Soil formation

Soil forms through a process called pedogenesis, shaped by location, surroundings, and time. Chemical and biological forces alternately build up and break down structure within the ground, creating distinct horizontal layers known as soil horizons. These layers differ in color, texture, structure, and chemical makeup, and their patterns across landscapes reflect variations in the factors that drive soil development. Scientists study pedogenesis as part of pedology—the broader field that examines soil in its natural setting, alongside soil morphology and classification. Understanding how soil forms helps explain where different soils appear today and how they were distributed in ancient geological periods.

The process begins when freshly deposited parent material undergoes weathering. Microbes such as bacteria, archaea, and fungi feed on simple nutrients released by weathering, producing organic acids and specialized proteins that further break down minerals. Their organic remains contribute to humus. Plant roots, aided by symbiotic mycorrhizal fungi, also extract nutrients from rock. New soil deepens through ongoing weathering and additional deposits—for instance, in Sicily’s Mediterranean climate, weathering adds roughly one-tenth of a millimeter per year. Dust deposition can also increase soil depth. Over time, soil supports more complex life, starting with pioneer species and progressing through ecological succession. Topsoil thickens as humus accumulates from dead plants and microbes, and as organic matter mixes with weathered minerals. As soils mature, distinct horizons develop through organic buildup, mineral weathering, and leaching.

Five classic factors interact to shape soil formation: parent material, climate, topography (relief), organisms, and time. Rearranged as climate, organisms, relief, parent material, and time, they form the acronym CLORPT.

Parent material is the mineral source from which soil forms. All soil minerals and plant nutrients—except nitrogen, hydrogen, and carbon—originate from igneous, sedimentary, or metamorphic rock. Through physical and chemical weathering, transport, deposition, and precipitation, rock transforms into soil. Common parent minerals include quartz (SiO₂), calcite (CaCO₃), feldspar (KAlSi₃O₈), and mica (biotite, K(Mg,Fe)₃(AlSi₃O₁₀)(F,OH)₂). Parent materials are classified by how they were deposited: residual materials weather in place from bedrock; transported materials are moved by water, wind, ice, or gravity; cumulose material is organic matter that grows and accumulates on site. Residual soils develop from underlying bedrock and share its chemistry, often found on mesas and volcanoes—though only about three percent of U.S. soils are residual. Most soils come from transported materials, carried long distances by wind, water, ice, or gravity. Wind moves silt and fine sand hundreds of miles, forming loess (60–90 percent silt) in the U.S. Midwest, Canada, northwestern Europe, Argentina, and Central Asia; clay rarely moves by wind because it forms stable aggregates. Water-transported materials include alluvial (moved by flowing water), lacustrine (settled in lakes, such as around the Great Lakes and ancient Lake Bonneville), and marine deposits (former seabeds, like soils along the Gulf Coast and California’s Imperial Valley). Ice deposits parent material as terminal and lateral moraines from stationary glaciers, smoother ground moraines from retreating glaciers, and outwash plains from meltwater. Gravity moves material down steep slopes, forming talus cones and colluvial deposits. Cumulose parent material, such as peat and muck, comes from organic matter preserved by low oxygen in high water tables; peat can yield sterile soils, while muck may be very fertile.

Weathering of parent material includes physical disintegration, chemical decomposition, and chemical transformation. It usually affects only the top few meters of geologic material, as stresses and fluctuations diminish with depth. Physical disintegration occurs when rocks formed deep underground swell and become unstable upon exposure to lower pressure near the surface. Chemical decomposition depends on mineral solubility; its rate doubles with every 10°C temperature rise and requires water. Rocks that decompose in a few years in tropical climates may remain unchanged for millennia in deserts. Structural changes result from hydration, oxidation, and reduction. Chemical weathering mainly results from these processes.