Chapter 10 Global Soils

Introduction Soil- the uppermost layer of the land surface that contains organic matter and support for plants Vegetation Climate Chain reaction: if soil loses portions of any of three, then plants struggle, animals that rely on plants are affected, and animals that depend on those animals are in trouble

Important Factors in Soil Formation Parent material – mineral material suitable for transformation into soil Organisms – all flora and fauna living on/in soil; trees, grasses, bacteria, worms, prairie dogs, etc. Climate – mainly precip and temp that lead to physical and chemical breakdown of parent material; wind also plays a role Topography – elevation, slope, aspect of surface

The Nature of Soil Contains all three phases of matter Solid Mineral matter: mainly from rock material Organic matter: dead biological material and excretions from living biological objects Hummus Liquid: H2O and dissolved substances (nutrients) Gas: CO2, CH4, O2, etc. _______is the physical and chemical process by which soils develop their properties and characteristics. Weathering

Average Composition of Soil

The Nature of Soil Is soil static or dynamic? DYNAMIC! Numerous processes in soil involve the constant/continual interaction of minerals, organic matter, water, and gases at various scales Weathering All physical and chemical processes that lead to formation of soil properties and characteristics Occurs on parent material both above and below current soil layer; below – bedrock and regolith; above – rocks, gravel, sand, silt, clay

Cross-section of a Patch of Land

Soil Color and Texture Why are soils different colors? Soil texture Inherited from parent material (composition) Dominant processes w/i soil (evaporation, oxidation) Soil texture Proportion of particles within soil Described using a series of names emphasizing dominant particle size (particles include sand, silt, and clay) Loam refers to mixture in which above particles each account for > 10% of total composition Important b/c major determinant in water-retention capacity of soil

Soil Particles and Associated Sizes

Some Soil Textures

Soil Texture Triangle

Structure and Colloids Soil structure – how soil grains are lumped or held together Colloids Smallest particles (mineral or organic) in soil (< 0.00001mm) and are extremely important in soil structure, as well as attracting nutrients that are used by plants Hold nutrients due to different charges (colloids are negative, nutrients are positive)

Soil Particles and Associated Sizes

Acidity and Alkalinity How acidic or alkaline (basic) a soil is will affect the fertility of the soil If positively-charged acidic ions get into soil, can replace positively-charged basic ions (the nutrients)  reduced soil fertility Measured using pH scale Ranges from 0 to 14 7 is a neutral substance, e.g., distilled water < 7 is acidic, e.g., wine, lemon juice > 7 is alkaline or basic, e.g., baking soda, ammonia Best soil pH ranges from about 6.0 to 7.2

pH Scale for Soil Acidity and Alkalinity

Soil Minerals Two main types Primary: compounds found in unaltered rock; play no major role in sustaining life Secondary: develop from primary minerals via chemical weathering; necessary for soil development and fertility Important minerals include clay for fertility and mineral oxides (Al2O3 and Fe2O3) for structure

Soil Moisture Importance: determine ability of soil to support plants and can lead to weathering How does moisture get to the soil? Precipitation, condensation, deposition, snow and ice melt, irrigation, subterranean water (aquifers), surface water Then three options: runoff, infiltration, evaporation/evapotranspiration Infiltration  soil water recharge  saturation  soil reaches storage capacity (no more water drains due to gravity) Remember: storage capacity affected by texture

Soil Moisture and Texture

Soil Water Balance Similar concept to water balance in hydrologic cycle and energy balance in entire earth Recharge – water added to soil Disposal/lowering – water removed from soil Evapotranspiration Actual (AE): amount of water used Potential (PE): amount of water needed If AE > PE, then have shortage or deficit and water can be pulled from previous storage OR artificially added If PE > AE, then have an abundance of water which can be stored as excess

Soil Water Budget “Accounting” of water into and out of a system Pos. values: Precip (P), Change in Storage (ΔST), Surplus (S) Neg. values: Actual Evapotranspiration (AE), Potential Evapotranspiration (PE), Water Deficit (D) Over course of year, hopefully balances; if neg. are higher during growing season, need irrigation to help crop growth See Fig. 10.9, p. 348

Soil Horizons Distinctive horizontal layers with different physical or chemical composition or different organic content or structure Develop by interaction of factors discussed at beginning of class (parent material, organisms, climate, topography, time) Distinguished by color Soil profile – cross-section of a sample of soil that shows horizons Two types of horizons Organic: develop from accumulating organic matter, O Mineral: combination of organic and mineral matter, A, E, B, C, (R)

Soil Horizons O – uppermost soil layer, only organic matter A – minerals, organic matter, plant roots, and humus E – sand, silt, and plant roots B – clay, oxides, organic matter, plants roots C – regolith, clay, oxides, little organic matter; not really part of soil R – (not shown) bedrock, little, if any, organic matter; not really part of soil

Soil-forming Processes Enrichment – material added to soil via runoff, flooding, wind, humus accumulation Removal – surface erosion of sediment Translocation – material moved w/i soil, usually from one horizon to another Eluviation vs. Illuviation Transformation – conversion of minerals from primary to secondary types

Soil Temperature Helps determine chemical development of soils and formation of horizons by controlling biologic activity, organic decomposition, and intensity of chemical processes Affected by topography, latitude, longitude, and season Plant growth generally require temp > 41°F

Soil and Topography Elevation Influences temp and moisture of soil Higher elevations tend to have cooler temps, higher moisture content (Catalina Mountains) Slope Affects drainage of water and soil thickness Steep slopes have more drainage, thinner soil horizons Aspect Influences temp and moisture content Toward sun (south slope in NH) means warmer and dryer, away from sun means cooler and more moisture

Soil and Biologic Factors Humus – “finely divided, partially decomposed organic matter;” affects soil fertility, maintains structure for infiltration of water and air Plants – protect vs. erosion, litter becomes humus Animals (non-human) – affect accumulation time, accumulation rate, mixing, nutrient cycles via burrowing/digging, travel, etc. Humans – same effects as animals, but also via cultivation, irrigation, chemical and mineral fertilizers, urbanization, recreation, etc.

The Global Scope of Soils Soil varies at many spatial scales – globally, country to country, region to region, locally Type of soil linked to climate, parent material, biologic processes (plant and animal), time, and topography/landforms Classified into 3 groups, which contain 12 orders and numerous sub-orders, all of which are differentiated by unique combinations of physical and/or chemical properties (see Fig. 10.2, p. 354)