Mining the Midden

Lately I’ve been contemplating the core of the ecological niche for human cultivated crops. Let’s explore one of the key factors that started before the official origins of agriculture.

In the majority of terrestrial habitats plant growth is limited by access to nitrogen or phosphorus. Only in arid zones does water become the limiting factor. In some peculiar regions with unusual geology a deficiency or surplus of other elements may come into play. For example there are natural zones with high heavy metal levels that evolved their own specialist flora. Deficiencies are more common though, such as how agriculture in West Australia was initially crippled by selenium deficiencies.

Local imbalances in soil mineral profiles have long been evened out by the vertical activity of plants (which make the minerals bioavailable) coupled to the horizontal activity of large, migratory animals which would consume the minerals in a place of excess, and deposit them in a place of deficiency. Far from being entirely random, this process would be driven by the animals own sophisticated sense of taste as they sought out balanced nutrition. The terrestrial migration was also supplemented by the movement of fish and seabirds inland, to distribute key elements like iodine which are rapidly lost from the land.

These great mineral cycles were irreversibly broken over much of the planet, tens of thousands of years ago when the majority of the world’s megafauna became extinct (usually coinciding with the arrival of humans, right up to the relatively recent extinction of the moa of New Zealand and elephant birds of Madagascar within a century of settlement).

By contrast, how do humans engage in horizontal mineral cycles? Most hunter-gatherers settled in relatively small home ranges, undergoing an annual migration around their territory to coincide with the availability of prime food resources. This is distinct from the often continent scale migration of megafauna. Along the way humans formed a series of regular camps, to which they carried harvested food, along with large quantities of wood for burning (also containing minerals). The consequence of thousands of years of this pattern is visible in the form of middens- monstrous mounds of shells, bones, excrement and plant material, usually concentrated close to the periodically disturbed ground of human camps.

This concentration of minerals would have represented a prime resource for the evolution of human associated weed species, many of which would eventually form the ancestors of crops (especially vegetables). Adaptable fruit species would also find their seeds deposited in this region, undisturbed until the humans returned.

The wider region around the camp would also be affected, through persistent wood collection at a convenient distance to camp. The creation of deforested habitats may have created niches for non-woody species that tolerated the periodic disturbance by humans, especially in swamps and waterways. Perennial tuberous crops seem especially suited to this niche since they have a fairly wide harvest window (unlike grains which must be harvested as soon as they are ripe). Another early domesticate ideal for this system is the bottle gourd, a vigorous annual vine which produces woody fruit that are invaluable for carrying water.

Agricultural humans intensified this new pattern, further shrinking home ranges. They often spread their crops at the expense of trees until the vertical mineral cycle was significantly damaged. Agriculture seems to be the most stable in the long run in regions where the majority of the landscape is unsuitable for cropping (with only limited pockets of flat, silty soil), leaving the remainder of the hilly landscape to continue to grow trees. Uniformly flat regions tend to experience cyclic population explosions, deforestation, then crashes. Regions of cropping could be viewed as a kind of wound upon the skin of the world, only tolerable in isolated patches (comparable to some animal species that gnaw wounds in the bark of a great tree to regularly feed on the oozing sap). The ancient practice of trading dried sea and mineral salt over vast distances likely had significant impacts on local mineral cycles (many animals are limited from living in some regions permanently due to lack of salt).

Industrial humans have further transformed the planet’s mineral cycles in recent generations. Particularly the application of mineral phosphorus to soils has permanently changed their capacity to support plant life (usually supporting weedy species over the indigenous types that adapted for lower mineral levels). Most of the world’s industrial agriculture is dependent on phosphorus imported over vast distance and global reserves are estimated to face depletion in 80 years at current levels of consumption. In my estimate it is more likely that the economics of processing and transporting this vast quantity of material is more likely to sputter out before then (already farmers in the developing world are cutting back on fertilizer applications due to rising prices).

Phosphorus in particular has a tendency to become insoluble as soon as it is applied to soils. In doing so it binds up trace elements, often exacerbating deficiencies of key micronutrients. Plants that are well adapted to local soil structure and climate usually have little difficulty tapping into this buried treasure (and many weed species are phosphorus accumulators, making them a potentially useful component of a diverse system, provided their biomass is harnessed and the resulting nutrient flows directed at desirable species).

In our own systems we need to consider both vertical and horizontal mineral flows. Moving large amounts of material across the surface ideally requires multiple end uses beyond enrichment of cultivation spaces, otherwise it is difficult to motivate people to perform the function. Livestock is a particularly efficient way to concentrate nutrients close to the point of use. Stands of coppiced trees for livestock feed and firewood also stack multiple end uses and provides mineral rich ash and biochar for soil enrichment. But ultimately there is a limit to how much local mineral balances can be perturbed by human activity without the support of mechanisation. Finding crops and livestock that are already compatible with local soil and climate by pulling out all of the industrial era life-support systems is the only long-term sustainable solution.

Better that we go through the sometimes painful process of turning off the hose, throwing away the bag of fertiliser and letting the weaker crops die while the consequences are not catastrophic.

A modern day midden- a monstrous pile of buffalo skulls, representing a concentrated calcium and phosphorus resource.