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SOIL PROFILE

This lesson is on: A soil profile is a vertical section of a well developed soil showing distinctive horizontal bands(horizons) which differ both in appearance and chemical composition. Soil horizons are grouped in O,A, E, B, C and D. O horizons(organic horizons) It forms above the mineral soil and is formed from dead plant and mineral matter. It commonly occur in forested areas. It can be divided into: Oi – original forms plant and animal residues which are slightly decomposed Oe – constitutes of residues that are intermediately decomposed Oa – constitute of organic residues that are highly decomposed Significance of O horizon in crop production • Provide nutrients for plant growth • Provide organic matter that improves soil structure • Reduces rate of soil erosion since the top moist soil is the on vulnerable to erosion Eluvial horizons(A and E) These are mineral horizons that lie at or near the surface and characterized by maximum leaching eluviation. Eluviation is removal of material in suspension from a layer. A horizon A is the top most mineral horizon containing a mixture of humified organic matter which impart a dark colour on lower horizons. The A-Horizon is the second layer of the soil. It is referred to as the top-soil. It is composed of highly decaying organic matter and minerals, and its color ranges from gray to brown. It is the layer that hosts many plants and supports many agricultural and grassland areas. Seed germination and the growth of plant roots take place in this soil layer. It is distinguishable by the different soil types including loam, clay, sand, and silt. It is also the soil layer that is most vulnerable to water and wind erosion. E horizon This is the third and the thinnest soil layer, and it is lighter in color compared to the A-Horizon. It is termed as the layer of maximum eluviation due to the leaching of clay, iron, aluminium oxides and resistant minerals such as quartz. As a result, this layer is depleted of aluminum, clay, organic compounds, and other soluble elements. It is mainly made up of silt and sand after losing most of its minerals as they move down through the soil. Importance of eluvial horizon in crop production • Supplies nutrients to plants • Anchors plants • Contains majority of macro and micro organisms which decompose and supply nutrients B horizon The B-horizon is the fourth layer and is also referred to as the sub-soil. It is mainly made up of large rocks, clay, and bedrock. The color of the sub-soil is lighter than the layers above it and ranges from rust to tan. It is also composed of minerals and little organic matter. The sub-soil accumulates a lot of silicate clays, aluminum and iron oxides, and organic compounds from the layers above it a process known as illuviation. Hence, it is sometimes called the illuviation zone. It is the layer where the roots of big trees end. . It contributes less to plant growth because the roots of plants can’t reach the layer and it lacks organic matter. C horizon (regolith) This is the fifth layer of the soil and it also known as the regolith. Plant roots do not go through this layer and it has very little organic matter. It may or may not be the same as material from which it was formed. The color of the C-Horizon is gray and is made up of fragmented pieces of bedrock. Also, it has compacted sediment and cemented geological material. Very little activity is present in this layer but there may be instance of additions or losses of minerals and other soluble compounds. It is the outside the zones of major biological activities. Plant roots do not grow through this layer. R horizon The R-Horizon is the sixth and final layer of the soil. It is also known as the bedrock. The materials here are mainly solid unweathered rock that are cemented and compacted by the weight of the overlying layers. The color of this layer is gray with several hard rocks. This layer is beyond 50 meters from the top-soil. Some of the rocks found here include granite, limestone, and basalt. Crop science 6049