Why Does Soil Reflect Climate?

Soil Formation

• Soils form from parent material such as in-place bedrock, or sediment brought in from elsewhere by way of wind or water. The parent material weathers to form a "weathered mantel." The weathered mantel then undergoes leaching and deposition, separating the soil into multiple layers called horizons. The lowest horizon is just above the parent material and contains bits of the parent material. The middle horizons are more mature parts of the soil containing clays, silts, sands and minerals. The top layer typically consists of organic material that has fallen to the surface.

Physical Weathering

• Particles of rock and sediment break down via two processes, physical weathering and chemical weathering. In physical weathering, the material breaks into smaller and smaller pieces but does not change in composition. When rocks get warmer, they expand, and when they get colder, they contract. This small change in size causes cracks to form in the material. Cracks in rocks can fill with water and undergo a freeze-thaw cycle in which water freezes and expands, making the fracture larger. Over time, this breaks the rock apart into smaller pieces. Smaller pieces of rock have greater surface area to volume and therefore more surfaces to weather, and the process continues.

Chemical Weathering and Decomposition

• Chemical weathering changes the chemical composition of the parent material. Usually this results from exposure to water and air. For example, when a rock containing iron comes in contact with oxygen in the air, the surface oxidizes, forming a layer of iron oxide, or rust. Other minerals respond to oxygen similarly. Organic matter within the soil, such as leaves that fall to the ground, breaks down with the help of organisms such as bacteria and earthworms. This process adds nutrients to the soils, making fertile topsoil.

Climate

• The weathering process determines how quickly soils form and how mature they become. In dry climates, physical weathering typically dominates, and soils form more slowly and incompletely. In wet climates, chemical weathering typically dominates, and rich soils form quickly. Climate also influences what flora and fauna populate an area, which then influence what organic matter is present in soils, as well as how quickly it decomposes. Some parent material, such as limestone, is highly susceptible to chemical weathering, and breaks down quickly during rain events. However, in dry climates, limestone weathers quite slowly.