Basalt fertility (why red ground grows rainforest)

Deep red earth on a range is the surest sign of rich country there is. It is red for exactly the reason rust is red: iron, oxidised. The old lava that once flowed here was loaded with the stuff, and as the rock rots down the iron reddens like a nail left out in the rain, staining the whole soil the deep, warm ochre you see cut into every road bank on the Blackall Range.

That colour is a promise, and for once the country keeps it. Basalt is young rock, geologically speaking, and it arrives full of the minerals that make a soil fertile: iron, and more to the point phosphorus, the nutrient that runs everything and, unlike nitrogen, can never be pulled from the air but only ever weathered out of rock. (The poor sand is where that asymmetry bites; here on the fresh basalt the mineral bank is still full.) So the rock hands its nutrients over to the developing soil, and the soil hangs on to them. That holding is the point. Basalt weathers to the right kind of soil.

It breaks down to a deep, well-structured red clay: a Ferrosol, in the language of the modern soil surveys, though your grandfather would have called it a krasnozem. The clay is what does the work. Where sand lets water and nutrients run straight through, clay clings to both with a miser’s grip. So a basalt soil manages what few soils do: it holds its water and its nutrients both, deep down where roots can reach them, instead of letting the rain sluice them away. This is the ground of Buderim, Mapleton and the Blackall Range, and it is the most fertile country in the region by a wide margin. It is the plainest story the coast has to tell: where the rock is rich, the soil is rich, and the growth follows.

That fact explains a long chain of others. It is why the basalt uplands carried the original subtropical rainforest, the “big scrub,” the tallest and densest forest the coast ever grew. It is why, once that forest came down, the same red ground carried the dairy herds that replaced it, and why today it grows the avocado and macadamia orchards that in turn replaced them. The good soil has only ever changed tenants, handing its fertility from one land use to the next. What it has not grown since the 1890s is much rainforest, because the rainforest was the one tenant that had to be cleared off before any of the others could take the ground.

The tidy story has a complication — the one worth carrying away. Stand in one of those rainforest survivors, kneel to the floor, and you will find the leaf litter coming apart at speed: fungi and insects rotting each fallen leaf in the warm, wet, windless air far faster than in the dry forest upslope, and the shallow, hungry roots and their fungal partners snatching the freed nutrients back almost before they touch the ground. The basalt soil beneath is genuinely rich; that is why the dairy farmers wanted it. Yet the forest still runs its nourishment on this fast, hand-to-hand cycle overhead, barely banking anything in the soil at all. Most of the wealth is in the living forest itself.

Which is why clearing a basalt rainforest does such lasting harm. Fell the forest and the wealth does not politely stay behind for the paddock. Most of it walks away in the logs; the rest washes off the bared red slope in the first summer storm, running the country’s fertility down to the creeks in a single wet season. The red ground is real, and it is the best on the coast. But the richest thing standing on it was the forest, not the soil. That was the real loss when the range was cleared.