8) Soils Overview

What are Soils?

  • Composed of minerals, soil organic matter, living organisms, gas, and water.
  • Soil minerals: clay, silt, and sand.
    • Percentages of these determine soil texture.
  • Mineralogy is diverse (e.g., smectite, quartz).
  • Soil organic matter: plant, animal, and microbial residues.
    • Indicates agricultural soil quality.

Soil Formation

  • Soils can be young (from landslides, volcanic eruptions, glacial retreat) or very old.
  • Change over time through biological, chemical, and physical processes (horizon formation, weathering, nutrient leaching).
  • Five factors of soil formation: climate, organisms, topography, parent material, and time.

Soil Horizons

  • O Horizon: high percentage of soil organic matter.
  • A Horizon: darkened by organic matter accumulation.
  • E Horizon: removal (eluviation) of clays, organic matter, iron, or aluminum; lightened color.
  • B Horizon: subsurface, transformed by soil formation processes (color/structure development, deposition of materials).
  • C Horizon: minimally affected by soil formation processes.
  • R Horizon: Bedrock.

Major Soils: Humult and Mor

  • Decomposition altered due to soil acidification (pH < 4.2).
  • Reduced decomposition leads to accumulation of leaves and organic matter.
  • Sandy or sandy-clay loam mineral base.
  • Tropical humid podspoil: litter accumulates, soluble organic compounds precipitate, forming a humic pan.
  • Soil pH < 4.2 affects vegetation and animal species.

Peat Swamp Soil

  • Seasonal or periodic flooding from eutrophic river water.
  • Anoxic subsoil conditions confine roots near the surface.
  • Anaerobic condition increases soil acidity (pH: 3.7-3.2) and slows decomposition.
  • Litter layer develops above flood level.

Soil Fertility

  • Plants need major and minor elements (Nitrate N+N^+, Sodium NaNa, Potassium KK, Phosphorous PP).
  • Elements originate from atmospheric N or decomposed organic matter.
  • Element availability to plants can be limited (Calcium, Potassium, Phosphorous).
  • Lowland tropical soils are often poor due to leaching.

Nutrient Cycling

  • Rainforests have poor, infertile soil (spodosoil and oxisol).
  • Soil carbon determines nutrient-holding capacity and is an index of fertility.
  • Fine-grained organic matter improves soil structure and pH buffering.
  • Organisms decompose soil quickly, maintaining nutrients at the surface.