Chapter twelve
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    Chapter twelve Chapter twelve Presentation Transcript

    • Title Page Photo “ A nation that destroys it's soils destroys itself.” — Franklin D. Roosevelt (
    • Vocabulary Chapter 12 loam (p. 364) O horizon (p. 367) parent material (p. 355) peds (p. 364) regolith (p. 354) R horizon (p. 367) salinization (p. 371) soil (p. 353) soil order (p. 373) soil profile (p. 367) Soil Taxonomy (p. 372) soil-water balance (p. 362) wilting point (p. 362) A horizon (p. 367) B horizon (p. 367) calcification (p. 371) cations (p. 359) C horizon (p. 367) colloid (p. 366) E horizon (p. 367) eluviation (p. 362) gleization (p. 370) horizon (soil horizon) (p. 367) humus (p. 357) illuviation (p. 362) laterization (p. 368) leaching (p. 362)
    • Soil and Regolith
      • Soil —zone of plant growth.
        • The upper portion of lithosphere characterized by its ability to produce and store plant nutrients.
          • Average depth is about 15 centimeters (6 inches).
        • An infinitely varying mixture of weathered mineral particles, decaying organic matter, living organisms, gases, and liquid solutions.
        • Stage in a never-ending continuum of physical–chemical–biotic activities.
      • Regolith —a layer of broken and partly decomposed rock particles that covers bedrock; its upper part is soil.
    • Soil-Forming Factors
      • Five factors are involved in forming soil:
        • Geology
        • Climate
        • Topography
        • Biology
        • Time
    • The Geologic Factor
      • Parent material —the source of the weathered fragments of rock from which soil is made; solid bedrock or loose sediments that have been transported from elsewhere by the action of water, wind, or ice.
        • Influences chemical composition of soil and plays role in soil development.
          • This influence diminishes with time, as other factors become increasingly important.
    • The Climatic Factor
      • In long run, climate is generally the most influential factor.
        • Temperature and moisture are most significant.
          • High temperatures and abundant moistures accelerate chemical and biological processes in soil.
    • The Topographic Factor
      • Slope and drainage are main features in this factor.
      • Change through lowering of bottom (because of rock weathering and plant root extension) and top of soil layer (because of erosion).
        • Slope and soil drainage
          • Waterlogged soils in valley bottoms
        • Slope and soil depth
            • - Fig. 12-5 Slope is a determinant of soil depth.
    • The Biological Factor
      • Organic matter only small fraction of soil volume, but of utmost importance.
        • Gives life to soil.
        • From living and dead plants and animals
    • The Biological Factor
      • Vegetation provides passageways for drainage and aeration.
      • Pedoturbation —mixing of soil provided by animals, which counteracts tendency of other soil-forming processes to accentuate the vertical differences among soil layers.
      • Microorganisms generate estimated 75% of soil’s metabolic activity.
        • Create humus —decomposed organic matter of utmost important to agriculture.
        • Loosens soil structure, lessens density, and promotes root development.
          • Provides reservoirs for plant nutrients and soil water.
    • The Biological Factor
      • Earthworms
        • Of large variety of animal life in soil, earthworm is most important to soil formation and development.
          • Tunnels facilitate drainage and aeration, deepening of soil profile.
          • Movement creates a crumbly soil structure, which is favorable for plant growth.
          • Movement brings in leaf litter, which fertilizes subsoil.
          • Digestive actions and tunnels increase porosity and help soil impact of raindrops, which helps deter erosion.
          • Casts, which are the inorganic material worms excrete, increase nutrients of soil through their physical and chemical nature.
          • Movement also brings deeper material to surface, where it can be weathered more rapidly.
          • Movement, digestive action, and decomposition of own bodies help promote nitrification.
    • The Biological Factor
      • Microorganisms in the Soil
        • An estimated three quarters of a soil’s metabolic activity is generated by microorganisms.
        • Microbes decompose organic material into humus (a dark adhesive of minute particles).
        • This makes nutrients usable by plants.
    • The Chronological Factor
      • Most soil develops with geologic slowness: changes imperceptible within human lifespan.
        • Nonrenewable resource:
          • Can be degraded through erosion or depletion of nutrients in just a few years.
    • Soil Components
      • Four neutral components to soil:
        • Inorganics
        • Organics
        • Air
        • Water
    • Inorganic Materials
      • Bulk of soil is mineral matter.
      • Half of average soil is small, granular mineral matter called sand and silt.
        • Mineral composition depends on parent material.
