Chapter twelve
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  • 1. Title Page Photo “ A nation that destroys it's soils destroys itself.” — Franklin D. Roosevelt (Brainyquote.com)
  • 2. 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)
  • 3. 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.
  • 4. Soil-Forming Factors
    • Five factors are involved in forming soil:
      • Geology
      • Climate
      • Topography
      • Biology
      • Time
  • 5. 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.
  • 6. 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.
  • 7. 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).
  • 8.
      • Slope and soil drainage
        • Waterlogged soils in valley bottoms
      • Slope and soil depth
          • - Fig. 12-5 Slope is a determinant of soil depth.
  • 9. 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
  • 10. 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.
  • 11. 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.
  • 12. 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.
  • 13. 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.
  • 14. Soil Components
    • Four neutral components to soil:
      • Inorganics
      • Organics
      • Air
      • Water
  • 15. 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.
  • 16. 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.
  • 17. 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.
  • 18. 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.
  • 19.
      • Four forms of soil moisture
          • - Fig. 12-12
    • Gravitational Water
    • (Free Water)
    • Capillary Water
    • (Water of Cohesion)
    • Hygroscopic Water
    • (Water of Adhesion)
    • Combined
  • 20. 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.
  • 21. 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.
  • 22. Soil Properties
    • Color
      • Most conspicuous property, and can provide clues to nature and capabilities.
      • 175 gradations of color.
  • 23. 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.
  • 24. 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.
  • 25. Soil Chemistry
    • Intricate series of chemical reactions determine the presence and availability of nutrients.
  • 26. 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.
  • 27.
      • Structure types (continued)
          • - Fig. 12-18
  • 28. 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.
  • 29. 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.
  • 30. Soil Profiles
    • Four processes deepen and age soils:
      • Addition, loss, translocation, and transformation.
      • Five soil-forming factors influence the rate of these processes.
  • 31. 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)
  • 32.
    • 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
  • 33. 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.
  • 34.
          • -Fig. 12-25: Map of Major Soil Orders
  • 35.
          • 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
  • 36.
    • Entisols
      • “ Ent”, from rec ent formation
      • Very little profile development
        • Thin and sandy
        • Low fertility
    • Global Distribution of Major Soils
          • - Fig. 12-27
  • 37.
    • Inceptisols
      • Latin Inceptum , “beginning”, young
      • Few Diagnostic Features
        • Faint horizons
      • Tundra, mountains, old valley bottoms
          • - Fig. 12-28
  • 38.
    • Andisols
      • “ Andi”, andesite (a lava rock) named after the Andes Mts.
      • Volcanic ash soils
      • Mild weathering
      • Inherently fertile
          • - Fig. 12-29
  • 39.
    • Gelisols
      • Latin gelatio , “freezing”
      • Permafrost layer
      • Young soils
      • Arctic and subarctic regions
        • Cryoturbation
          • - Fig. 12-30
  • 40.
    • Histosols
      • Greek histos , “living tissue”
      • Organic soils
      • Waterlogged conditions
        • Glaciated areas
        • Poorly drained coastal areas
          • - Fig. 12-31
  • 41.
    • Aridisols
      • Latin aridus , “dry”; dry soils
      • Thin, low organic content
      • High in soluble minerals
      • Unproductive due to lack of moisture
          • - Fig. 12-32
  • 42.
    • Vertisols
      • Latin verto, “turn”
      • Swelling and cracking clays
      • Alternating wet and dry climate
        • Churning effect inhibits soil-horizon development
          • - Fig. 12-33
  • 43.
    • 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
  • 44.
    • Alfisols
      • “ al” for aluminum, “f” for iron
      • Moderate leaching
        • Subsurface clay accumulation with high nutrient bases
      • Second to Mollisols in fertility
          • - Fig. 12-35
  • 45.
    • 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
  • 46.
    • Spodisols
      • Greek spodos , “wood ash”
      • Light color due to heavy leaching
        • Notoriously infertile
      • Acid, sandy forest soils
        • Forms under coniferous forest
          • - Fig. 12-37
  • 47.
    • Oxisols
      • “ Ox”, large amount of ox ygen containing compounds
      • Highly weathered and leached
        • laterization (Alt. : Latosols)
        • Infertile
      • Humid tropics
          • - Fig. 12-38
  • 48.
    • Distribution of Soils in the United States
          • - Fig. 12-39