Soil Science

Importance of Soil

• Soil provides mechanical support to plants by allowing roots to grow through it
• Soil holds water, nutrients, and air essential for plant growth

Soil Definition

• Thin layer of the Earth's crust
• Unconsolidated mixture of disintegrated rock with:
  • Organic matter
  • Water
  • Air
  • Microorganisms

Soil Components

• Mineral particles of weathered rocks
• Water and dissolved mineral salts
• Air in spaces between particles
• Dead organic matter (humus)
• Microorganisms

Soil Texture

• Classified into four types based on particle size:
  • Clay (< 0.002 mm)
  • Silt (0.02 mm - 0.002 mm)
  • Fine sand (0.2 mm - 0.02 mm)
  • Coarse sand (2.0 mm - 0.2 mm)

Types of Soils

• Clayey soil: heavy, poor drainage, and moldable
• Sandy soil: light, good aeration, and fast drainage
• Loam soil: composed of equal amounts of clay, silt, and sand
• Silt soil: medium pore size, good drainage, and fertile

Soil Profile (Layers)

• Every soil has a profile with multiple layers:
  • 0 horizon (soil surface): few centimeters thick, contains fallen leaves
  • A horizon (topsoil): 10-30 cm below the surface, contains 10-15% organic matter
  • B horizon (subsoil): large soil particles, little organic matter, and minerals washed from topsoil
  • C horizon (bedrock or hardpan): compacted soil, no organic matter, and poor drainage

Soil Organisms

• Beneficial organisms:
  • Form part of the soil complex
  • Improve drainage and aeration
  • Aid in plant nutrient and water uptake
  • Some are harmful, causing plant injuries or diseases
• Examples of soil organisms: nematodes, insects, earthworms, fungi, and bacteria

Organic Matter in Soil

• When plants and animals decompose, they form humus
• Soils lacking organic matter and humus are light in color
• Humus is important for:
  • Holding moisture
  • Absorbing and holding plant nutrients
  • Feeding soil organisms

Soil Erosion

• Movement of soil particles from one place to another due to:
  • Water (heavy rainfall or irrigation)
  • Wind
• Can be controlled by growing plants with extensive, fibrous roots

Soil pH

• Measures the hydrogen ion concentration in soil
• Affects soil characteristics and plant growth
• pH scale: 0 (extreme acidic) to 14 (extreme alkaline)
• Optimum pH range for most plants: 6.5 - 7.0

Soil Improvement

• Can be improved by adding amendments:
  • Inorganic amendments (e.g., vermiculite, perlite)
  • Organic amendments (e.g., compost, humus)
• Amendments should be used in large amounts (about 25% of the soil volume)

Soil of Desert and Kuwait

• Desert soils: mostly sandy, saline, and poor in some elements
• Kuwait soils: mostly sandy, calcareous, and with accumulations of carbonate, gypsum, and salts

Water and Soil

• Importance of water to plants:
  • Carries soluble minerals
  • Essential for photosynthesis
  • Essential for transpiration and regulating internal temperature
  • Keeps cells turgid
• Water availability:
  • Field water capacity: maximum amount of water held by a soil
  • Permanent wilting capacity: minimum amount of water available for plant growth
  • Water availability = Field water capacity - Permanent wilting capacity

Irrigation and Drainage

• Irrigation: watering plants by rainfall or other sources
• Drainage: rate at which water flows through the soil or potting medium
• Good drainage is recognized by fast water flow and low retention