What Is Soil Erosion? Losing Something That Takes Centuries to Make
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Soil forms at a rate measured in fractions of a millimetre per year, as rock weathers and organic matter accumulates. Under cultivation it can be lost at rates orders of magnitude faster, carried off by water and wind, which makes it a resource that is renewable in principle and non-renewable on any timescale that matters to the people farming it.
How soil is lost
Erosion proceeds by identifiable mechanisms, and which one dominates depends on the soil, the slope and the weather:
- •Splash erosion, where raindrops hitting bare soil break aggregates apart and throw particles into the air, which is the first stage and the one that seals the surface
- •Sheet erosion, a thin layer removed uniformly across a slope, which is the most insidious because it is almost invisible until the subsoil starts showing
- •Rill erosion, where flow concentrates into small channels that can still be ploughed out
- •Gully erosion, where those channels deepen beyond the reach of cultivation and permanently divide a field
- •Wind erosion, which removes the finest and most fertile fraction and requires dry, bare, loose soil and open ground
- •Tillage erosion, a mechanism frequently overlooked, in which repeated ploughing moves soil progressively downslope by gravity regardless of any rainfall
Why it matters more than the volume suggests
The material lost is not average soil. Erosion preferentially removes the surface layer, which holds the organic matter, the nutrients and the biological activity, and it preferentially carries the finest particles, which hold most of the nutrient exchange capacity. A field that has lost a centimetre of topsoil has lost considerably more than a hundredth of its productivity. The losses compound, since removing organic matter weakens the aggregates that resist erosion, which increases the rate of further loss. And soil formation is slow: rates vary enormously with climate and parent material and are generally in the range of a fraction of a millimetre per year, while erosion rates under conventional tillage on sloping ground are frequently ten to a hundred times that. Global assessments have estimated that a substantial share of agricultural land is degraded to some degree, and while the headline figures are contested in detail, the direction is not.
Where it has already happened
The historical record contains several unambiguous cases. The American Dust Bowl of the 1930s followed the ploughing of grassland whose deep-rooted native vegetation had held the soil, and when drought arrived the exposed soil blew, with dust storms reaching the eastern seaboard and an agricultural population displaced on a large scale, which produced the first national soil conservation programme. The Mediterranean basin has been eroded over millennia of clearance and grazing, with soils on many hillsides thin or absent and a corresponding accumulation of sediment in valley floors and harbours. The Loess Plateau in China suffered some of the highest erosion rates ever recorded, colouring the Yellow River and contributing to its flooding, and a large-scale restoration programme involving terracing, tree planting and grazing restrictions has since produced measurable reductions. Madagascar, Haiti and parts of the Sahel present current cases where erosion, deforestation and poverty reinforce each other.
What reduces it
The measures that work all address the same principle of keeping soil covered and slowing water. Maintaining living cover through cover crops between cash crops is among the most effective single interventions, since bare soil is the precondition for almost all erosion. Reduced and zero tillage leaves residue on the surface and avoids breaking aggregates. Contour ploughing and strip cropping slow downslope flow. Terracing converts a slope into steps and is the most effective and most expensive option, representing an enormous historical labour investment in many regions. Windbreaks and shelter belts reduce wind speed at the surface. Grassed waterways carry concentrated flow without scouring. Buffer strips along watercourses trap sediment before it reaches them, which also addresses the downstream problem, since eroded soil carries phosphorus and pesticides into rivers and is a leading cause of water pollution from agriculture. The obstacle is rarely knowledge and usually economics, since the cost falls on the farmer now and much of the benefit accrues later or to someone else.
The takeaway
Soil forms at a fraction of a millimetre per year and is lost under cultivation at rates ten to a hundred times faster, through raindrop splash, sheet and rill flow, wind and the gravitational movement of repeated ploughing. What is removed is the organic-rich surface and the finest particles, so losing a centimetre costs far more than a hundredth of productivity. The Dust Bowl and the Loess Plateau are the clearest cases, and keeping soil covered is the single most effective remedy.