The slow work of soil
Of all the resources on which human societies depend, few are as widely overlooked as the thin layer of soil that covers much of the land surface. Formed over centuries by the gradual breakdown of rock and the accumulation of decaying plant and animal matter, topsoil supports almost all of the crops that feed the world's population, yet it is routinely treated as little more than dirt. The assumption, rarely stated but widely shared, that soil will simply be there for the next harvest has shaped farming practices for generations.
Only in recent decades have scientists begun to appreciate the extent to which soil is a living system rather than an inert medium. A single handful of healthy topsoil contains billions of microorganisms, many of which have never been formally described. These organisms, interacting with plant roots in ways that are still poorly understood, break down organic matter, release nutrients and bind mineral particles into the crumbly structure that allows water and air to move through the ground. Earthworms play their part too, dragging dead leaves below the surface and opening channels through which rain can drain. It is this structure, rather than the minerals themselves, that largely determines how well a soil can hold water during a drought.
The pressures on this system are considerable. Repeated ploughing, by exposing soil to wind and rain, accelerates erosion; heavy machinery compacts the ground, reducing the space available for water and roots; and the removal of crop residues after harvest deprives soil organisms of the material on which they feed. Estimates of the rate at which soil is being lost vary widely, depending on the region and the methods used, but there is broad agreement that in many intensively farmed areas it is being eroded considerably faster than it is being formed. Since the formation of a few centimetres of topsoil can take centuries, losses of this kind are, on any human timescale, effectively permanent.
Not all the news is discouraging, however. Farmers who have adopted practices designed to protect soil structure, such as reducing ploughing, keeping the ground covered with plants between harvests and rotating a wider range of crops, have in many cases reported better water retention and, after an initial period of adjustment, yields comparable to those achieved by conventional methods. Such results, though encouraging, are not universal: what works on the clay soils of one region may fail on the sandy soils of another, and the transition can be costly for farmers who cannot afford a temporary fall in income. Some governments now pay farmers to adopt these practices, treating healthy soil as a public good rather than a purely private concern.
What emerges from this research is a picture of soil not as a fixed asset but as something closer to a bank account, from which withdrawals can be made only for as long as deposits continue. A field that has lost much of its topsoil may go on producing harvests for years, sustained by fertilisers, but it does so on borrowed time. Had this been understood earlier, much of the damage done to farmland over the last century might have been avoided. Whether it is understood widely enough now, and whether the costs of protecting soil will be shared fairly between farmers and the societies that depend on them, remains to be seen.