Rangeland Soils Lecture Notes

Rangeland Soils

Introduction

  • Soils are crucial to life on Earth; without productive soils, life would perish.

Soil Formation

  • The Earth's surface has experienced various influences from five soil-forming factors:

    • Climate

    • Relief (topography)

    • Parent material

    • Time

    • Biota (living organisms)

  • These factors cause different layers, known as horizons, to form in each part of the landscape.

  • Soils are characterized by a sequence of these horizons, which can be exposed to show a soil profile.

Soil Horizons

  • Most soils have three major horizons:

    • Surface Horizon (A): Top layer where organic activity occurs.

    • Subsoil (B): Often where minerals leach and accumulate.

    • Substratum (C): Parent material layer below the subsoil.

  • Some soils may contain an Organic Horizon (O) on the surface, which can be buried under other layers.

  • The E horizon signifies a loss of minerals through the process known as eluviation.

  • R horizon refers to bedrock that is not considered soil.

Horizon Descriptions

  • O Horizon: Organic layer rich in humus.

  • A Horizon: Eluvial layer, topsoil enriched with organic material.

  • B Horizon: Illuvial layer characterized by leached materials.

  • C Horizon: Unconsolidated layer representing parent material.

  • R Horizon: Consists of bedrock.

Soil Composition and Properties

  • Pedons: The smallest sampling unit of soil showing its full range of properties; could occupy 1 to 10 square meters.

  • Polypedons: Groups of closely associated pedons, providing a broader view of soil properties.

  • Soil Texture: Describes the composition of sand, silt, and clay in soil.

  • Soil Structure: Refers to the arrangement of sand, silt, and clay particles into larger particles or aggregates.

  • Aggregates: Clusters of soil particles bound together, significantly affecting water retention capability.

Factors Affecting Soil Structure

  • Influenced by:

    • Parent Material: The mineral content from which the soil originates.

    • Transport Processes: Includes wind, water, ice, and gravity.

    • Weathering: Physical and chemical processes that break down parent material.

    • Biological Activity: Roots, microbes, and soil fauna play essential roles in structure formation.

  • Control of:

    • Aggregate shape (e.g., single-grained, blocky, granular).

    • Porosity and permeability: Influences how well water infiltrates and drains.

    • Water infiltration and drainage coupled with root penetration abilities.

    • Nutrient retention represented by Cation Exchange Capacity (CEC).

Soil Structure Types

  • Examples include:

    • Granular: Small, rounded aggregates.

    • Platy: Thin, horizontal plates.

    • Single Grain: Individual grains without aggregation.

    • Blocky (Subangular/Angular): Irregular blocks, with subangular having rounded edges.

    • Prismatic: Tall, vertical columns.

    • Columnar: Rounded tops of vertical columns.

    • Massive: Soil behaves as one coherent mass with no visible aggregates.

Implications of Soil Structures for Agriculture

  • Single Grained:

    • Characteristics: High aeration, good drainage, low nutrient retention, quick drying.

    • Agricultural Implications: Requires frequent fertilization and irrigation.

  • Granular Structure:

    • Aggregate size: 1–10 mm.

    • Implications: Excellent for crops due to high microbial activity and good texture for root growth.

  • Blocky Structure:

    • Aggregate size: 5–50 mm.

    • Implications: Can restrict roots and may retain moisture better in dry conditions.

  • Platy Structure:

    • Issues with root penetration and water infiltration; management through deep tillage is necessary.

  • Prismatic Structure:

    • Restricted horizontal root growth; vertical drainage is moderate.

  • Columnar Structure:

    • Difficult for root penetration and requires management strategies like gypsum application.

  • Massive Structure:

    • Poor aeration and root growth restriction; management through subsoiling necessary.

Soil Infiltration

  • Infiltration: The rate at which water enters soil; varies based on texture, structure, and surface condition.

  • Soil types affect infiltration rates:

    • Single-grained: rapid infiltration.

    • Platy: slow infiltration.

    • Granular: moderate infiltration efficiency.

Soil Classification

  • Twelve Soil Orders: Including Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols are briefly described in their characteristics and where they form.

Notable Soil Orders

  • Alfisols: Clay-rich with high fertility, mainly found in humid climates.

  • Andisols: Volcanic soils with excellent root growth potential due to low density; nutrient management is crucial.

  • Aridisols: Low fertility, arise in arid conditions, facing significant salinity issues.

  • Vertisols: Clay-rich, characterized by shrink-swell phenomena affecting agricultural practices.

Effect of Cultivation on Soil Structure

  • Long-term cultivation decreases aggregate formation and pore space, often resulting in compacted layers that hinder water movement.

  • Understanding these effects enables better management practices to maintain soil health such as organic matter additions and avoiding practices that lead to compaction.

Conclusion

  • The study of rangeland soils encompasses understanding soil horizons, structures, and the implications of these properties in agricultural practices. Effective management relies heavily on recognizing these factors and employing sustainable techniques to maintain soil productivity.