Soils are classified based on their parent material, texture, structure, and profile. Parent material is what the soil was made from, usually mostly inorganic rocks, so a soil that has <20% organic matter (O.M.) is a mineral soil while one with >20% O.M. is an organic soil (peat or muck). Texture refers to the proportion of sand, silt, and clay in the soil; sandy soils are called light or coarse-textured, whereas clay soils are called heavy or fine-textured. Clay tends to increase the water-holding capacity of the soil. Loamy soils have a balanced sand, silt, and clay composition and are thus superior for plant growth. Structure refers to the aggregation of soil particles into platy, prismatic, blocky, spherical, or crumblike clods. A soil profile is a vertical cross-section of a soil, showing the various layers or horizons, beginning at the surface with the A horizon and continuing downward through the B, C, and D horizons to the parent material.
Soil profile in hills of Manisa Province, Turkey, showing a distinct
horizon of accumulated red-purple metallic (manganese and/or iron?)
oxides.
There are five key factors in soil formation: i) type of parent material; ii) climate; iii) overlying vegetation; iv) topography or slope; and v) time. Type of parent material influences the soil pH, structure, color, etc., in a profound way. High-rainfall climates tend to have less-fertile soils, due to rainwater's effect in leaching nutrients down to lower levels of the soil profile, and have more acidic soils. Low-rainfall climates tend to accumulate salts near the surface and have generally higher soil pH. Soils that form under coniferous forests tend to be more acidic than those under deciduous forests, and root action is also critical in soil formation. Soils generally have a harder time forming on steep slopes, due to runoff of soil particles during rain events. The longer a soil has to form, the deeper its profile is going to be.
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