Why Is Some of the Best Farmland Made of Dust? Wind-Blown Silt, Metres Deep
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Deposits of fine wind-blown silt cover large parts of several continents, in places hundreds of metres thick, and they carry some of the most productive soils on Earth. They also collapse spectacularly when wet.
Where the material came from
The deposits are made of silt, a grain size between sand and clay, produced overwhelmingly by glacial grinding during the ice ages. Glaciers crushed bedrock into rock flour, meltwater carried it out and spread it across braided outwash plains, and those plains dried between floods leaving vast expanses of bare fine sediment. Cold dry winds blowing off the ice sheets picked that material up and carried it, sometimes for hundreds of kilometres, depositing it where vegetation or terrain slowed the wind. Desert margins produced comparable material by a different route. The process operated repeatedly through the glacial cycles, building deposits that in parts of China exceed two hundred metres in thickness.
Why the soils are so good
Several properties combine to make these among the most productive agricultural soils:
- •A uniform silt grain size that holds water well and drains adequately
- •High mineral nutrient content, since the material is freshly ground rock
- •Easy root penetration and easy cultivation
- •Great depth, so roots and water are not limited by underlying rock
- •A crumbly structure that resists compaction
- •Flat or gently rolling terrain, since the deposits mantle the landscape evenly
Where these regions are
The distribution follows the former ice margins and the desert belts closely. The Chinese plateau of this material is the largest and thickest anywhere, supporting an enormous agricultural population and giving the Yellow River both its colour and its name, since the river carries the silt in vast quantities. The American Midwest carries deposits of tens of metres that underlie the corn belt directly. Central and eastern Europe, Ukraine and southern Russia hold extensive areas that are among the most productive farmland in the world. Argentina, New Zealand and central Asia hold further deposits. The correlation between these regions and historically dense agricultural settlement is direct rather than coincidental.
What the layers record
These deposits are an archive of past climate, and reading them has become a substantial field. Dust accumulated fastest during cold dry glacial periods when the source plains were bare and the winds strong, and slowest during warm wet periods when vegetation covered the ground and soils formed on the surface instead. The result is an alternating sequence of dust layers and buried soils, each pair recording one glacial cycle, and the Chinese sequence contains more than thirty such cycles extending back over two million years. Grain size within a layer indicates wind strength, since stronger winds carry coarser particles further. Magnetic properties of the buried soils track rainfall. The record is continuous on land in a way that few others are, which makes it complementary to ice cores and deep sea sediments.
The problems they cause
The same properties that make the material good to farm make it dangerous in other respects. It erodes extremely easily by both wind and water, since the particles are fine and weakly held, which produced the American dust storms of the 1930s after the protective grassland was ploughed and which causes severe gully erosion on the Chinese plateau. It can hold a vertical face when dry, which is why these regions are full of steep-sided gullies and why cave dwellings have been cut into them for millennia, and it loses that strength abruptly when saturated. That collapse under wetting causes ground subsidence beneath buildings and roads, and it contributes to the catastrophic landslides that earthquakes trigger in these regions, including one in 1920 that killed enormous numbers of people.
The takeaway
Glaciers ground bedrock into silt, meltwater spread it and cold winds carried it away and dropped it, building deposits up to hundreds of metres thick. The uniform grain size, high nutrient content, depth and easy cultivation make these among the most productive soils anywhere, which is why the regions coincide with dense agriculture. The same material erodes catastrophically and collapses when saturated.