Soils are more than just dirt. Plants depend on them for physical support, water, and nutrients. They are also home to diverse underground organisms. Creatures living in soils range from microscopic bacteria to the largest organism known—a honey fungus, Armillaria ostoyae, in eastern Oregon, which is 3 times the size of New York’s Central Park (Figure 7.5). Soil development and soil structure vary around the world largely because of differences in climate and the types and abundance of organisms present (Figure 7.6).

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If you were to dig into a mature soil, you would find a series of layers, called soil horizons. In a temperate deciduous forest, these are known as the O, A, E, B, C, and R horizons (Figure 7.7).

The surface layer, or the O horizon, is a site of active decomposition of organic matter, such as leaves, twigs, and bark. Burrowing organisms and physical processes, such as freezing and thawing, mix this decomposing organic matter with the inorganic clays and sands found in the lower soil horizons, producing the crumblike structure characteristic of many fertile soils.

The A horizon, which sits beneath the the O horizon, is typically referred to as the topsoil. It is rich in essential plant nutrients, such as nitrogen, phosphorus, and potassium, on which much plant production is dependent. Although it contains significant amounts of organic matter, which is generally dark in color, the A horizon is predominantly an inorganic layer, consisting of a mixture of various proportions of sand, silt, and clay.

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The proportions of sand (coarse and gritty), silt (intermediate grain size), and clay (small grain size) in the topsoil determine soil texture. Soils dominated by one of the three types of mineral soil particles are called sandy, silty, or clay soils. Meanwhile, a soil consisting of approximately equal proportions of sand, silt, and clay is a loam. A loam soil, which has properties intermediate between sandy and clay soils, is considered one of the most desirable for agriculture.

In well-developed soils, there can be an E horizon below the A horizon. The light-colored E horizon results from clays and dissolved organic matter flowing downward to deeper soil layers, leaving pale-colored sand and silt. The subsoil, or B horizon, is a depositional layer, rich in materials that leached out of the E horizon. The C horizon, the deepest soil layer, typically contains unconsolidated weathered rocks from the parent material from which soil develops. Parent material can be made up of rock, windblown or water-transported sand, or organic matter, such as peat. At the base of a soil developed on rock is the R horizon, which is partially weathered bedrock.

Soil forms as the environment interacts with parent material. The factors important to soil formation include climate, organisms, the nature of the parent material, the topography (or form) of the land surface, and time (Figure 7.8).

Climate directly influences soil development through temperature and precipitation. Temperature fluctuations, freezing and thawing in cold climates, and heating and cooling in hot climates, promote weathering of rocks. Weathering begins with fracturing and fragmentation of large rock, which eventually reduces even great rock formations to small soil particles. The rate of soil development reaches maximum levels in the warm, humid tropics. Climate also indirectly affects soil development through its influences on the activity of soil organisms and plant roots.

Wind and rain add nutrients as they deposit dust on a landscape (Figure 7.9). Nitrogen-fixing bacteria and plants, such as legumes, produce most of the biologically available nitrogen in soils. Runoff from rainfall can cause erosion of soils from the landscape, whereas rainfall that percolates into soils can remove nutrients and carry them down the soil profile and into groundwater.

Though all soils are subject to erosion by wind or water, those on steep slopes are more vulnerable. As soils erode from higher ground, the soil remaining on the slopes is thin and prone to drying. Soils washed into valleys and other low points in a landscape, such as swales, cause a thickening and moistening in these depositional areas.

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