Red-brown earths — the plains' difficult soils

Every plain wears its climate on its underside, and Adelaide’s is written into a single act of digging. Push a spade into almost any paddock on the Adelaide Plains and for the first thirty-odd centimetres it goes in easily, through a pale, loose, sandy loam. Then, often within that same spade’s depth, it stops — not tapers, stops — against a heavy, reddish clay you have to prise out in wet, gluey slabs. Soil scientists call this a texture-contrast or “duplex” soil: two quite different soils, stacked, with an abrupt line between them instead of a gradual blend. Under the older Great Soil Group system this particular stack had a name of its own, the red-brown earth, and South Australia’s soil survey still carries it, plainly labelled: Hard Red-Brown Texture-Contrast with Alkaline Subsoil (Hall et al. 2009).

The abruptness is the tell of a whole climate at work. Water soaking through the loose sandy top meets the dense clay and stalls, so the ground waterlogs in a wet winter and sets like a road once that same clay dries out over summer. Worse, the clay itself is usually sodic, alkaline and laced with lime — and the reason is not the rock but insufficient rain. In a wetter country, winter rain would flush salts and carbonate down through the profile and away. Adelaide’s Mediterranean climate almost never delivers enough to finish that job. James Prescott, the soil chemist who put Australian pedology on the map from a chair at the Waite Institute, spent years reducing that idea to a number: a “leaching index,” the ratio of rainfall to evaporation that predicts, climate by climate, how much of a soil’s salt and lime survive underneath it (Prescott 1950). Where evaporation wins, as it does across almost all of this dry continent, the leaching never finishes, and the salts stay exactly where they fell.

These are difficult soils to love: hard to dig, quick to waterlog, quicker still to set like brick. They are also, inconveniently, the soils that grew the wheat that built colonial South Australia, and the ones the market gardens of the Northern Adelaide Plains still work today. Read a fence line or an old wheat paddock on the plains and you are reading this — not bad luck, but a climate’s arithmetic, banked in clay.