Like all living things, soils age. Exposure to wind, rain, sun, and fluctuating temperatures combine to push soils through four stages of development: parent material, immature soil, mature soil, and old-age soil.
As noted above, parent materials are loose materials weathered from rocks. As plants establish themselves in parent material, organics accumulate, and the upper soil layer becomes richer and darker, and evolves into an A horizon. At this point, the soil has only A and C horizons and is in the immature stage, which it usually reaches in less than 100 years.
Through continued weathering and plant growth, the soil gathers more nutrients, and can support more demanding species. Soils break down into smaller particles such as clay, and as water moves down through the matrix, it carries these fine soil particles with it. As they accumulate in the underlying layer, these particles form a B horizon. Soils that have A, B, and C layers are described as mature.
Gradually, as weathering continues, plant growth and water percolation remove nearly all of the mineral nutrients from soil, and acidic by-products begin to develop. When a soil lacks the nutrients or contains enough acids that plant growth is slowed, the soil has reached old age.