Concept · Ch 4
Basalt fertility (why red ground grows rainforest)
Basalt is young, nutrient-rich rock, and it weathers to a deep red clay — a Ferrosol, the old 'krasnozem' — that holds water and nutrients better than any other soil on the coast, which is why the richest forest grows on it.
First, meet: The nitrogen–phosphorus asymmetry
Deep red earth on a basalt range tells you the richest soil going — rainforest and orchard country. It is red because it is rusty: the iron in the old lava, oxidised as the rock rots down. And it is fertile because basalt is young, mineral-rich rock, and weathers to a deep, well-structured clay that holds on to water and nutrients where sand would let both drain straight through.
Set it against the poor sand and the logic is plain. Phosphorus, the nutrient that runs everything and can never be replaced from the air, comes only out of rock. Fresh basalt is still full of it; ancient leached sand has lost it to the sea. So the basalt uplands grow the tallest, densest forest on the coast — and, once that forest was felled, the dairy farms and the orchards that came after. Rich rock, rich soil, rich growth.
There is one twist worth keeping. A rainforest on this rich ground still holds most of its wealth overhead, in the living forest, recycling each fallen leaf almost before it lands rather than banking it in the soil. So the red ground is genuinely fertile, but it is the forest, not the dirt, that is rich — which is exactly why clearing it does such lasting harm.
In depth
Basalt fertility is the "rich" pole of the region's soil story, and it makes most sense read against the N–P asymmetry that dooms the sand. Phosphorus, the nutrient that cannot be replaced from the air, comes only from rock — so a soil is fertile for exactly as long as it still holds unweathered mineral phosphorus, and poor once that phosphorus has leached out to sea for good. Basalt is a young, mineral-rich volcanic rock, and where it rots down it releases iron, phosphorus and other nutrients into the developing soil.
It also weathers to the right kind of soil. Basalt breaks down to a deep, well-structured red clay — a Ferrosol, which an older generation of soil scientists called a krasnozem. It is red for the same reason rust is red: oxidised iron. Clay clings to water and nutrients with a miser's grip, where sand lets both run straight through as through a sieve, so the basalt soils are deep, moist and nutrient-holding all at once. On the Sunshine Coast this is the ground of the Blackall Range, Buderim and Mapleton, and it is the most fertile country in the region by a wide margin.
That single fact explains a chain of things: why the basalt uplands carried the original subtropical rainforest; why, once that was felled, they carried the dairy farms and now the avocado and macadamia orchards; and why the "big scrub" was doomed by the very thing that made it grand — it grew on the best soil, which was worth clearing. The honest caveat: a rainforest on basalt still runs its nutrients through the living forest on a tight, fast overhead cycle rather than banking them in the soil, so "fertile ground" and "wealth held in the biomass, not the soil" are both true — and it is why clearing a basalt rainforest loses the wealth to the logs and the eroding slope rather than leaving it behind for the paddock.
Primary sources & further reading — the doorway
- Willmott, W. (2007). Rocks and Landscapes of the Sunshine Coast (2nd ed.). Geological Society of Australia. (Local basement = Amamoor & Booloumba Beds; North Arm Volcanics; Landsborough Sandstone. NB: Neranleigh–Fernvale Beds are the Gold Coast/Brisbane equivalent, not the Sunshine Coast basement.) — Rocks and Landscapes of the Sunshine Coast — basalt geology to red Ferrosol/krasnozem.
- Walker, T.W. & Syers, J.K. (1976). The fate of phosphorus during pedogenesis. Geoderma 15: 1–19. — Why fresh, mineral-rich rock means a fertile soil (and why old, leached ground does not).