Soils - Chapter 14 - Geography

Soil

A thin layer of loose material on Earth’s surface containing mineral matter, air, water, humus and living organisms.

Provides nutrients for plants to grow

The Composition of Soil

Mineral matter - 45%

Water - 25%

Air - 25%

Humus - 4%

Living Organisms - 1%

Mineral Matter

• Comes from bedrock that has been broken down by weathering and erosion.

• Includes stone, silt, clay and sand → Help nourish plants

Humus

• A dark jelly-like substance that is rich in nutrients

• Forms when dead plants and leaves decay on top of the soil - humification

• Increases soil fertility

Living Organisms

• Earthworms and insect → burrow through the soil, allowing water and air to pass through

• Microorganisms and fungi → help break down dead plants to form humus and increase soil fertility

Water

• Helps plants grow

• Dissolves the minerals and nutrients in soil, which allows plants to absorb them through their roots

Air

• Essential for plant growth because it supplies oxygen and nitrogen

• Helps living organisms to survive in soil

How Bedrock Influences How Soil Forms

• The rock beneath the soil.

• Soil forms when it is broken down by weathering.

• Some rocks weather slowly (e.g. granite) → soil forms slowly.

• Some rocks weather quickly (e.g. limestone) → soil forms faster.

• The type of bedrock affects soil quality.

Climate

• Rainfall and temperature determine the rate at which the parent rock is broken down at by weathering and erosion

Relief

• The shape of the land - up/lowland

• Highland areas → wetter → leaching or waterlogged soil → less plant life

• Lowland → well-drained soils → soil is more fertile

Human Activity

• Affects the composition and quality of soil

• Ploughing, irrigation schemes and fertilisers → makes soil fertile

• The removal of vegetation and overuse of soil can strip off nutrients and make it less fertile

Soil Profile

A vertical section of soil showing its different layers

Horizon

Layers of soil

O Horizon

The surface we can see with plant litter, which decay to form humus

A Horizon

• Topsoil

• Has the most humus and living organism

B Horizon

• Subsoil

• Has less humus than A horizon and is lighter in colour

• Made up of partially weathered parent rock

C Horizon

• Parent rock

• Consists of rock particles and bedrock

Podzol

• Form in cold, wet upland areas with coniferous trees

• Poor humus - little leaf litter

• High levels of rainfall - leaching - causes the nutrients to form a hard layer called hardpan - impermeable - soil becomes waterlogged

• Nutrients washed through A horizon - soil is grey

• Wexford

Brown Soils

• Formed in well-drained areas with deciduous trees

• Rich humus

• Moderate rainfall - little leaching -nutrients stay in A horizon

• Fertile, good from growing crops and grazing cattle and sheep

• Most common in Ireland - midlands

Soil and Vegetation

• Soil provides nutrients for plants to grow

• Soil holds water, which plants need

• Plants add organic matter to soil when leaves fall and decay → creates humus, which makes soil more fertile

• More vegetation = healthier, richer soil