Soils of the Pacific Islands: A Comprehensive Overview

5.1 What is Soil?

  • Soil is the surface layer of the earth where plants grow.
  • It's a mixture of minerals, organic matter, water, and air.
  • Soil is a crucial resource.
  • It is consolidated, meaning it can be broken up by hand.
  • Soil has distinct horizontal layers called horizons.

Why Study Soils?

  • Soil supports plant and animal life.
  • It serves as both a source and a sink for sediments.
  • Soil plays a role in landscape development.
  • It acts as a filter for water and chemicals, removing contaminants.
  • Soil provides a foundation for buildings and infrastructure.

5.2 Soil Forming Processes

  • Pedogenesis: The process of soil formation on the earth's surface.
  • Four main processes:
    • Additions: Materials added to the soil (e.g., sediments from rivers, humus).
    • Losses: Materials lost from the soil through leaching (washing away of minerals by water).
    • Transformation: Change of materials from one form to another (e.g., decomposition of organic matter into inorganic nutrients).
    • Translocation: Movement of minerals from one part of the soil to another (e.g., eluviation – movement of fine particles downwards).

5.3 Factors Influencing Soil Formation

  • Soil formation is influenced by six factors: climate, organic matter, relief, parent material, time, and human influence.

1. Climate

  • Soil varies with climate (latitude and longitude).
  • Climate controls biological and chemical activity in the soil.
  • Soil temperature: Varies based on weather and climate.
  • Soil moisture: Balance between rainfall and evapotranspiration (water vapor lost from leaves).

2. Organic Matter

  • Includes plant and animal matter in and on top of the soil.
    • E.g., earthworms, ants, bacteria, and fungi provide energy to the soil system.
  • Processes:
    • Cycling: Animals ingest soil material, move, and excrete it elsewhere; humus (dead plants/animals) decomposes and returns nutrients.
    • Additions: Adds basic elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and silica) to the soil.
    • Modification of chemical processes: Decomposing plant material forms organic acids, changing the soil's pH (acidity or alkalinity).
    • Mixing (Bio-turbation): Trees falling over pull up soil from lower layers and deposit it on higher layers; animals mix the layers.
    • Insulating effects: Vegetation cover keeps soil cooler than exposed soil.
    • Modification of erosion and deposition processes: Vegetation reduces physical weathering.

3. Relief (Topography)

  • Shape and slope of the land.
    • Elevation: (height above sea level) Influences soil temperature and moisture.
    • Slope Aspect: (direction slope faces) affects radiation on the soil. In the Southern Hemisphere, a north-facing slope is sunnier and drier with more flora and fauna.
    • Slope Steepness: Determines soil drainage. Steeper slopes lead to less infiltration, faster runoff, and more landslides, soil slumps, and soil creep.

4. Parent Material

  • Unconsolidated matter on the earth’s surface (e.g., rock, loess, alluvium), not necessarily the underlying bedrock.
  • Affects the minerals, structure, and color of the soil.
  • Its influence diminishes over time.
    • Physical Weathering: Parent material breaks into pieces through freezing, thawing, abrasion, heating, and cooling.
    • Chemical Weathering: Rock and mineral matter decompose.

5. Time

  • Biological, physical, and chemical reactions in soil formation take time.
    • Isotropic: Initially, soil properties are the same throughout.
    • Anisotropic: Over time, soils develop thickness, horizons, and differences in properties throughout the profile.

6. Human Activities

  • Activities modify soil properties and characteristics, including settlements, agriculture, waste dumping, irrigation, excavation, mining, deforestation, and burning.
  • Different soil types influence human activities.
    • Fertile soils support more farming.
    • Infertile soils may have manure or fertilizers added.
    • Dry soils may have irrigation projects.

5.4 Soil Description

  • Systematic identification of the physical and chemical properties of a soil profile.
  • Understanding soil types and properties helps in utilizing soil effectively.
  • Soil is described through studying the soil profile.

a) Soil Profiles

  • Soil profile: Cross-section view of soil horizons.
  • Soil horizons: Distinctive horizontal layers indicating mature soil.
  • Soil porosity: Cracks or spaces in the soil.
  • Soil permeability: Rate at which air and water move through the soil.

Soil Horizons

  • O Horizon: Litter or humus layer on top of the soil.
  • A Horizon: Topsoil or zone of leaching below the O Horizon.
  • B Horizon: Composed of inorganic materials; zone of accumulation (leached and weathered parent rock).
  • C Horizon: Broken down or weathered material on top of the underlying bedrock.

b) Physical Properties of Soil

  • Soil Colour: Indicates soil properties (e.g., black soils = high organic matter, reddish soils = high iron content, light grey soils = waterlogged).
  • Soil Texture: Size differences of soil particles (e.g., small particles = silts, fine particles = clay, mixture = loam).
  • Soil Consistency: Cohesion of particles.
    • Stickiness: How sticky wet soil particles are.
    • Plasticity: Ability to roll wet soil into a rod without breaking.
    • Hardiness: How hard the soil particles are when dry.

