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 deeper as 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.