5.1 Introduction to Soil Systems

What is a soil profile?

A visual representation of the different horizons present in a soil system

What is a layer known as on a soil profile?

Horizon

What are the 6 main horizons?

• O horizon

• A horizon

• E horizon

• B horizon

• C horizon

• R horizon

Describe the O horizon

Organic horizon (uppermost layer)

• Composed of mainly organic matter e.g. leaf litter, decaying plant material, organic debris

What is the function of the O horizon?

Rich in nutrients and serves as a site for nutrient cycling and organic material decomposition

Describe the A horizon

Topsoil

• Mineral matter mixed with humus

• Darker in colour

What is the function of the A horizon?

Important for plant growth as it contains nutrients and provides favourable environment for root development

Describe the E horizon

Eluvial/leached horizon

• Leaching or removal of nutrients due to downward movement of water

• Lighter in colour

Describe the B horizon

Illuvial/deposited horizon

• Where minerals and nutrients leached from upper horizons accumulate

• Has different colours, textures or chemical properties compared to horizons above and below it

What do the properties of C horizon influence?

Development and characteristics of upper horizons

Describe the R horizon

Bedrock

• Unweathered

• Unaffected by biological activity

What does the R horizon represent?

The original geological material from which the soil formed

Who does the soil profile help?

Scientists, farmers, land managers to understand:

• properties and fertility of soils

• informed decisions for land use, crop selection, soil conservation practices

What are the storages in a soil system?

• Organic matter

• Organisms

• Nutrients

• Minerals

• Air

• Water

What are the inputs of soil systems?

• Organic matter

• Inorganic matter from rock material

• Precipitation

• Infiltration

• Energy

What are the outputs of soil systems?

• Leaching

• Uptake by plants

• Mass movement

• Soil erosion

What are the transfers in soil systems?

• Biological mixing

• Leaching

• Eluviation

• Illuviation

• Translocation

What are the transformations in soil systems?

• Decomposition

• Weathering

• Nutrient cycling

• Humification

• Mineralisation

Describe sand soils

• Large particles of sand (silica etc)

• Low organic matter and nutrients (as sand particles don’t have high capacity for nutrient retention)

• Good drainage due to large pore spaces between sand particles, allowing water to move quickly through soil

• Low water holding capacity as sand particles limited ability to retain water

• Air spaces due to large particle size, facilitating oxygen availability for plant roots + soil organisms

• Low microbial activity and diversity of organisms

• Low potential to hold organic matter as sand particles don’t have strong binding capacity

Describe clay soils

• Composed of small clay particles with minerals e.g. silicates, aluminium oxides

• High nutrient content and strong cation exchange capacity

• Poor drainage due to compactness and small particle size

• High water-holding capacity

• Limited air spaces, restricting oxygen availability for roots + organisms

• Supports diverse soil organisms due to retained water and nutrients

• Strong potential to hold organic matter

Describe loam soils

• Composed of balanced mix of sand, silt, clay, diverse mineral content

• Moderate nutrient content with adequate cation exchange capacity for nutrient retention + availability

• Moderate drainage, balancing water movement and retention

• Moderate water-holding capacity, allowing plants to access water while preventing water logging

• Balanced air spaces for root respiration and soil organism activity

• Supports diverse biota, including microorganisms and earthworms

• Moderate potential to hold organic matter, facilitating decomposition and nutrient cycling

Soil properties summary table

How do soils influence plant growth?

Structure + properties affect nutrient content, water availability, air spaces, biota → promote plant growth

Why is nutrient content important in soil?

Provides essential elements needed for plant growth

How do water holding capacity and drainage affect plants?

Provides water for photosynthesis, while drainage prevents water logging, which can harm plants

Why are air spaces important in soil?

Ensure oxygen is available for plant roots and soil organisms to breathe

What role do soil organisms (biota) play in soil health?

Help recycle nutrients, break down organic matter and form relationship with plants (e.g. nitrogen-fixing bacteria)

How does organic matter affect soil fertility?

Adds nutrients to soil and improves its structure, helping plants to grow

How do different soil types affect primary productivity?

Balance of nutrients, water, air spaces, biota, organic matter determines how well a soil can grow

What is soil structure?

Refers to how soil particles are arranged, which impacts plant growth

What is soil texture?

Size of soil particles e.g. sand, silt, clay

How does soil texture affect plants?

Influences moisture content, aeration, nutrient retention and root penetration

What are properties of clay soils?

Holds nutrients well but can become waterlogged and shrink during droughts

What are sandy soils good at?

Excellent drainage but may not hold onto nutrients well

What happens to silt soils if ploughed when wet?

Can become compacted, harming growth

Why are loam soils considered ideal for cultivation?

Balanced texture, good drainage, moisture retention and aeration, making them highly productive for plant growth