The GEOGRAPHY of SOILS. Physical Geography (Geog. 300) Prof. Hugh Howard American River College

The GEOGRAPHY of SOILS Physical Geography (Geog. 300) Prof. Hugh Howard American River College

SOIL and SOIL- FORMING FACTORS

SOIL and FORMING FACTORS Soil Natural surface layer of Earth Composed of mineral and organic matter Essential to life of the planet and to us Medium for plant growth Habitat for soil organisms (animals) Engineering medium Pedology: soil science

Soil

SOIL and FORMING FACTORS Soil-Forming Factors Parent Material Organisms Climate Time Topography

SOIL and FORMING FACTORS Parent Material Rock (the mineral component of soil) Residual Parent Material Bedrock weathers in place Transported Parent Material Bedrock is weathered elsewhere and is delivered by streams, wind, glaciers, etc. Regolith Weathered parent material

SOIL and FORMING FACTORS Organisms Vegetative cover dies, decomposes, and gets incorporated into soil Burrowing insects and animals rearrange soils and get incorporated after they die Microorganisms contribute Humus Decomposed organic matter a vital component of soil

SOIL and FORMING FACTORS Climate Soils are influenced by temperature and precipitation Time Soils take thousands of years to develop And can be ruined in a single person s lifetime Topography Influences the type of soil and its thickness

SOIL TEXTURE

Grain Size SOIL TEXTURE Size distribution of mineral particles in soil Sand: 0.5 2.0 mm Individual grains can be seen Gritty feel Porous

Silt 0.002 0.5 mm SOIL TEXTURE Only the largest grains can be seen Intermediate feel Intermediate porosity Clay < 0.002 mm Grains can be seen using a microscope Smooth feel Not porous water is retained within

Loam SOIL TEXTURE Soil consisting of a mixture of sand, silt, and clay The soil texture triangle can be used to define loams Sandy loam has abundance of sand Silt loam has abundance of silt Clay loam has an abundance of clay

Soil Texture Triangle

SOIL CHEMISTRY

Soil Colloids SOIL CHEMISTRY Particles smaller than 1/100,000th of a mm (0.00001mm) Mineral or organic Large surface area per volume Lots of surfaces to bond with other materials One spoonful of clay contains enough surface area to cover a football field

SOIL CHEMISTRY Mineral Exchange Most colloids have negative electrical charges that attract water and nutrients Colloids allow the exchange of nutrients with plant roots H is exchanged for Ca, Mg, Potassium, Sodium Cation-Exchange Capacity

Soil Colloids and Mineral Exchange

SOIL CHEMISTRY Soil Acidity and Alkalinity Acid soils <=5 ph Alkaline soils >=10 ph Nutrient exchange acidifies soils by introducing H ions Soil acidity can be reduced through the application of lime, burning, etc.

Soil Acidity and Alkalinity

SOIL MOISTURE

SOIL MOISTURE Soils attract and retain water Negative electrical charges on colloids attract positive poles of water molecules Soils help prevent water from Sinking into the ground beyond plant roots Being lost through evapotranspiration Storage Capacity The maximum amount of water that a soil can retain

SOIL MOISTURE Available Water Capacity Amount of water in soil that is actually available to plants Vast surface area and negative charge of colloidal particles out compete plant roots for some moisture A wet soil can allow plants to die

Soil Moisture

SOIL STRUCTURE, PROFILES, and HORIZONS

STRUCTURE, PROFILE, HORIZON Soil Structure Manner in which soil particles are held together Structures Granular Platy Blocky Prismatic

Soil Structure

STRUCTURE, PROFILE, HORIZON Soil Profile Vertical section of soil layers Soil Horizon Individual layer of soil in a profile Horizons O, A, E, B, C, R

Soil Profile and Horizons

STRUCTURE, PROFILE, HORIZON O Horizon Organic, at top Most prevalent in cooler climates, where decomposition is slow Nutrient rich A Horizon Organic and mineral Dark colored Nutrient rich

STRUCTURE, PROFILE, HORIZON E Horizon Leached layer Clays, oxides, and organic matter are washed downward by water Light colored B Horizon Accumulation layer

STRUCTURE, PROFILE, HORIZON C Horizon Weathered parent material not really soil yet Regolith R Horizon Unweathered parent material Bedrock

SOIL TAXONOMY

SOIL TAXONOMY A classification system has been devised by the NRCS Natural Resources Conservation Service Soil Taxonomy Based upon Diagnostic Horizons Organized into Orders, Suborders, etc. Only Orders are covered here Soil types reflect the four spheres

Soil Taxonomy

Oxisols SOIL TAXONOMY Equatorial, tropical, and subtropical Rainforest soil Lack distinctive horizons Extremely weathered Easily eroded w/o vegetation

Oxisols SOIL TAXONOMY Nutrient-poor soils Vegetation takes nutrients from soil and gives H in return, leading to acidic soil Most dead vegetation gets eaten by bacteria before it can get reintegrated into soil Nutrients are washed downward (leached), out of the reach of plant roots Associated with slash & burn agriculture

Oxisol

Aridisols SOIL TAXONOMY Found in arid climates Desert soils Little organic matter Can be nutrient rich Productive if properly irrigated

Aridisol

Mollisols SOIL TAXONOMY Found in midlatitude grasslands Dark & soft Mollic Epipedon A Horizon rich in organic matter Very nutrient rich

Mollisol

Alfisols SOIL TAXONOMY Found anywhere from equatorial regions to upper mid-latitudes Nutrient rich Well developed: older landscapes Predominant in the Central Valley Xeralfs Tall grassland & trees Do well with ample water

Alfisol

Ultisols SOIL TAXONOMY Similar to Oxisols in location and character, but have a clay-rich B Horizon Clay minerals leached from E Horizon and deposited in the B Nutrient poor Slash & burn agriculture

Ultisol

Spodosols SOIL TAXONOMY Found in upper latitudes (cold) in formerly glaciated regions Nutrient poor (acidic) Entisols No distinct distribution Undeveloped soils, w/o distinct horizons Nutrient poor in arid regions; nutrient rich in wetter regions (floodplains)

Spodosol

Entisol

Inceptisols SOIL TAXONOMY Found near Oxisols and Ultisols Young soils with few diagnostic features Gelisols Found in extremely cold climates Permafrost within 2 meters of the surface Lack of microbial activity leads to buildup of organic matter over time

Inceptisol

Gelisol

Andisols SOIL TAXONOMY Found near volcanoes volcanic ash Nutrient rich Vertisols Dark, shrink-swell clays Nutrient rich, but difficult to work with Great examples in Texas

Andisol

Vertisol

Histosols SOIL TAXONOMY Found near Spodosols Organic soils Formed in shallow lakes or ponds Peat and muck can form Nutrient poor (limited decomposition)

Histosol

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The GEOGRAPHY of SOILS Physical Geography (Geog. 300) Prof. Hugh Howard American River College