Soil and Soil Forming Processes

By HO Pui-sing

Soil and Pedogenesis

Soil as a Dynamic BodyPhysical and Chemical Properties of SoilsSoil ProfileFactors affecting Soil FormationSoil 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 MattersSolidOrganic and Inorganic

WaterComplex chemical solution from precipitation, runoff, seepage and groundwater

AirPore spaces – air or water

Field CapacityThe 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 ColourSoil TextureSoil StructureColloids, Bases and AciditySoil Water

Soil ColourBlack – much humus

(Cool climate) – temperate grasslandLight – little humus

(Hot climate) - DesertReddish – feric compounds

(Well drained) - TRFGreyish or bluish – reduced ion compounds

(Poorly drained) - Swamp

Soil TextureName of Grade Diameter (mm)

Coarse gravelFine gravel

Above 21.0 - 2

Coarse sandMedium sandFine sandVery fine sand

0.5 - 10.25 - 0.50.1 - 0.25 0.05 - 0.1

Silt 0.002 - 0.05

Clay Below 0.002

Soil Texture

Soil Texture

Ratio of sand, silt and claySoil Texture Water-retention properties

Loam textures are the best for plant growth

Soil Texture

Soil Structure

Soil Structure

Individual soil particles aggregate together in lump or peds.

Important for the ease of cultivationCrumb structure: best for seed germination

Soil Colloids, Bases and Acidity

ColloidsInorganic: 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 – fertilizerBases are essential as nutrients for plant growth.

AcidityAcidity 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 ProfileLayer-like horizons of differing texture, colour and consistency

Soil horizonsRegolithBedrock

Soil Profile

Horizon AZone of eluviation

Horizon BZone of illuviation

Horizon CWeathered material

Horizon DUnweathered rock

Factors affecting soil formation

Passive FormersParent materialsTopography/ LandformTime

Active FormersClimateBiotic factor

Passive FormersParent Material

Topography / Landform

TimeThe clear horizons are absent or poorly developed in young soils

Active Soil Former - Climate

MoistureChemical and biological activitiesExcess precipitation – eluviation and illuviationExcess evaporation – capillary action

Active Soil Former - Climate

TemperatureChemical reactionBacterial activitiesRate of decomposing

Active Soil Former - Climate

Wind: Minor factorIncrease evaporationReallocation of soil (removal and accumulation)

Active Soil Former – Biotic FactorVegetation

Provide humus, bind soil and counteract percolation by transpirationMaintain fertilityDifferent types of vegetation require different proportions of bases.Tree: little calcium and magnesiumGrass: much calcium and magnesium

Active Soil Former – Biotic FactorFrom bacteria to large animalsMechanical

Eg. Burrowing by earthwormsEg. Excavation by man

ChemicalEg. Organic acid

Soil Forming ProcessesWeathering

Physical, chemical and biologicalSoil 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 SoilWell developed and reflect the influence of climate as the major factor

Azonal SoilImmature or poorly developed soil

Intrazontal SoilWell developed by the local factors dominant

World Pattern of Soil