How is the age of soil figured?

Question: How is the age of soil figured?

Answer: Soil scientists agree that soils form over time as the climate (temperature, rain, wind, etc.), topography (shape of the landscape), parent material (weathered rocks or stuff deposited by wind, water, ice, or gravity), and living organisms (plants and animals that live in the soil) interact.

juniper-in-soil
To determine the age of a soil, scientists look for clues. For example, a soil can’t be younger than the oldest tree growing in it. Photo of tree growing in Arches National Park, Utah, USA, by Kim Seng.

So, when soil scientists want to know how old a soil is, they look for clues.

Though the climate affects how fast weathering and erosion occur, it usually does not give many clues about how long a soil has been in a given spot. The landscape, parent materials, and trees provide better clues.

A soil can’t be younger than the oldest trees growing in it. It can’t be older than the materials in which it forms or the landscape on which it is found. Soil scientists work with geologists to determine how old the landscape is, and how long the parent materials have been there. Since most deposition of parent materials occurred before written history began, geologists make educated guesses, estimating the age of the landscape (and materials in it) relative to periods of known glaciation, volcanic activity, floods, and similar events.

Soil scientists also know that soils tend to form more rapidly on certain positions on the landscape than on others. More soil formation occurs on flat landscapes in upland positions, for example, than on slopes. Erosion on the slopes limits the rate of soil formation. Soil formation in lowland positions may be slowed by deposition of new materials on the surface by floods or gravity.

–Answered by Clay Robinson, a.k.a., Dr. Dirt

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5 thoughts on “How is the age of soil figured?

  1. I do not entirely agree with the answer. Considering soil consists of more living organisms than minerals, the age and the soil itself would be ever changing. Also soils can become rocks ( clay to shale) making the unconsolidated material older.

  2. Soil’s age as measured using radiocarbon (which poses its own significant problems in its use as a soil clock) would fluctuate in response to changes in the organism factor through time. That is true. However, we consider the organism factor a dependent variable – especially in older soils. Consider ecosystem retrogression, for instance. It happens in places like the Amazon basin and old soils in Hawaii where vegetation relies on aeolian inputs of phosphorus because the soil there is depleted. That is, as the soil itself develops, the organism factor develops and thus, the rate of photosynthates’ flux into soil changes. So, the age you are referring to as a soil’s age – as determined from radiocarbon methods for assessing organisms’ activities in soil – isn’t exactly correct. You see, these P depleted soils once weren’t P depleted and the organism factor has a lot to do with that (perhaps, everything to do with that since ~430 million years ago when vegetation colonized land). When the soils weren’t depleted in P is when they were ‘young’ soils and, now that they are ‘old soils, they are depleted. Biota played a role in the depletion but time remains an independent factor – it marches on irrespective of what plant communities subside on a soil. This is why the first argument of your statement is incorrect: soil continuously increases in age with time irrespective of what biological (or geological) processes are acting. Regarding your second statement, yes, soils (maybe sediments would be a better word here) do form sedimentary rock. However, the nature of formation of this rock requires pressure, i.e., forming greywackes, and, in the case of meta-sedimentary rock like schists, heat (perhaps present because of the immense pressure). In either case, the soil is no longer at the Earth’s surface and, as such, is no longer forming or developing as soil’s by definition do. Therefore, it is no longer a soil. Kind of like how a mountain will erode to form sand-sized particles which eventually become beaches in Florida. These beaches in Florida aren’t mountains anymore just as sediments that are being formed into sedimentary rock are no longer soil. Another mistake in your second argument is that these sedimentary rocks are consolidated – they’re rocks. So, yes, the soil that became a rock is old but it is not soil anymore – like the beaches in Florida aren’t mountains anymore. Now, the once-was-soil-but-now-is-rock material serves as the parent for soil forming from it (so long as the material is at the Earth’s surface where soils form)

  3. A bit brief imho (certifiable soil dork here) and your last paragraph is not accurate. Soil forms at much higher rates on steep slopes, i.e., > 30 degrees, than in flat positions (as was demonstrated at least 2 years prior to this posting) because of the dependence of soil production rates on soil depth, i.e., the hump-shaped function. The soil doesn’t stick around for as long in steep settings (at least theory holds and observations have, so far, backed up) so there isn’t as much soil observably present, however, the total amount of soil formed – over the same time period as determined from soil production rates – can still be greater in mass.

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