MEV 013: Unit 07 - Soil Chemistry

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UNIT 7: SOIL CHEMISTRY

7.0 Introduction

Soil is the thin outer layer of the Earth's crust that supports plant life and forms a key component of terrestrial ecosystems. It is a dynamic system composed of minerals, organic matter, water, air, and living organisms. Soil chemistry deals with the chemical composition, reactions, and properties of soil in relation to its fertility, structure, and environmental interactions.

7.1 Objectives

After studying this unit, you will be able to:

  • Understand the origin, nature, and composition of soil.
  • Explain the processes of soil formation and the factors involved.
  • Describe the chemical properties of soil including pH, ion exchange, and nutrients.
  • Differentiate between soil macronutrients and micronutrients.
  • Understand soil fertility and the importance of maintaining soil health.

7.2 Origin and Nature of Soil

Soil originates from the weathering of rocks and the decomposition of organic materials over time. It acts as a medium for plant growth, a reservoir for nutrients and water, and a habitat for organisms.

7.2.1 Soil Composition

Soil is composed of:

  • Mineral Matter (45%): Sand, silt, clay derived from parent rock.
  • Organic Matter (5%): Decomposed plant and animal material (humus).
  • Water (25%): Held in pores, essential for chemical reactions and plant uptake.
  • Air (25%): Occupies the remaining pore space, supports microbial activity.

7.3 Soil Formation

Soil formation, or pedogenesis, is a slow, continuous process influenced by several environmental factors.

7.3.1 Soil Forming Processes

  • Weathering: Physical, chemical, and biological breakdown of rocks.
  • Leaching: Downward movement of dissolved substances.
  • Humification: Formation of humus from organic matter.
  • Mineralization: Conversion of organic nutrients into inorganic forms.
  • Aggregation: Binding of soil particles into clusters.

Five Factors of Soil Formation:

  1. Parent material
  2. Climate
  3. Topography
  4. Biological activity
  5. Time

7.4 Soil Chemical Properties

Soil chemistry influences nutrient availability, soil fertility, and crop productivity.

7.4.1 Soil Acidity

  • Soil pH: Indicates hydrogen ion concentration.
    • Acidic (< 7), Neutral (= 7), Alkaline (> 7)
  • Sources of Acidity:
    • Leaching of bases
    • Decomposition of organic matter
    • Use of ammonium-based fertilizers
  • Importance: Affects nutrient solubility and microbial activity.

7.4.2 Ion Exchange and Cation Exchange Capacity (CEC)

  • Ion Exchange: Reversible exchange of ions between soil colloids and soil solution.
  • Cation Exchange Capacity (CEC):
    • A measure of soil’s ability to hold positively charged ions (cations).
    • Units: meq/100g of soil
    • High CEC indicates high fertility potential.

Common exchangeable cations: Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺

7.5 Macro and Micronutrients in Soil

Plants require essential nutrients for growth, broadly divided into:

7.5.1 Soil Macronutrients

Required in large amounts:

  • Primary Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K)
  • Secondary Macronutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S)

Functions:

  • Nitrogen: Protein synthesis
  • Phosphorus: Energy transfer (ATP)
  • Potassium: Enzyme activation and water balance

7.5.2 Soil Micronutrients

Required in trace amounts:

  • Examples: Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn), Boron (B), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni)

Functions:

  • Fe: Chlorophyll synthesis
  • Zn: Enzyme cofactor
  • B: Cell wall strength

Deficiency or excess of these nutrients affects plant growth, yield, and quality.

7.6 Soil Fertility

Soil fertility is the ability of soil to supply nutrients in adequate amounts and proportions for plant growth.

Factors affecting fertility:

  • Nutrient availability
  • Soil pH
  • Organic matter content
  • Microbial activity
  • Water retention and drainage

Improvement of Soil Fertility:

  • Organic amendments (compost, manure)
  • Crop rotation and green manuring
  • Balanced fertilization
  • pH correction using lime or gypsum

7.7 Let Us Sum Up

This unit introduced the foundational aspects of soil chemistry, focusing on the nature, formation, and chemical behavior of soil. We covered important properties like pH, cation exchange capacity, and the distinction between macro- and micronutrients. A comprehensive understanding of these factors is essential for managing soil fertility and ensuring sustainable agricultural practices.

7.8 Key Words

  • ·         Pedogenesis-The process of soil formation.
  • ·         Soil Profile-Vertical section showing different soil horizons.
  • ·         Cation Exchange Capacity (CEC)-The ability of soil to hold and exchange positively charged ions.
  • ·         Macronutrients-Essential elements required in large amounts by plants (e.g., N, P, K).
  • ·         Micronutrients-Essential elements required in small quantities by plants (e.g., Fe, Zn).
  • ·         Soil Acidity-Degree to which soil has a low pH, often limiting nutrient availability.
  • ·         Ion Exchange-Exchange of ions between soil and soil solution, critical for nutrient dynamics.
  • ·         Soil Fertility-Capacity of soil to support plant growth and yield by providing nutrients.

 

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