1.2 Prelab Soil Profile - Soil Science and Rock Identification Notes

Objectives of Soil Profile Exercise

  • Visual Differences in Soil Horizons:

    • Identify major soil horizons: A, B, and C and their characteristics in the field.

  • Parent Material:

    • Familiarize with common rocks and minerals that constitute parent materials in important Philippine soils.

  • Distinguishing Properties:

    • Recognize properties of rocks and minerals (composition, contribution to soil).

  • Influence on Soil Formation:

    • Understand how rocks and minerals influence soil types.

Soil Horizons Overview

  • O Horizon:

    • Surface litter, includes fallen leaves and decomposed organic debris.

  • A Horizon (Topsoil):

    • Contains organic matter (humus), living organisms, and inorganic minerals.

  • E Horizon (Zone of Leaching):

    • Area where dissolved materials move downward.

  • B Horizon (Subsoil):

    • Accumulated aluminum, iron, clay, and humic compounds leached from A and E horizons.

  • C Horizon (Weathered Parent Material):

    • Composed of partially broken-down inorganic minerals.

  • R Horizon (Bedrock):

    • Impenetrable layer beneath the soil.

Distinguishing Properties of Soil Horizons

  • Depth:

    • Thickness of each horizon.

  • Texture:

    • Soil coarseness: gritty, smooth, sticky, etc.

  • Structure:

    • Shapes of soil aggregates: granular, blocky, platy, prismatic.

  • Presence of Roots:

    • Indicates biological activity.

  • pH Levels:

    • Indicates soil acidity or alkalinity.

  • Coloring:

    • Descriptive coloring or Munsell color chart measurements.

  • Consistency:

    • Cohesion of soil particles.

  • Boundary Characteristics:

    • Form (smooth, wavy, broken) and distinctness (clear, gradual).

  • Presence of Mottles:

    • Color variations indicating soil composition.

  • Concretions:

    • Grains or pellets cemented by CaCO3 or FeO.

  • Iron Content:

    • Indicates weathering stages of soil.

  • Organic Matter Content:

    • Estimated by the darkness of soil surface.

How to Describe Soil Boundaries

  • Distinctness:

    • Sharp, abrupt, indistinct, diffuse.

  • Shape:

    • Smooth, wavy, irregular, convolute.

Soil Structure Types

  • Granular Structure:

    • Common in A horizon, has crumb-like aggregates.

  • Blocky Structure:

    • Angular blocks that fit together; common in B horizon.

  • Platy Structure:

    • Flat plates of soil aggregates, layered appearance.

  • Prismatic Structure:

    • Vertical columns of soil, often found in subsoil/B horizon.

Rock Description and Identification

  • Rocks:

    • Aggregates of one or more minerals.

    • Three classifications:

    • Igneous: Formed from magma.

    • Sedimentary: Formed from fragments of other rocks.

    • Metamorphic: Transformed by heat and pressure.

Classes of Rocks

  • Igneous Rocks: Formed from solidification of magma.

  • Sedimentary Rocks: Created from sediment compaction.

  • Metamorphic Rocks: Altered igneous or sedimentary rocks due to heat and pressure.

Igneous Rocks Examples GDBO

  • Granite: Quartz, K-feldspar, biotite.

  • Diorite: Plagioclase, amphibole, quartz.

  • Basalt: Pyroxene, plagioclase, olivine.

  • Obsidian: Volcanic glass.

Sedimentary Rocks Examples SSL

  • Sandstone: Medium-sized fragments.

  • Shale: Small clay particles.

  • Limestone: Composed of CaCO3.

Metamorphic Rocks Examples GMS

  • Gneiss: Derived from granite.

  • Marble: Derived from limestone.

  • Slate: Derived from shale.

Weathering of Rocks

  • Physical Weathering: Mechanical breakdown without chemical alteration.

  • Chemical Weathering: Alteration of minerals due to chemical reactions.

  • Biological Weathering: Breakdown due to biological organisms and processes.

Physical Weathering Processes

  • Frost Wedging: Freeze-thaw cycles cause cracks.

  • Thermal Expansion: Expansion and contraction from temperature changes.

Chemical Weathering Processes

  • Dependent on water presence and environmental conditions (temperature, precipitation).

Biological Weathering Processes

  • Involves tree roots, animal activity, and microorganisms/lichens.

Mineral Description and Identification

  • Composition: Defined chemical make-up of minerals.

  • Crystal Form: Geometric arrangement of crystal components.

Physical Properties of Minerals

  • Color: Reflective property, less reliable for identification.

  • Cleavage: Tendency to break along smooth planes.

  • Fracture: Breaks irregularly.

  • Streak: Color of mineral powder.

  • Luster: How light interacts with the mineral surface (metallic vs. non-metallic).

  • Specific Gravity: Weight ratio to water volume.

  • Hardness: Resistance to scratching (Mohs Scale).

Mohs Hardness Scale

  • Ranks minerals based on scratch resistance, from talc (1) to diamond (10).

Rock Forming Minerals

  • Igneous: Feldspar, mica, amphibole, quartz.

  • Sedimentary: Quartz, calcite, feldspar.

  • Metamorphic: Quartz, feldspar, micas.

Conclusion

  • Understanding soil profiles and the influence of rocks and minerals is crucial for soil science and agricultural practices. Incorporating knowledge of mineral identification and weathering processes aids in comprehending soil characteristics and behavior.