The Most Fertile Soil in the World Is Made of Dust Blown Off a Glacier
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Wind-deposited silt covers large parts of several continents in layers tens of metres deep, and it feeds an enormous share of the world while collapsing under anybody who digs into it.
What it is and how it got there
The material is silt, meaning particles finer than sand and coarser than clay, deposited by wind rather than by water. Its usual source is rock ground to powder beneath glaciers and washed out onto broad plains by meltwater, where it dried and was lifted by wind from the ice sheet and carried until the wind slowed, usually where vegetation or hills obstructed it. Deposits accumulate over tens of thousands of years and reach depths of tens and occasionally hundreds of metres, blanketing whatever landscape was there before.
Why it is so fertile
Several properties combine to make exceptional farmland:
- •Particles are uniform in size, which gives excellent drainage
- •And simultaneously good water retention
- •Freshly ground rock is rich in unweathered minerals
- •The material is deep, so roots are never obstructed
- •It is soft enough to work with simple tools
- •And it lies on flat plains rather than on slopes
The strange way it stands
The material has a mechanical property that catches people out repeatedly. Freshly deposited grains are held by weak bonds and by their angular shapes, and the deposit is therefore able to stand in vertical faces many metres high, which sand cannot do. Cut into it and the face stays put. That property made it possible to excavate dwellings directly into it, and millions of people in northern China have lived in such dwellings. The same property makes it lethal, since a vertical face that has been wetted or shaken loses its cohesion entirely and collapses without warning.
What it records about the past
Because the deposits build up steadily and bury whatever was on the surface, they preserve a long climate record readable from a single vertical section. Layers deposited during cold dry periods are thicker and coarser, since more dust was available and winds were stronger, while warmer wetter intervals produced thin layers and soils formed on top of them. Alternating bands of dust and buried soil therefore record successive glacial and interglacial cycles. Sections in northern China cover more than two million years and are among the longest continuous land-based climate records anywhere.
Where it matters most
The deposits concentrate in a few regions and their significance is hard to overstate. The largest is in northern China, where a plateau covering several hundred thousand square kilometres is buried under it to depths reaching two hundred metres, and the river draining it carries so much of it that the water is opaque and the river is named for the colour. The American Midwest, the Ukrainian and Russian black earth belt, parts of central Europe and the Argentine pampas all sit on it. Those regions together produce a substantial fraction of the world's grain.
The takeaway
Silt ground by glaciers, washed onto plains and lifted by wind builds deposits tens of metres deep, with uniform particles that drain and retain water well and unweathered minerals that make exceptional farmland. It stands in vertical faces, which allowed cave dwellings in northern China, and collapses without warning when wetted or shaken. The world's main grain regions sit on it.