Which Soil Horizons Form First and How They Develop Over Time?

The very first stage of soil formation begins with the parent material (rock or sediment). The development of horizons follows a general sequence over time.

1. The C Horizon (Parent Material): This is the starting point. It is the unweathered or slightly weathered geologic material (rock, sand, glacial till) from which the soil will form. It is not considered "true soil."
2. Formation of the A Horizon (Topsoil): This is the first "true soil" horizon to develop.
   • Pioneer organisms like lichens and mosses colonize the C horizon.
   • They physically and chemically break it down, and when they die, they add the first small amounts of organic matter.
   • This mixture of weathered mineral material (C horizon) and accumulated organic matter creates the A horizon. This is also called the "zone of additions."
3. Development of the B Horizon (Subsoil): As the A horizon thickens and time passes, the four soil-forming processes become more pronounced.
   • Water moving through the A horizon begins to dissolve materials and wash fine clay particles downward (translocation).
   • The B horizon forms below the A horizon as the "zone of accumulation." This is where the leached clays, iron oxides, and other materials (illuviation) build up.
   • This B horizon is often denser, finer-textured, and differently colored (often redder from iron) than the horizons above or below it.
4. Development of the E Horizon (Eluviated Horizon): In older, well-developed soils (especially in forests with high rainfall), a distinct horizon may form between A and B.
   • This is the E horizon, a "zone of loss" (eluviation).
   • Intense leaching has stripped this layer of its clay, iron, and organic matter, leaving behind only the most resistant minerals, like light-colored sand and silt (often appearing bleached).
5. The O Horizon: In forested areas, an O horizon (purely organic matter like leaves, needles, and moss) forms on the very surface, above the A horizon.

A "young" soil may only have an A horizon over a C horizon. A "mature" soil will have a well-developed A, B, and C profile. An "old" soil will show distinct O, A, E, B, and C horizons.