Lecture 5 SOIL FORMING PROCESSES
Soil horizons Horizons in the soil profile are the results of soil formation processes
PROCESS OF SOIL FORMATION The layers in soil, called soil horizons, develop their characteristics not only because of the geological materials at a given site, but also because of soil forming processes at work, over time, in that zone of the soil. The soil forming processes fall into the following categories: 1. Addition 2. Removal 3 Mixing 4. Translocation 5. Transformation
1. Additions water (from the surface, and by ground water discharge) suspended and dissolved materials carried by water solids transported by wind gases from the air energy from the sun organic carbon by plants in form of roots and root-derived material organic carbon by photoautotrophic bacteria organic nitrogen by nitrogen-fixing bacteria plant and animal remains on and in the soil.
2. Removal materials removed by erosion ; wind, water, ice, gravity dissolved and suspended material may be leached out from the bottom of a soil profile uptake of nutrients from the soil by plants carbon dioxide gas produced by plant root, microbial and faunal respiration other gases such as nitric oxide, nitrous oxide and nitrogen produced by denitrifying bacteria other gases such as methane which are produced under anaerobic conditions
3. Mixing By animals ; earthworm, insects, rats etc
4. Translocation Translocation of materials within the soil profile is primarily due to gradients in water potential and chemical concentrations within the soil pores. Soluble minerals (calcium carbonate), colloidal material(clays), organic compounds, and iron may move up or down the profile, between horizons, with water movement. Biological activity may cause gradient in the chemical composition of the water and air-filled pores of the soil.
Translocation Processes: E.g. i. Clays= lessivage ii. Fe Oxide + humus= podzolization iii. Calcium carbonate= calcification iv. Laterization = Fe, Si
i) (Clay movement)
3. Change in chemical composition of soil - Flocculation of clay
Clays -> lessivage =formation of Bt horizon
ii) Podzolization Podzolization develop in cool and moist climates under pine forests. They are typical of the colder portions of the humid continental and Subarctic climates. The E horizon is heavily leached and basically composed a of light colored layer of sand. Typical ashy, gray layer left from leaching of sesquioxides is apparent in this podzolized soil. The upper portion of the B horizon is stained reddish color from the accumulation of sesquioxides. The profile gets lighter in color as depth increases.
Albic and Spodic Horizon i) (Albic)
ii) Spodic horizon Accumulation B horizon ( Bh, Bir, Bhir )
iii) Calcification e.g. Mollisol soil enriched with calcium carbonate. Calcification occurs in warm, semi-arid environments, usually under grassland vegetation. Soil tends to be rich in organic matter and high in soluble bases. The B horizon of the soil is enriched in calcium carbonate precipitated from water moving downward through the soil or upward movement of capillary water from below.
Exposure of the soil to the hot tropic sun by deforestation bakes the soil dry, reducing infiltration, increasing runoff, and reducing fertility. iv) Laterization The deep red to bright orange-red soils of the tropics are a product of laterization.(movement of Fe 2+ and oxidation Fe oxides. Laterization occurs in the hot, rainy tropics where chemical weathering proceeds at a rapid rate. This ultisol displays the typical features of a soil having undergone laterization Soils subject to laterization end to be acidic and lack much organic matter as decomposition and leaching is extreme.
Transformation Soil components are transformed by chemical and biological reaction. Organic compounds decay, some minerals dissolve, other minerals precipitate. These transformation result in the development of soil structure, and in changes in colour, relative to the parent material.
Secaondary minerals (Fe, Mn, Al) (hematite, geotite)