1. Chapter 10
Global Soils

2. 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

3. 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

4. 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

5. Average Composition of Soil

6. 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

7. Cross-section of a Patch of Land

8. 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

9. Soil Particles and Associated Sizes

10. Some Soil Textures

11. Soil Texture Triangle

12. Structure and Colloids
Soil structure – how soil grains are lumped or held together
Colloids
Smallest particles (mineral or organic) in soil (< mm) 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)

13. Soil Particles and Associated Sizes

14. 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

15. pH Scale for Soil Acidity and Alkalinity

16. 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

17. 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

18. Soil Moisture and Texture

19. 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

20. 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

21. 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)

22. 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

23. 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

24. 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

25. 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

26. 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.

27. 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)