Soil and Soil Forming Processes By HO Pui-sing

Soil and Pedogenesis Soil as a Dynamic Body Physical and Chemical Properties of Soils Soil Profile Factors affecting Soil Formation Soil Forming Processes – Pedogenesis World Pattern of Soil

Soil as a Dynamic Body Soil is a dynamic layer in which many complex chemical, physical and biological activities are going on constantly. Soils become adjusted to conditions of climate, landform and vegetation, and will change internally when those controlling conditions change.

Soil Matters Solid Organic and Inorganic Water Complex chemical solution from precipitation, runoff, seepage and groundwater Air Pore spaces – air or water Field Capacity The percentage of water remaining in the soil 2 or 3 days after the soil has been saturated and free drainage has practically ceased

Physical and Chemical Properties of Soils Soil Colour Soil Texture Soil Structure Colloids, Bases and Acidity Soil Water

Soil Colour Black – much humus (Cool climate) – temperate grassland Light – little humus (Hot climate) - Desert Reddish – feric compounds (Well drained) - TRF Greyish or bluish – reduced ion compounds (Poorly drained) - Swamp

Soil Texture Name of GradeDiameter (mm) Coarse gravel Fine gravel Above Coarse sand Medium sand Fine sand Very fine sand Silt Clay Below 0.002

Soil Texture

Ratio of sand, silt and clay Soil Texture Water-retention properties Loam textures are the best for plant growth

Soil Texture

Soil Structure

Individual soil particles aggregate together in lump or peds. Important for the ease of cultivation Crumb structure: best for seed germination

Soil Colloids, Bases and Acidity Colloids Inorganic: chemical properties of clay (Clay minerals) Organic: Humus

Bases Clay minerals which are negatively charged. This is neutralized by the attraction to their surface of positively-charged ions (cations) of minerals. Calcium (C), magnesium (Mg), potassium (K) and sodium (Na) - Bases

Bases Bases are recycled to the soil by the decomposition of plants and animals. Bases can be replenished by artificial supply – fertilizer Bases are essential as nutrients for plant growth.

Acidity Acidity is measured on the pH scale.

Soil Water It is a complex chemical solution. It is a dilute solution of such substances as bicarbonates, sulfates, chlorides, nitrates, phosphates and silicates of calcium, magnesium, potassium, sodium and iron ……..

Soil Profile Layer-like horizons of differing texture, colour and consistency Soil horizons Regolith Bedrock

Soil Profile Horizon A Zone of eluviation Horizon B Zone of illuviation Horizon C Weathered material Horizon D Unweathered rock

Factors affecting soil formation Passive Formers Parent materials Topography/ Landform Time Active Formers Climate Biotic factor

Passive Formers Parent Material Topography / Landform Time The clear horizons are absent or poorly developed in young soils

Active Soil Former - Climate Moisture Chemical and biological activities Excess precipitation – eluviation and illuviation Excess evaporation – capillary action

Active Soil Former - Climate Temperature Chemical reaction Bacterial activities Rate of decomposing

Active Soil Former - Climate Wind: Minor factor Increase evaporation Reallocation of soil (removal and accumulation)

Active Soil Former – Biotic Factor Vegetation Provide humus, bind soil and counteract percolation by transpiration Maintain fertility Different types of vegetation require different proportions of bases. Tree: little calcium and magnesium Grass: much calcium and magnesium

Active Soil Former – Biotic Factor From bacteria to large animals Mechanical Eg. Burrowing by earthworms Eg. Excavation by man Chemical Eg. Organic acid

Soil Forming Processes Weathering Physical, chemical and biological Soil genesis Additions, losses, transformation, translocation Refer to CUHK Powerpoint

Oxisols and Aridisols Refer to CUHK Powerpoint Soil Formation Factors with special reference to Oxisols and Aridisols

World Pattern of Soil Zonal Soil Well developed and reflect the influence of climate as the major factor Azonal Soil Immature or poorly developed soil Intrazontal Soil Well developed by the local factors dominant

World Pattern of Soil