Soil Characteristics (Leaving Cert Geography): Revision Notes
Soil Characteristics
Soil can be identified and classified using several key characteristics that work together to determine different soil types. These features directly influence how fertile and productive a soil will be for plant growth. Understanding these characteristics helps us predict which crops will grow well in different areas and how to manage soils effectively.
The six main soil characteristics are:
- Texture
- Structure
- pH value
- Organic matter
- Moisture content and water retention
- Colour
Texture
Soil texture describes how a soil feels when you touch it and depends on the proportions of sand, silt, and clay particles it contains. The texture affects how well soil can absorb and hold water, retain nutrients, and allow plant roots to penetrate through it.
Texture is determined by the relative amounts of sand, silt, and clay particles in the soil. This characteristic controls water absorption, nutrient retention, and root penetration.
The four main texture types
Clay soils Clay consists of extremely small, tightly packed particles. Soils with more than 50% clay particles feel smooth and sticky when wet because the particles bind together. This creates several challenges:
- Water and air struggle to move through the compact structure
- During wet periods, clay soils become waterlogged and sticky
- When dry, the soil shrinks and cracks, making root penetration nearly impossible
- These conditions make clay soils better suited for grassland rather than crop cultivation
Sandy soils Sandy soils contain large, loose particles that don't stick together even when wet. The coarse texture creates large spaces between particles, allowing:
- Water and air to pass through very easily
- Rapid drainage, which can be too fast for plants to absorb enough water
- Nutrients and minerals to wash away quickly (leaching)
- The need for regular irrigation and fertiliser application
Silty soils Silt particles are intermediate in size between sand and clay, giving silty soils balanced properties:
- Generally quite fertile and support diverse plant growth
- Small pores help retain moisture but can cause waterlogging during heavy rainfall
- When dry, the soil becomes powdery and easy to cultivate
- Particles can stick together when wet, similar to clay
Loam soils Loam is considered the ideal soil for agriculture because it contains roughly equal proportions of clay, sand, and silt. This perfect balance provides:
- A crumbly texture that retains moisture while allowing free drainage
- Easy cultivation conditions
- Natural resistance to leaching, maintaining high humus content
- Optimal conditions for most crops
Worked Example: Identifying Soil Texture
Step 1: Take a handful of moist soil and squeeze it Step 2: Observe the result:
- If it forms a sticky ball that holds its shape → High clay content
- If it crumbles and won't hold together → High sand content
- If it forms a loose ball that breaks apart easily → Balanced loam soil
Step 3: Feel the texture:
- Smooth and slippery → Clay dominant
- Gritty and rough → Sand dominant
- Smooth but not sticky → Silt dominant
Structure
Soil structure refers to how individual soil particles clump together to form larger units called peds. The shape and size of these peds determine how much space exists for air, water, roots, and living organisms within the soil.
Peds are small clumps of soil particles that have bound together. The structure of soil is determined by the shape of its peds.
Types of soil structure
Crumb or granular structure
- Peds are small and rounded, similar in size to breadcrumbs
- Provides excellent drainage and air movement
- Commonly found in fertile loam soils
- Creates ideal conditions for root growth and soil organisms
Blocky structure
- Peds form cube-shaped particles that pack tightly together
- Naturally provides good drainage
- Can become compacted easily, restricting air and water movement
- Often found in sandy soils
Platy structure
- Peds are thin and flat, creating a layered appearance
- Found in compacted soils, particularly clays
- Severely limits air and water movement
- Makes root penetration extremely difficult
The structure of soil can be improved through proper management practices such as adding organic matter, avoiding cultivation when soil is too wet, and using cover crops to protect soil from compaction.
pH value
The pH scale measures how acidic or alkaline a soil is, ranging from 1 (extremely acidic) to 14 (extremely alkaline). This characteristic is crucial because it controls which plants and organisms can survive in the soil.
pH measures how acidic or alkaline something is. Acidic soils have a pH below 7, neutral soils have a pH of 7, and alkaline soils have a pH above 7.
pH and soil fertility
Optimal pH levels Most fertile soils are slightly acidic with a pH around 6.5. This level allows the greatest variety of plants to thrive and nutrients to remain available.