5.5 Soil Structure

  • Level of aggregation within soil particles.
  • A single aggregate is called a ped.
  • Important for agriculture as it determines water and air infiltration.
  • Determines a soil's proneness to erosion and ease of cultivation.

Types of Soil Structure:

  • Granular: Resembles cookie crumbs (0.5 cm).
  • Blocky: Irregular blocks (1.5 – 5.0 cm).
  • Prismatic: Vertical columns of soil.
  • Platy: Thin, flat plates lying horizontally (compacted soil).
  • Columnar: Vertical columns with a soil cap.
  • Single Grained: Individual particles that don't stick together.
  • Massive: No visible structure; hard to break apart.

5.6 Soil Degradation and Conservation

  • Soil degradation: Abuse of soils that reduces its productive capacity.
  • Normal functions: Maintaining or supplying nutrients, air & water, root support, habitat.

Main Causes of Soil Degradation:

  • Deforestation: Removal of plants/forest cover exposes soil to erosion, leaching, salinization, and desertification.
  • Overgrazing: Compacts soils, causes erosion and desertification.
  • Over Cultivation: Depletes soil fertility and structure, leading to erosion, leaching, and desertification.
  • Rubbish and Chemical Dumping: Prevents natural filtering, impacts water quality and food chain.
  • Human induced landscape changes: Excavation for mining and roads results in landslides and erosion.
  • Over Population: Increases demands on soils for agriculture, housing, and urban sprawl.

Soil Conservation Methods:

  • Methods to reduce erosion, prevent nutrient depletion, and restore lost nutrients.
  • Types: Terracing, contour farming, strip cropping, agroforestry, shelter belts, etc.

Ways to Carry Out Soil Conservation:

  • Reforestation
  • Declaration of nature reserves
  • Rotational grazing and cultivation
  • Increased use of natural fertilizers
  • Reduction in chemical dumping
  • Increased waste recycling
  • Community awareness programs
  • Government implementation of strict soil conservation programs during development.

5.7 Soils of the Pacific

  • Range from the most fertile to the most infertile.

Main Types:

  • Highly weathered iron-rich soils.
  • Mountain soils.
  • Dark, sticky clay-rich soils.
  • Young volcanic, thin easily eroded sloped soils.
  • Poorly drained swamp or Hydromorphic soils.
  • Alluvial and colluvial soils.
  • Sandy coral-limestone soils.

a. Highly Weathered Iron Rich Soils

  • Reddish to yellow soils (ferralitic or lateritic).
  • Iron-rich, susceptible to leaching.
  • Best kept under forest cover.
  • Found in Rarotonga (Cook Islands), Solomon Islands, and Fiji.

b. Mountain Soils

  • Also known as mountain Podzols.
  • Found in the highlands of Irian Jaya and Papua New Guinea.
  • Cold climate slows decomposition, resulting in high organic matter and humus.
  • Dark brown to grey in color.
  • Highly acidic; useful with lime addition.

c. Dark Brown to Black Clay Rich Soils

  • Also known as 'vertisols'.
  • Found in low lying areas between hills.
  • Common in western Fiji; good for sugar cane farming.

d. Young Volcanic Soils

  • Black clay rich soils.
  • Recently developed soils of the Pacific.
  • Richest soils in the world (andosols).
  • Made up of andesitic and basaltic soils providing many plant nutrients.
  • Found in volcanic active islands in Samoa, Tanna (Vanuatu), Taveuni (Fiji), New Britain (PNG), Rarotonga (Cook Islands), Tahiti (French Polynesia), and the Hawaiian Islands.

e. The Easily Eroded Slope Soils

  • Thin soils found on steep slopes.
  • Thin due to high erosion rates.
  • Vegetation cover should be maintained to avoid complete erosion.
  • Known as lithosols or skeletal soils.

f. Poorly Drained or Hydromorphic Soils

  • Swamp soils known as 'Hydromorphic or gley soils'.
  • Found in lowland areas with high water levels.
  • Plant nutrients are not leached out, so the soil is very fertile.
  • Usually grey due to the gleying process.
  • Good for rice, taro, and sugar cane.
  • Found in Papua New Guinea, Fiji, Samoa, Cook Islands, and Tahiti.

g. Alluvial and Colluvial Soils

  • Product of erosion and deposition processes.
  • Young soils, fertile and rich in minerals.
  • Useful for subsistence and commercial agriculture.

h. Coral Limestone Soils

  • Found on low lying coral atoll islands.
  • Poorest soils of the Pacific Islands (calcareous soils).
  • People in Tuvalu and Kiribati add mulch and use baskets (coconut leaves) to plant giant swamp taro.
  • Other vegetation: breadfruit, pawpaws, pandanus, bananas, and coconut trees.