Time

Soil Profile Evolution

Soil profiles develop over time, and their properties depend on how long the soil-forming processes have been operating.

In this example, you will see how a profile might form in a humid, leaching environment following a landslide, which deposits a mixture of soil materials on a bare rock substrate.

The sequence of animations illustrates the pedogenic evolution of a mixture of clay, sand, and gravel, which has been deposited on a bare rock surface by a mudslide and then subjected to a humid, leaching environment.

For simplicity, the scenario shows soil profile development from a single deposit of transported parent material. In reality, most soils have a more complex genesis. They may develop in situ (in place) from weathered rock and/or may be influenced by a number of deposition and erosion cycles.

Leaching of Soluble Salts:

Soluble salts will rapidly leach (months to years) as water percolates through the profile. Sparingly soluble salts like calcium carbonates may dissolve and leach over decades.

Organic Matter Accumulation:

Within a few years, plants colonize the deposit, soil microbial activity increases, and humus starts to accumulate in the soil. Eventually, an equilibrium will be reached where Soil Organic Matter (SOM) formation and decomposition rates balance. This might take from centuries to millennia.

Separation of Sand and Clay:

Illuviation and other processes will tend to differentiate the profile into a sandy A horizon and a clay-rich B horizon. This might occur over tens of thousands of years.

Weathering of Rock:

Rock weathering to produce new soil material is relatively slow. For example, in a high P/E environment (like Boss Yellow Earth in the Australian Soils module), it has been estimated that it would take 1000 years of weathering to form 15 mm of new soil material.