Acidic soils (pH below 7)
- Severely acidic soils have been heavily leached, losing essential minerals
- Support very few living organisms or plants
- Often require lime to be added to raise the pH to more suitable levels
Alkaline soils (pH above 7)
- Contain high levels of calcium
- Develop in areas with chalk or limestone bedrock
- Commonly found in desert regions or drought-stricken areas with permeable bedrock
Farmers can test soil pH using simple test kits and adjust it by adding lime (to raise pH) or sulphur (to lower pH) depending on their crops' needs.
Organic matter
The organic content in soil comes from the decomposed remains of plants and animals. This material undergoes breakdown by bacteria and fungi, eventually forming humus - a dark, nutrient-rich substance that dramatically improves soil quality.
Humus is the dark, decomposed organic matter in soil that has been broken down by bacteria, fungi, and soil organisms like earthworms.
How humus improves soil fertility
Living organisms process organic matter and create humus, which benefits soil in several ways:
- Nutrient supply: Rich in essential plant nutrients like carbon and nitrogen
- Soil binding: Helps hold soil particles together, reducing erosion risk
- Moisture retention: Allows soil to hold more water
- Nutrient protection: Absorbs nutrients and prevents them from being washed away
The amount of humus directly relates to soil fertility - more humus means more fertile soil. Irish brown earth soils are particularly rich in humus because they developed under thick deciduous forests. The humus content also affects soil colour, with higher humus levels creating darker soils.
Worked Example: Estimating Organic Matter Content
Step 1: Observe soil colour
- Very dark brown/black soil → High organic matter (5-10%)
- Medium brown soil → Moderate organic matter (2-5%)
- Light brown/grey soil → Low organic matter (less than 2%)
Step 2: Check for visible organic debris
- Presence of decomposing leaves, roots, or plant material indicates active organic matter input
Step 3: Assess soil texture
- Well-structured, crumbly soil often indicates good organic matter content
Moisture content and water retention
The amount of water a soil can hold depends on its structure, humus content, and texture working together. This characteristic is vital for plant growth and soil development.
Factors affecting moisture retention
Soil texture effects
- Clay and other platy-structured soils retain large amounts of water, sometimes leading to waterlogging
- Sandy soils with large pores retain very little water and suffer from drought in dry climates
- Loam soils achieve the perfect balance - well-drained but holding enough moisture to remain highly fertile
External influences
- Precipitation: Areas with high rainfall naturally have higher soil moisture content
- Underlying rock: Permeable rocks like limestone allow water to drain away, while impermeable rocks hold moisture
- Hardpan development: Heavily leached soils may develop an impermeable hardpan layer that causes waterlogging
Importance of moisture for soil health
Adequate moisture is essential because it:
- Allows plants to absorb suitable minerals from the soil
- Enables different soil horizons to develop properly
- Binds soil particles together, preventing erosion
Soil moisture can be managed through techniques such as mulching, terracing, and choosing appropriate irrigation methods based on soil type and climate conditions.
Colour
Soil colour is the most easily identifiable characteristic and often provides immediate clues about soil fertility and composition.
What soil colours indicate
Dark brown or black soils
- Typically very fertile with high humus content
- The dark colour comes from decomposed organic matter
- Tend to be warmer as they absorb and retain heat
- Warmer temperatures promote seed germination
Grey soils
- Usually infertile due to heavy leaching of nutrients
- Have a "washed out" appearance from lack of humus and minerals
- Often suffer from poor drainage and waterlogging
- May appear blue-grey when waterlogged (called gley soils)
Red soils
- Found in warm, moist tropical or equatorial climates
- Red colour develops from high iron oxide content in the soil
- High temperatures and rainfall levels cause intense chemical weathering
- Typically low in nutrients and organic matter due to rapid decomposition
Iron oxide (rust) gives red soils their distinctive colour and forms due to chemical weathering in warm, wet climates.
Key Points to Remember:
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Texture depends on particle size: Clay (small, sticky), Sand (large, loose), Silt (medium), Loam (perfect mix for farming)
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Structure affects drainage: Crumb peds allow good drainage, Platy peds block water movement, Blocky peds can become compacted
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pH controls plant life: Most fertile soils are slightly acidic (pH 6.5), while very acidic or alkaline soils support fewer plants
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Humus makes soil fertile: Dark, decomposed organic matter improves nutrition, water retention, and soil binding
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Colour reveals fertility: Dark soils are usually fertile, grey soils are often leached and infertile, red soils form in tropical climates with high iron content