4.4 - Soil Horizon Development Over Time

Because of the diversity of landscapes and controlling variables of soil formation – such as glaciations, flooding, erosion, and tectonics – soil horizons are often found in multiple combinations of vertical arrangement. To simplify the concept, consider a landscape where a river has deposited several meters of silty, gray, limestone-rich sediments (alluvium). At this stage, the profile would be C; since no soil forming processes have occurred yet, it is not yet a soil. The annual precipitation at this location is 75 to 100 cm (30 to 40 inches). The climate is temperate with warm summers and cool winters. Over geologic time these sediments develop a soil profile:

0 – 50 years: Plants (grasses and forbs) begin to establish on the surface. Plant growth is good, roots proliferate below ground and senesced plant material falls to the surface in the fall. Worms, ants, beetles, and other organisms break these leaves into smaller pieces and mix them in the upper layers of the soil. A thin, dark A horizon begins to form (addition) over the existing C horizon. Profile horizons are A-C.

50 – 500 years: The plants continue to grow, and the A horizon becomes thicker (addition). The silty sediments contain calcium carbonate, CaCO₃, which is easily dissolved in the temperate, humid environment. The CaCO₃ begins to leach out of the upper part of the soil and move deeperas rain and snowmelt move down through the soil (translocation). Some of the CaCO₃ moves all the way through the soil profile and into the groundwater (loss). Some of the iron in the sediments begins to oxidize (rust) and the soil starts to turn from gray to yellow. This is the formation of a weak B horizon. Profile horizons are A-B-C.

500 – 5,000 years: The A horizon has stabilized and is fairly thick and dark for this soil. High productivity and lots of biotic activity continually recycle the organic matter. The original minerals in the soil begin to weather and form new clay minerals. This clay is moved by water flowing downward through the soil from the A horizon and accumulates in the B horizon (translocation). New clay minerals precipitate from minerals dissolved in the soil water thickening the B horizon and increasing its clay content (transformation). The depth to the C horizon increases as the combined thickness of the A and B horizons increases. Profile horizons are A-B-C.

5,000 – 50,000: The above sequence of events continues. Most of the CaCO₃ is leached out of the soil or is very deep in the C horizon (loss). The continued accumulation of clay thickens and makes the B horizon more dense (transformation, translocation). Viewing the C horizon requires a lot of digging.

50,000 – 500,000: The A horizon has been continually mixed by the plants and animals, possibly even a plow. But it is still thick and dark and weathering. Below where the organic matter makes the soil dark, iron, clay and other soil pigments have been stripped away; a lighter, coarser textured E horizon may become evident. The clay and iron from the A and E horizons have moved downward and accumulated in a very thick, very dense B horizon. To reach the C horizon, a backhoe is needed. Profile horizons are A-E-B-C.

Then, one day, a very large sheet of ice, a glacier, comes out of the cold north, and scrapes the weathered glacial till away, creating a blank slate for a new soil to form when things warm up again.

The following questions build on the ideas of soil profile development in this lesson and also the soil forming factors of Lesson 3.

Quiz

Question

A soil is developing in the Southeastern United States, on a geologically old, forested landscape, the temperatures are warm, and precipitation plentiful but the soil is well drained. Which of the following soil profile arrangements will best fit this scenario?

A. O, A, C

Incorrect: An A over C implies very little weathering or soil development. The scenario given implies that much more development (clay formation and translocation) would be expected.

B. O, A, B, C

Correct: In this scenario, the older landscape with warm, moist conditions will encourage weathering of the soil minerals which will result in the formation and translocation of clays within the soil profile, resulting in a B horizon. The vegetation almost assuredly will provide an A horizon, and somewhere deep within the profile there will be parent material, the C horizon.

C. A, R

Incorrect: An A over R implies very shallow soils, unlikely in a old geology that has optimum conditions (climate) for soil development.

D. O, R

Incorrect: An O over R implies little development in a well drained system. The scenario described suggests optimum conditions (time and climate) for soil development.

Looks Good! Correct: In this scenario, the older landscape with warm, moist conditions will encourage weathering of the soil minerals which will result in the formation and translocation of clays within the soil profile, resulting in a B horizon. The vegetation almost assuredly will provide an A horizon, and somewhere deep within the profile there will be parent material, the C horizon.

Question

This soil is developing in an active floodplain in central China. The silty alluvial soils are somewhat poorly drained and the climate is on the cool side. The vegetative cover is grasses and other small forbs. Which of the following is the most likely horizon scenario?

A. O, R

Incorrect: In a grassy environment, organic horizons are rare. Also, the fact that this is a floodplain with silty, alluvial soils implies that there would at least be one mineral horizon over a bedrock layer.

B. A, B, C

Incorrect: Floodplains are constantly being replenished with silt when they flood. And, active floodplains are young geologically speaking so there will be less of a chance for soil development and therefore a B horizon is less likely. However, take comfort in knowing that, if the flooding stopped on this floodplain, this horizon scenario would be the next best choice.

C. A, E, B, C, R

Incorrect: Several horizons in this selection imply good drainage, and extensive soil development. The scenario says these are somewhat poorly drained soils in a cool and young landscape. These conditions do not encourage much soil development.

D. A, C

Correct: The system described means that the soil is constantly being renewed by the silts that are deposited when the floodplain is flooded. This addition of material constantly resets the soil development clock. And, since this floodplain is active, it would be geologically young, so there has not been much opportunity for mineral weathering. The climate (cool, moist) also is not conducive for soil development in a short timeframe.

Looks Good! Correct: The system described means that the soil is constantly being renewed by the silts that are deposited when the floodplain is flooded. This addition of material constantly resets the soil development clock. And, since this floodplain is active, it would be geologically young, so there has not been much opportunity for mineral weathering. The climate (cool, moist) also is not conducive for soil development in a short timeframe.