          • Quartz (silica, SiO2) most common.
      • Clay provides an important reservoir for plant nutrients and soil water.
        • Only clay particles take part in the intricate chemical activities that occur in soil.
        • Negatively charged, so attracts positively charged nutrients.
          • Cation —an atom or group of atoms with a positive electrical charge.
    • Organic Matter
      • Varies from alive to dead, partially decomposed to completely decomposed.
      • Litter —the collection of dead plant parts that accumulate at the surface of the soil.
      • Decomposition rates depend on climate.
    • Soil Air
      • Pore spaces make up more than half the volume of average soil.
        • Allow water and air to penetrate.
          • Soil air is saturated with moisture, rich in carbon dioxide, and poor in oxygen.
            • Plants, roots, and soil organisms remove oxygen from and respite carbon dioxide into pore spaces.
    • Soil Water
      • Water performs number of important functions:
        • Dissolves essential nutrients for plant roots;
        • Helps complete necessary chemical reactions;
        • Assists microorganisms producing humus;
        • Mixes soil particles.
        • Four forms of soil moisture
            • - Fig. 12-12
      • Gravitational Water
      • (Free Water)
      • Capillary Water
      • (Water of Cohesion)
      • Hygroscopic Water
      • (Water of Adhesion)
      • Combined
    • Soil Water
      • Leaching —the process in which gravitational water dissolves soluble materials and carries them downward in solution to be redeposited at lower levels.
      • Eluviation —the process by which gravitational water picks up fine particles of soil from the upper layers and carries them downward.
      • Illuviation —the process by which fine particles of soil from the upper layers are deposited at a lower level.
    • Soil Water
      • Soil–Water Budget —the relationship between gain, loss, and storage of soil water (percolation of rainfall or snowmelt vs. evapotranspiration).
        • Field capacity —the maximum amount of water that can be retained in the soil after the gravitational water has drained away.
          • Most of the pore spaces are filled with water.
        • Wilting point —the point at which plants are no longer able to extract moisture from the soil because the capillary water is all used up or evaporated.
        • Soil–water budget —an accounting that demonstrates the variation of the soil – water balance over a period of time.
        • Four forms of soil moisture: gravitational water, capillary water, hygroscopic water, combined water.
          • Gravitational water is mostly superfluous to plant development, while capillary water is the most important.
    • Soil Properties
      • Color
        • Most conspicuous property, and can provide clues to nature and capabilities.
        • 175 gradations of color.
    • Soil Properties
      • Texture
        • No soil is made up of particles of uniform size.
        • Texture is determined by the relative amounts of various separates present.
          • Separates —the size groups within the standard classification of soil particle sizes.
          • Three principal types of soil separates:
            • Sand
            • Silt
            • Clay
          • Loam —a soil texture in which none of the three principal soil separates— sand, silt, and clay—dominates the other two.
    • Soil Properties
      • Structure
        • Ped —a larger mass or clump in which individual soil particles tend to aggregate; determines the structure of the soil.
          • Four basic ped shapes:
            • spheroidal, plate-like, block-like, prism-like.
        • These four shapes give rise to seven generally recognized soil types (see Fig. 12–16, page 349).
        • Structure is key in determining soil’s porosity and permeability.
    • Soil Chemistry
      • Intricate series of chemical reactions determine the presence and availability of nutrients.
    • Colloids
      • Colloids —organic and inorganic microscopic particles of soil that represent the chemically active portion of particles in the soil.
        • Smaller than about 0.1 micrometer.
        • Inorganic colloids consist of clay in thin, crystalline, platelike forms.
        • Organic colloids consist of decomposed organic matter (humus).
        • Major determinants of water-holding capacity of soil.
        • Structure types (continued)
            • - Fig. 12-18
    • Cation Exchange
      • Colloidal complex —the combination of colloid and attached cations.
        • Created by colloid’s negative charges attracting swarms of nutrient cations (positively charged).
        • Too weak of bond allows nutrients to leach away; too strong means plants won’t be able to absorb.
        • Cation exchange capacity (CEC) —capability of soil to attract and exchange cations.
        • Generally, the higher the CEC, the more fertile the soil.
        • Most fertile soils tend to be those with a notable clay and humus content; both have high-CEC activity.
    • Acidity/Alkalinity
      • Acid —chemical compound that produces hydrogen ions or hydronium ions when dissolved in water.
      • Base —chemical compound that produces hydroxide ions when dissolved in water.
      • Acidity —measure of dissolved acids in a solution.
