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A soil horizon is a unique horizontal layer of soil that can be identified as O, A, E, B, C, or R, having distinct physical and chemical properties.
Definition of Soil Horizon A soil horizon is a parallel layer of soil to the ground surface formed by pedogenic processes. Every horizon has a different appearance compared to the others.
Types of Soil Horizons O horizon – Organic soil rich in decomposed plants. A horizon (Topsoil) – A combination of mineral soil and humus.
E horizon – A layer where clay and organic matter are removed and is light in colour. B horizon (Subsoil) – A layer of deposition of clay, oxides and mineral layers. C horizon – The horizon where parent material of soil is found. R horizon – Solid and compact bedrock layer.
How Soil Horizons Form Soil horizons form as a result of the activities of weathering, leaching, biological actions, and the accumulation of materials.
Soil Profile Soil profile is a succession of horizons forming the soil profile that reflects on the formation of the soil.
Soil Horizons form the basis of the soil classification system used in both scientific and practical purposes.
Soil formation is a function of five primary natural factors: climate, organisms, parent material, topography, and time.
According to soil science principles (often represented by Jenny’s famous state factor equation \(S = f(cl, o, r, p, t)\)), these factors interact to break down rocks and mix organic matter:
Climate (Weather): Temperature and precipitation control the rate of chemical and physical weathering. Warm, humid conditions accelerate rock breakdown and biological activity.
Organisms (Biota): Plants, animals, fungi, and bacteria add organic matter through debris and root systems, while earthworms and microbes mix the soil and cycle nutrients.
Parent Material (Mother Rock): The underlying geological rock or deposit (such as granite, limestone, or basalt) provides the basic mineral composition, texture, and pH of the soil.
Topography (Relief): The shape, slope, and elevation of the land influence water drainage, sun exposure, and erosion rates. Steep slopes shed soil quickly, whereas flat lowlands accumulate deeper layers.
Time: Soil formation is a slow, continuous process. It takes hundreds to thousands of years for distinct soil layers (horizons) to develop from raw rock.
