🌱 Soils for Plant Growth

🌍 1. What is Soil?

Soil is a natural resource formed from weathered rock and organic matter.
It develops over many years and supports plant life.

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🧱 2. Soil Composition

Soil has four main parts:

• Mineral particles (45%)
  • Sand (large)
  • Silt (medium)
  • Clay (small)
• Organic matter (humus) (5%)
  • Decayed plants and animals
  • Improves fertility and structure
• Water (25%)
  • Dissolves nutrients for plant uptake
• Air (25%)
  • Provides oxygen for root respiration

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🪨 3. Soil Texture

Texture = size of soil particles

Soil Type	Properties	Effect on Plants
Sandy	Large particles, loose	Drains quickly, low nutrients
Clay	Very small particles, sticky	Holds water, poor drainage
Silt	Medium particles	Fertile but can compact
Loam	Balanced mix	Best for growth

🌿 4. Soil Structure

Structure = how soil particles are arranged into clumps (aggregates)

• Good structure:
  • Allows air circulation
  • Helps root growth
  • Improves water movement
• Poor structure:
  • Causes compaction
  • Reduces plant growth

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💧 5. Water in Soil

• Field capacity: amount of water soil holds after excess drains
• Wilting point: when plants cannot get enough water

👉 Plants grow best when soil moisture is between these two levels

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🌬️ 6. Soil Air

• Air spaces between particles contain oxygen
• Roots need oxygen for respiration
• Waterlogged soil → less air → roots may die

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🧪 7. Soil Nutrients

Main nutrients needed by plants:

• Nitrogen (N) → leaf growth
• Phosphorus (P) → root growth
• Potassium (K) → overall health

Other nutrients:

• Calcium, magnesium, iron

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🌱 8. Soil pH

• Measures how acidic or alkaline soil is
• Scale: 0–14
  • 7 = neutral
  • <7 = acidic

  • 7 = alkaline

👉 Most plants grow best at pH 6–7

• Acidic soil → add lime
• Alkaline soil → add organic matter

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🌾 9. Soil Fertility

Fertile soil has:

• Good nutrients
• Good structure
• Proper pH
• Active organisms

Improving fertility:

• Fertilizers (chemical nutrients)
• Organic manure/compost
• Crop rotation
• Cover crops

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🪱 10. Soil Organisms

Examples:

• Earthworms
• Bacteria
• Fungi

Their role:

• Decompose organic matter → form humus
• Improve soil structure
• Help nutrient cycling (e.g. nitrogen cycle)

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🌦️ 11. Drainage and Water Retention

Good agricultural soil should:

• Drain excess water (avoid waterlogging)
• Retain enough moisture for plants

Problems:

• Too much water → lack of oxygen
• Too little water → drought stress

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🌍 12. Soil Profile (Layers)

Typical layers (horizons):

• O Horizon: organic layer (leaf litter)
• A Horizon (Topsoil):
  • Rich in humus
  • Most plant roots grow here
• B Horizon (Subsoil):
  • Fewer nutrients
  • More minerals
• C Horizon:
• Weathered rock

Mr. Amir Salih—Social Studies—Environmental