Soil as Ecological Factor

Introduction

• Soil consists of weathered layer of Earth’s crust with products of decay of living organisms intermingled.
• In soil, thus, abiotic & biotic components are intimate.
• Soil is considered to be chief organizing centre for land ecosystems.
• Pedosphere: Outermost layer of earth that is composed of soil & subject to its formation.
• Pedogenesis: Process of formation of soil.
• Pedology: Study of soil.
• Soil is said to be composed of five categories of components:
  • Mineral matter
  • Soil organic matter (Humus)
  • Soil water
  • Soil Atmosphere
  • Biological system.

Formation of Soil

Process of soil formation is generally divided into two stages:

1. Weathering:
   • Breakdown of large rocks into smaller & fine particles.
   • It can be brought about by:
     • Mechanical factors (e.g. wind, water, ice, temperature, oceanic waves, sun energy etc.)
     • Biological factors (e.g. Plant roots, soil organisms, cultivation by man, animal movements etc.)
     • Chemical factors (e.g. Hydration, hydrolysis, oxidation, reduction, carbonation, chelation etc.)
2. Soil Development:
   • The interaction of above weathering factors further & especially interaction of biological phenomenon causes rock debris to convert into true soil.
   • Lichens, bacteria, fungi, algae, arthropods, molluscs etc. release various enzymes, acids etc. + addition of organic matter after their death, brings about geochemical, biochemical and biophysical processes leading to true soil formation.

Soil activity is said to be concentrated in root zones (rhizospheres), organic aggregates etc. termed “hot spots”.

Soil Profile

• Vertical section of well-developed soil shows presence of distinct layers called horizons & their sequence from surface down is called soil profile.
• There are five major horizons which are further subdivided:
  • O – horizon (Organic horizon): Composed of fresh or partially decomposed organic matter.
    • O₁ (A₀₀) region – (Litter)
    • O₂ (A₀) region – Consists duff (fairly decomposed matter).
  • A – horizon (Mineral horizon): First mineral layer. Characterized by accumulation of organic matter. Mineral horizons show downward loss of soluble salts, clay, iron, aluminium, being consequently rich in silica or other resistant minerals – the phenomenon termed eluviation/leaching.
    • A₁ region: (Humus) Dark, rich in organic matter. Here, amorphous, finely divided organic matter become mixed with minerals.
    • A₂ region: Light coloured layer with more mineral particles and little organic matter. Also called eluvial zone.
  • B – horizon (Mineral horizon): Composed of mineral soil in which organic compounds have been converted to inorganic compounds by process of mineralization and thoroughly mixed. Silica rich organic compounds, hydrated oxides of aluminium, iron etc. are present here. The horizon is also called illuvial zone, as chemicals leached from A₂ collect here.
  • C – horizon (Mineral horizon): Contains more or less unmodified parent material.
  • R – horizon (Bed rock): Consists of parent rock from which soil is formed.
• A₁ + A₂ + B horizons = Solum or Mineral Soil.
• Thickness of horizons: Vary with different climatic regions & topographical conditions.
• Grassland soils: Subject to rapid humification, slow mineralization. Little litter or duff but more humus present.
• Forest soils: Subject to rapid mineralization. Litter, root decay is slow, therefore, less humus present.

Classification

Major types of soil in world:

1. Aridisols (desert soils)
2. Inceptisols (weakly developed soils)
3. Alfisols (moderately weathered forest soils)
4. Entisols (recent soils, profile undeveloped)
5. Oxisols (Tropical soils)
6. Mollisols (grassland soils)
7. Ultisols (highly weathered forest soils)
8. Spodosols (northern conifer forest soils)
9. Vertisols (expandable clay soils)
10. Histosols (organic soils)
11. Miscellaneous soils (e.g. Steep Mountains etc.)

Soil Structure

Primary soil particles – sand, slit and clay, usually occur in form of aggregates contributing to structure of soil.

Peds – Aggregates of soil formed as a result of pedogenesis, i.e. natural process. Broken peds are called fragments.

Clods – Aggregates of soil formed artificially.

Four types of soil structures are observed, based on shapes of final aggregates:

1. Prism-like: Vertical axis more pronounced. Flattened sides. Pillar like appearance.

Two types –

• Columnar: When top of the pillar is round.
• Prismatic: When top of the pillar is flat.

2. Plate-like or Platy: Horizontal dimensions are more pronounced than vertical ones.

Two types –

• Platy: When units are thick.
• Laminar: When units are thin.

3. Block-like: All three dimensions are of same size and aggregates form a cubical structures with flat or rounded faces.

4. Sphere-like or spheroid: All axes of aggregates are more or less of same length with curved or irregular faces. These are of two types –

• Granular: When aggregates are less porous.
• Crumby: When aggregates are more porous.

