Title Page Photo “ A nation that destroys it's soils destroys itself.” — Franklin D. Roosevelt (Brainyquote.com)
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.
- Five factors are involved in forming soil:
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).
- Waterlogged soils in valley bottoms
- - 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.
- 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
- 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.
- Can be degraded through erosion or depletion of nutrients in just a few years.
- Four neutral components to soil:
- 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.
- 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.
- 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.
- Water performs number of important functions:
- Dissolves essential nutrients for plant roots;
- Helps complete necessary chemical reactions;
- Assists microorganisms producing humus;
- Four forms of soil moisture
- 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 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.
- Most conspicuous property, and can provide clues to nature and capabilities.
- 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:
- Loam —a soil texture in which none of the three principal soil separates— sand, silt, and clay—dominates the other two.
- Ped —a larger mass or clump in which individual soil particles tend to aggregate; determines the structure of the soil.
- 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.
- Intricate series of chemical reactions determine the presence and availability of nutrients.
- 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)
- 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.
- 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.
- Four processes deepen and age soils:
- Addition, loss, translocation, and transformation.
- Five soil-forming factors influence the rate of these processes.
- 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)
- 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.
- 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
- “ Ent”, from rec ent formation
- Very little profile development
- Global Distribution of Major Soils
- Latin Inceptum , “beginning”, young
- Tundra, mountains, old valley bottoms
- “ Andi”, andesite (a lava rock) named after the Andes Mts.
- Latin gelatio , “freezing”
- Arctic and subarctic regions
- Greek histos , “living tissue”
- Poorly drained coastal areas
- Latin aridus , “dry”; dry soils
- Thin, low organic content
- Unproductive due to lack of moisture
- Swelling and cracking clays
- Alternating wet and dry climate
- Churning effect inhibits soil-horizon development
- Latin mollis , “soft”; soft soils
- Central Eurasia, Pampas of Argentina, North American Great Plains
- “ al” for aluminum, “f” for iron
- Subsurface clay accumulation with high nutrient bases
- Second to Mollisols in fertility
- Latin ultimus, “last”; last of their nutrient bases leached out
- Low fertility due to leaching
- Possible fate of Alfisols
- Greek spodos , “wood ash”
- Light color due to heavy leaching
- Forms under coniferous forest
- “ Ox”, large amount of ox ygen containing compounds
- Highly weathered and leached
- laterization (Alt. : Latosols)
- Distribution of Soils in the United States