Factors Affecting Soil Formation (Leaving Cert Geography): Revision Notes

Factors Affecting Soil Formation

Soil development is a complex process influenced by several interconnected factors working together over time. Understanding these factors helps us appreciate why soils vary so dramatically across different regions and landscapes. The five main factors that shape soil formation are climate, parent material, topography, organisms, and time.

Climate

Climate stands out as the most significant factor influencing how soils develop. The combination of temperature and rainfall patterns directly controls the speed of rock breakdown and the level of biological activity within developing soils.

Hot climates

In tropical and subtropical regions, high temperatures accelerate chemical weathering processes. The warm conditions also boost the rate at which organic matter decomposes, creating nutrient-rich humus. These combined effects typically result in deep, fertile soils that support lush vegetation.

Cold climates

Colder regions experience much slower soil development due to reduced biological activity. The low temperatures limit the breakdown of organic matter, which means less humus formation occurs. Weathering processes are mainly restricted to freeze-thaw cycles, producing relatively thin soils with limited fertility.

Wet climates

Areas with heavy rainfall face a particular challenge called leaching. This process occurs when excess water washes nutrients downwards from the upper A horizon to the lower B horizon. Many wet-climate soils also develop a hardpan - a compacted layer that restricts drainage and makes soils prone to waterlogging.

Dry climates

In arid regions, drought conditions cause water to move upwards through the soil profile due to evaporation at the surface. This upward movement draws dissolved salts and calcium towards the top layers.

Parent material

The underlying rock or sediment from which soil develops plays a crucial role in determining soil characteristics. Different starting materials produce distinctly different soil types.

Rock types and soil formation

• Igneous rocks typically weather to form acidic soils with low pH levels
• Sedimentary rocks create diverse soil types depending on their composition:
  • Sandstone produces slightly acidic, well-drained sandy soils
  • Limestone creates calcium-rich, dark-coloured soils (such as terra rossa)
  • Shale tends to form dark-grey, wet clay soils with poor drainage characteristics

Non-rock parent materials

Not all soils develop from solid bedrock. Many Irish soils actually formed from glacial deposits left behind after the last ice age. These glacial materials - including boulder clay, sand, and gravels - have produced deep, fertile, and well-drained soils across much of the country.

Topography

The physical landscape features significantly influence soil development through their effects on drainage, temperature, and erosion patterns.

Flat upland areas

These locations tend to be cooler and wetter, often leading to waterlogging problems. The poor drainage encourages the formation of gley soils, and the low temperatures slow biological activity, resulting in organic matter accumulation that can form peat deposits.

Sloping land

Slopes experience faster soil erosion than soil formation can keep pace with, creating thinner soil layers. However, the improved drainage on slopes means these soils are typically well-drained and less prone to waterlogging.

Low-lying, flat areas

These areas collect soil eroded from higher ground, making them naturally deeper and more fertile. The warmer conditions at lower altitudes increase biological activity, which helps convert organic matter into valuable humus.

Organisms

Living organisms play a vital role in determining soil fertility by directly affecting how organic material decomposes and integrates into the soil structure.

Plant contributions

Plant roots serve multiple important functions in soil development. They break up and loosen compacted soil layers as they grow through the ground, whilst also binding soil particles together to prevent erosion. The plant canopy protects the soil surface from heavy rainfall and harsh weather conditions. When plants die, their decaying leaves and roots contribute organic matter that becomes humus.

Animal activity

Larger animals like rabbits create burrows that help aerate the soil and mix materials between the A and B horizons. However, earthworms and insects are far more numerous and play an even more important role in soil development. When these creatures die, their bodies decompose and add valuable nutrients to the soil.

Microscopic life

Fungi and bacteria are essential for breaking down organic matter through a process called humification, which converts dead plant and animal material into humus - the nutrient-rich component that gives soil its fertility.

Time

Soil formation is an extremely slow process, often requiring thousands of years to develop well-defined soil profiles. The age of soil directly affects its characteristics and fertility levels.

Young soils

Recently formed soils lack well-developed profiles and distinct horizons. They typically haven't undergone extensive weathering, so their mineral composition remains relatively unchanged from the parent material.

Mature soils

Older soils show well-developed profiles with clearly defined horizons. They've experienced prolonged weathering processes and often exhibit strong structural development.

Ireland's soils

Most Irish soils are less than 10,000 years old, as glaciers from the last ice age removed previous soil cover. This makes them postglacial soils, which are considered young in geological terms but have still had sufficient time to develop into productive agricultural soils.