all. Its entire supply is the apatite and other P-bearing minerals in the parent rock; as those weather, P is taken up, cycled, leached in drainage water, and progressively occluded — locked into iron and aluminium compounds that no root can prise open. Walker & Syers (1976) formalised this as the arc of phosphorus during pedogenesis: total and available P rise briefly, then decline monotonically and irreversibly over hundreds of thousands of years. Because there is no resupply, the oldest soils don't merely run low on phosphorus; they run out of it for good, and everything downstream (podzolisation's visible bleaching, the poverty→diversity paradox, sclerophylly) follows from this one asymmetry. It is the hinge on which the whole Cooloola story swings.
Primary sources & further reading — the doorway
- Walker, T.W. & Syers, J.K. (1976). The fate of phosphorus during pedogenesis. Geoderma 15: 1–19. — The fate of phosphorus during pedogenesis — the model this concept rests on.
- Chen, C.R. et al. (2015). Soil phosphorus fractionation and nutrient dynamics along the Cooloola coastal dune chronosequence, southern Queensland. Geoderma 257–258: 4–13. (The Cooloola P-decline study. Total P in the upper 30 cm falls from ~229–237 kg ha⁻¹ on the youngest dunes to ~24–28 kg ha⁻¹ on the oldest — a ~88–90% (roughly order-of-magnitude) decline. State the ~90% figure for total P, or qualitatively; the precise resin/available-P percentages are in the paywalled full text.) — Measured phosphorus decline along the Cooloola dune chronosequence (~90% loss of total P).