“To forget how to tend the soil is to forget ourselves.”
– Mahatma Gandhi
Human activities have a direct impact on the earth’s surface and the thin veneer of soil that sustains us. Scientific studies on agricultural erosion, logging and deforestation, and urban development have quantified human impacts across a wide range of geographic settings, confirming that human activities accelerate denudation rates and sediment delivery to drainage channels, which in turn leads to increased landslide and flood risks. Yet relatively little research has been conducted on the other half of the soil sustainability equation: soil production. We have seen the sensational results of erosion: the Dust Bowl of the 1930s, dust plumes spreading east from China, and the brown waters of the Amazon reaching far into the Atlantic Ocean. What we don’t know is whether this lost soil is replenished through soil production, or by what mechanisms and at what rates that might occur.
The simple truism underlying this relationship is that if erosion exceeds soil production, soil is stripped away and the landscape is laid bare. In order to predict what will happen on a landscape due to land use practices or climate changes, we must be able to quantify rates and processes of both erosion and soil production. If we could develop predictive capability for natural systems as well as for human impacts on the earth’s surface, we could calculate the potential for change of the earth’s surface and could hope to address the question of whether our soils can support us. Research quantifying these rates and processes is essential to human livelihood and global sustainability.
If both soil production and erosion rates were known for every point on Earth, we could immediately determine whether our soil resources can support us, assuming no changes to either rate. Natural processes of soil production from parent material include physical weathering such as freeze-thaw cycles and burrowing by animals and insects, and chemical weathering of the rock due to rainfall or hydrothermal fluids. Across a hilly, upland landscape, soil is continually produced by mechanical and chemical processes and is constantly being removed by erosional processes. In steady state this means that a finite soil mantle is in constant flux: Soil comes into the system from below and moves out of the system downslope. Our detailed knowledge of soil production is focused on upland landscapes, typically termed “marginal lands” due to their limited potential for supporting agriculture. In contrast, much of the world’s farmland is on soil deposited by rivers or other processes that bring the soil in from remote sources. These depositional areas received sediment delivered by flooding rivers for millennia before humans began farming them. Erosion often exceeds input of fresh material in these depositional areas, especially as dams and levees limit flooding. Where this is the case, the soil is essentially being mined. There is no soil production from beneath such deposits of farmland, and this mining leads to the progressive depletion of the soil. As an extreme example, in the farmland of rural Belgium, where centuries of farming have strip-mined the soil, some farmers have resorted to using bulldozers to rip up the rock to produce soil artificially.
For as long as humans have been farming, we have been battling erosion. The terraces visible in the first photograph are examples of one of the oldest and most widely used strategies to keep soil on hillslopes. More modern examples include contour plowing, no-till farming, and planting fallow fields with green manure crops to reduce wind erosion.
To address the question of whether our soil resources can support us, scientists must be able to extrapolate from their well-studied field sites to areas that are relevant on a policy scale. This is no simple task, and it will likely take years of coordinated efforts between scientists and policy makers. Nonetheless, there are several clear steps that we can take to integrate efforts across disciplines.
First, we must compile a comprehensive inventory of soil production rates and processes. This includes the difficult task of quantifying soil production rates for most of the prime agricultural lands that are on fluvial sediments. Second, we must compile an interdisciplinary inventory of erosion rates, which includes both natural and anthropogenic processes. This will be a considerable undertaking: Erosion rates have been measured extensively, using many different techniques, with varying degrees of accuracy. Third, we must compare the soil production and soil erosion inventories by region or land unit, where such comparisons are possible. This will enable the scientific community to draw local conclusions, informed by exact local conditions. Fourth, we must extend this mass balance approach to ever larger spatial scales until we determine the limits of the data that are available. Lastly, given the limits of the available data, we will be able to make simplifying assumptions that extend the analyses to the largest possible scales: continental and, finally, global. These last three steps will inform us of the expected lifespan of our soils at the global level.
Agriculture forms the backbone of our global economy, and soil is as critical as water and air for our survival, yet few people recognize its fragility in the face of human development. It is vital that we understand the sustainability of our soil resources. An ideal approach to this question will focus quantitatively on how the balance between soil production and erosion will determine the lifetime of our fertile soils. Understanding this balance, and understanding the projected lifetime of our soil resources, will arm our policy makers and natural resource managers with crucial information regarding the viability of local and global land use and populations.