The Soil Under Rooibos Cannot Tell It Is Farmed

A rooibos field in the Cederberg does not have much of an edge to it. On one side, the parallel rows of a planted crop, kept clean. On the other, sometimes within a few paces, the wild fynbos veld that has never been ploughed: the same sand, the same sun, the same thin winter rain arriving at the same time for both. It is one of the more companionable field boundaries in agriculture. It also raises a question a person might reasonably turn over during an evening cup. Does the ground itself know which side it is on?

The industry has taken real trouble over that neighbouring veld, putting more Cederberg land under formal conservation agreement than it actually cultivates, a story with a file of its own. And the case for that effort is usually made in a single breath: the wild land shelters the wildlife, and the wild land keeps the farm's soil healthy. The Ministry has always been fond of that sentence. As it happens, only the first half of it had ever been tested. The second half went and got tested, and did not come back the way anyone expected.

Somebody went out with sample bags

Anneke Postma's 2016 doctoral thesis, at Stellenbosch University's Department of Microbiology, compared the soil microbial communities of two commercially grown fynbos plants, honeybush and rooibos, at wild natural sites and at cultivated commercial ones¹. The rooibos bacterial half of that work was later published, with colleagues, in the journal Pedosphere² in 2020.

The method was not a matter of squinting at the dirt. The team sampled two kinds of soil: the rhizosphere, meaning the thin sleeve of earth that clings to a root when you lift it, and the bulk soil away from the roots. They read the bacterial DNA out of both directly, by high-throughput sequencing, rather than trying to grow anything in a dish. Then they set the results against the soil's own chemistry: its pH, its carbon content, its electrical resistance, its sodium, potassium, nitrate and ammonia. They sampled across seasons, so a summer answer would not be mistaken for a permanent one.

Here is what came back. No statistically significant difference was detected between the bacterial communities of the natural sites and those of the commercial ones. The fungal communities associated with natural and cultivated Aspalathus linearis did not differ significantly either. Whatever a plough, a planting row, and decades of commercial harvest do to a hillside in the Cederberg, they do not appear to redraw the roster of microbes living under a rooibos bush.

Two details are worth holding onto here, because they turn a plain null result into something more useful. The first is that season moved the picture when farming did not: the bacterial communities shifted noticeably from one time of year to another, so the study was plainly capable of detecting change. It just did not find the change it was looking for. The second is honeybush, sampled by the same researcher, in the same work, by the same methods. Its bacteria were likewise unbothered by cultivation, but its fungal communities did appear to differ between wild and commercial sites. Rooibos shrugged off farming in a way its fynbos cousin did not.

Why a legume might carry its own weather

There is a plausible reason, and it lives underground in the plant's own roots. Rooibos is a legume, and it runs an unusually particular arrangement with nitrogen-fixing bacteria housed in nodules on its roots, an arrangement so demanding that the plant alters the acidity of the soil immediately around itself to keep its partners alive (the red bush changes its own soil, then changes it back). Postma's conclusion pointed the same way: the plant's own selection of its microbial company looked strong, and was not significantly influenced by agricultural activity. A bush that particular about who it works with, and that willing to renovate the ground to keep them, may simply bring most of its own below-ground establishment with it. What is growing over the fence matters less to that establishment than the tidy version of the sustainability story implies.

None of which makes the conservation land pointless. It shelters wildlife, it holds the wild ancestors of the crop itself, and it cushions slopes against erosion and against a climate that is not being generous to the Cederberg. It also does one job for the farm directly and demonstrably: an unploughed strip left at the field edge keeps a pest's natural enemies living nearby rather than displaced, which is why growers leave it.

What the study narrows is something more specific. Do not assume that wild veld next door is quietly also maintaining the cultivated soil's own biology, because the one team that went and measured it did not find that happening. Land conservation and soil microbiology have been arguing on the same side for so long that they are easy to mistake for one argument. They are two, and only one of them has been checked.

Put the kettle on and consider the modesty of the finding. Years of a doctorate went into establishing that the ground beneath a rooibos field is, bacterially speaking, much the same ground it was before anyone put the bush in rows, and that the thing which moves it most is not the plough but the turn of the season. The red bush was growing in that sand long before the sand was farmed. So far as anyone has measured, the sand has not noticed.