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Name: Mary
Status: Educator
Age: K-3
Location: Texas
Country: United States
Date: January 1, 2005


Question:
How long does it take to make top soil, and how deep is the layer of topsoil on the earth?



Replies:
Let's start with soil depth. Where I'm from (Northern Illinois), bedrock is located about 700 feet beneath the surface. However, topsoil (the zone containing organic material) occupies just the upper three feet or less. Beneath that lies hard clay, then hundreds of feet of glacially deposited, unconsolidated material called till. This is a mixture of clay, sand, pebbles, gravel, boulders, ect. But just a few tens of miles away, bedrock can be found at or near the surface. It depends on the specific physical process that took place at that location. In the Upper Midwest, glaciation caused either scouring or deposition (or both) by the movement of the ice and the drainage pattern of its meltwater. But topsoil itself, by definition, ends just a few feet down at most. This is generally true everywhere soils exist because roots, fungi and organisms have limits on how deep they can or need to go due partially to the oxygen content in the ground.

How long soils take to form depends on the type of bedrock, climate, and vegetation. Let's suppose a large, flat, level area of bare, limestone rock was exposed today in a humid, temperate climate (such as the bottom of an abandoned quarry that is allowed to drain and not fill with water). Plant seedlings would sprout in cracks and crevices during the first warm season. Small amounts of eroded rock debris and wind-blown sediment would accumulate in these cracks and crevices and in other low spots. This would enable larger plants to take hold. Chemical weathering by dissolved CO2 and other compounds in rainwater, along with mechanical weathering by frost action, rain and wind would slowly disintegrate the rock surface. As vegetation increases, so would the amount of organic material to this loose sediment. After about 150,000 years, there may likely be about a foot of organic soil present, assuming no mass erosion or deposition took place (which shouldn't on flat, level ground in that amount of time). Keep in mind that soils build downward; they do not accumulate upward. The soils of the Midwest formed in less than 10,000 years because glaciers left a foundation of loose material as opposed to solid rock, enabling vegetation to proceed rapidly.

Tom Esposito



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