        • Highly acidic solution is likely to dissolve and leach away nutrients too rapidly for plants to absorb them.
      • Alkalinity —measure of dissolved bases in a solution.
        • Overly alkaline soil solution is inefficient in dissolving minerals and releasing nutrients.
      • Scale for measuring acidity and alkalinity ranges from 0 to 14 pH.
        • Based on relative concentration of hydrogen ions.
        • pH value of 7 is neutral, and that value is most suitable for great majority of plants and microorganisms.
    • Soil Profiles
      • Four processes deepen and age soils:
        • Addition, loss, translocation, and transformation.
        • Five soil-forming factors influence the rate of these processes.
    • Soil Profiles
      • Horizon —a more or less distinctly recognizable layer of soil, distinguished from another by differing characteristics and forming a vertical zonation of the soil.
        • Six different horizons: O, A, E, B, C, R.
          • O (organic litter; not typical for soils to have)
          • A (topsoil; mineral and organic)
          • E (eluvial layer; concentration of sand and silt particles)
          • B (subsoil; mineral layer that contains materials removed from E level)
          • C (unconsolidated regolith; no organic matter)
          • R (bedrock)
      • Soil Horizons
        • O horizon
          • Organic matter
        • A horizon
          • Top soil/dark color
        • E horizon
          • Eluviation layer
        • B horizon
          • Illuviation layer
        • C horizon
          • Beyond reach of roots
        • R horizon
          • bedrock
            • - Fig. 12-22
    • Soil Profiles
      • Soil profile —a vertical cross section from Earth’s surface down through the soil layers into the parent material beneath.
      • Solum —the true soil that includes only the top four horizons.
      • Water plays critical role in development of profile.
      • Time also important.
        • Formation of B horizon normally indicates mature soil.
            • -Fig. 12-25: Map of Major Soil Orders
            • Figure 12-28. The general relationship among the soil orders in terms of weathering, soil development, and broad environmental conditions.
      • Global Distribution of Major Soils
      • Entisols
        • “ Ent”, from rec ent formation
        • Very little profile development
          • Thin and sandy
          • Low fertility
      • Global Distribution of Major Soils
            • - Fig. 12-27
      • Inceptisols
        • Latin Inceptum , “beginning”, young
        • Few Diagnostic Features
          • Faint horizons
        • Tundra, mountains, old valley bottoms
            • - Fig. 12-28
      • Andisols
        • “ Andi”, andesite (a lava rock) named after the Andes Mts.
        • Volcanic ash soils
        • Mild weathering
        • Inherently fertile
            • - Fig. 12-29
      • Gelisols
        • Latin gelatio , “freezing”
        • Permafrost layer
        • Young soils
        • Arctic and subarctic regions
          • Cryoturbation
            • - Fig. 12-30
      • Histosols
        • Greek histos , “living tissue”
        • Organic soils
        • Waterlogged conditions
          • Glaciated areas
          • Poorly drained coastal areas
            • - Fig. 12-31
      • Aridisols
        • Latin aridus , “dry”; dry soils
        • Thin, low organic content
        • High in soluble minerals
        • Unproductive due to lack of moisture
            • - Fig. 12-32
      • Vertisols
        • Latin verto, “turn”
        • Swelling and cracking clays
        • Alternating wet and dry climate
          • Churning effect inhibits soil-horizon development
            • - Fig. 12-33
      • Mollisols
        • Latin mollis , “soft”; soft soils
        • Best agricultural soil
          • Rich clay-humus content
        • Central Eurasia, Pampas of Argentina, North American Great Plains
            • - Fig. 12-34
      • Alfisols
        • “ al” for aluminum, “f” for iron
        • Moderate leaching
          • Subsurface clay accumulation with high nutrient bases
        • Second to Mollisols in fertility
            • - Fig. 12-35
      • Ultisols
        • Latin ultimus, “last”; last of their nutrient bases leached out
          • Low fertility due to leaching
          • Reddish color throughout
        • Possible fate of Alfisols
            • - Fig. 12-36
      • Spodisols
        • Greek spodos , “wood ash”
        • Light color due to heavy leaching
          • Notoriously infertile
        • Acid, sandy forest soils
          • Forms under coniferous forest
            • - Fig. 12-37
      • Oxisols
        • “ Ox”, large amount of ox ygen containing compounds
        • Highly weathered and leached
          • laterization (Alt. : Latosols)
          • Infertile
        • Humid tropics
            • - Fig. 12-38
      • Distribution of Soils in the United States
            • - Fig. 12-39