Significance of Soil Structures:

• Chiefly influences amount & nature of porosity, regulating amount of moisture and air in soil.
• Platy structures said to hinder soil drainage.
• Spheroidal soils are supposed to be best for plant’s growth.
• Crumby and granular structures are said to be for favourable physical properties of soil.

Climate

Soil temperature: Dark exposed soils absorb heat more rapidly. Daily temperature fluctuations can be felt in upper layers whereas seasonal fluctuations in deeper layers. Absorption of heat by soil depends on exposure to sun rays (insolation), atmospheric conditions, local soil characteristics etc.

Soil moisture: Moisture movement and its storage in soil are affected by – nature of soil; soil structure; soil texture; physical forces like capillary, cohesive attractions, absorption, vaporization, condensation etc.

Soil spaces (Pore spaces): Air containing spaces or cavities present in soil influences by soil texture like coarse textured soil sand has more pore spaces than fine textured soil clay; type of vegetation and biota of soil. However, O₂ presence is less than CO₂ in these spaces, in contrast to atmosphere.

Soil Water

Amount of water present in the soil varies with structure & texture of soil, precipitation, evaporation etc. Some of the water falling during rainfall is lost and goes back to the source i.e. sea, ocean etc. is called run off. Remaining water percolates into ground.

Different types of soil water are described below:

1. Gravitational water: Also called free water. It moves downward into soil due to gravity till it reaches water table.
2. Capillary water: Water which fills the spaces between non-colloidal soil particles & is held by capillary forces. This water is available to plants.
3. Hygroscopic water: This water is held by the surface forces of soil particles forming a film around the particle. The water is held tightly by soil particles due to cohesive & adhesive forces and is not available to plants.
4. Combined water: This water is present in form of hydrated oxides of aluminium, iron, silicon etc. in soil.

Holard: Total amount of water present in soil is called holard.

Chresard: The amount of water (gravitational + capillary water) that plants can absorb out of holard is called chresard.

Echard: The amount of water (hygroscopic + combined) that plants can’t absorb is called echard.

Field Capacity: The amount of water that can be retained by soil after gravitational water has been drained away is called field capacity.

Field Capacity = Total amount of capillary water + hygroscopic water + combined water + water vapours

Permanent Wilting Point (PWP) or Wilting Coefficient: Percentage of water left in the soil when plant wilts permanently and can’t recover.

Soil Chemistry

Following elements are usually present in the soil:

Oxygen, Aluminium, Silicon, Calcium, Magnesium, Sodium, Potassium, Iron. Trace elements present: Cobalt, Boron, Iodine, Zinc, Arsenic, Barium, Strontium, Fluorine, Cadmium, Chromium, Lithium, Rubidium, Vanadium, Lead etc.

Soil is rich in calcium in limestone areas. But calcium presence retards leeching of magnesium & potassium to deeper layers, reducing acidity of soil.

Cation Exchange Capacity (CEC): Total capacity of soil to retain exchangeable cations. It affect soil’s ability to hold nutrients. Provides buffer against soil acidification. Organic matter, soils with high clay amount tend to have CEC.

Soil pH: Lies between 2.2 and 9.6. Soils may be neutral, acidic (e.g. in Rainy tropics) or alkaline (e.g. Dry areas).

• Most minerals are readily available to plants in neutral or near to neutral pH conditions.
• Strongly acidic soils result in poor plant growth & can be caused due to aluminium or manganese toxicity, calcium deficiency, magnesium deficiency, low levels of phosphorus, molybdenum etc.
• Soils with extremely high alkaline pH tend to have high sodium.

Enzymes

• Essential for soil health.
• Catalyse biological reactions, maintain soil fertility etc.
• Important role in decomposing organic matter, energy & NH₄ availability to plants.
• More than 100 enzymes are characterised in soil.
• Common enzymes present in soil are amylases, asparginase, catalase, dehydrogenase, glycerophosphatase, invertase, proteases, phosphatases, phenoloxidase, tyrosinase, urease etc.
• Saline soils and soils with high carbonate content show high urease activity.
• Dehydrogenase activity – absent in saline alkali soils but high in forest soils.

Biota

Soil supports various types of flora and fauna depending upon…

1. Nature of soil
2. Nutrient content of soil
3. Amount of vegetation present etc.

Microfauna: Includes mostly protozoans, mites, nematodes, rotifers, tartigrades etc.

Microflora: Includes bacteria, fungi actinomycetes, algae etc.

Mesofauna: Includes microarthropods, insect larvae etc.

Macrofauna: Includes earthworms, centipedes, millipedes, wood lice, hemipterans, annelids, termites, limbless lizards, snakes, rodents etc.