Soils began to form billions of years ago as rain washed minerals out of the once molten rocks that were cooling on the planet's surface. The rains leached potassium, calcium, and magnesium—minerals essential for plant growth from the rock, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished, and as each plant died and decomposed, it added nutrients and energy to the mineral mixture, making the soil more fertile for new plants.
Soil now covers the earth in depths from a few inches to several feet, and these soils are constantly forming and changing. Soils are created from "parent" material, loose earthy matter scattered over the earth by wind, water, or glacial ice, or weathered in place from rocks.
Parent material is turned into soil as other reactions take place on exposed rock surfaces. Water-borne acids react with elements in the rock and slowly change them into soil components. Minerals that break down relatively easily—feldspars and micas—become clay, the smallest soil particles with diameters less than 0.0002 mm, while harder minerals like quartz turn into sand (0.05-2.0 mm) and silt (0.0002-0.05 mm).
As the parent material weathers, the nutrients necessary for plant growth are released, and plants begin to establish themselves. As they die, they leave behind organic residues on which animals, bacteria, and fungi feed. Their consumption breaks down the organic matter further, enriching the parent material for plant growth. Over time, more and more organic matter mixes with the parent material.
Wherever soil is found, its development is controlled by five important factors: climate, parent material, living organisms, topography, and time.
A region's climate determines the range and fluctuation of temperature and the amount of precipitation that falls to the earth, which in turn controls the chemical and physical processes responsible for the weathering of parent materials. Weathering, in turn, controls the rate at which plant nutrients are released. Nutrient flow, along with temperature and precipitation, determines the types of plants a region can support.
A soil's parent material plays an important role in determining the chemistry and texture of soil (the size and shape of soil particles). The rate at which water moves through soil is controlled in part by the texture of the soil. Soils from some parent materials weather more or less quickly than others. Soils derived from quartz minerals, for example, weather more slowly than those derived from silicate materials.
The numbers and kinds of living organisms in a given region help determine the chemical composition of soil. Grassland soils are chemically different from those that develop beneath forests, and even within these broad categories of vegetative cover, soil profiles can differ; for example, different soils develop under conifers than under deciduous trees.
Topography, the configuration of the earth's surface, affects soil development because it determines the rate at which precipitation washes over soil and how soils erode. Smooth, flat lands hold water longer than hilly regions, where water moves more quickly down slopes. Swamps, marshes, and bogs are formed as low-lying areas hold water over time. Soil erodes, or wears away, more quickly on sloping land than flat.
Time plays an important role in soil development: soils are categorized as young, mature, or old, depending on how the above factors are combined, and the rate at which they work.