Soil

Composition of Soil

Soil is the loose, upper layer of the Earth's surface where plants grow. It provides essential nutrients and support for plant roots. Soil is made up of several components, each playing a vital role in its structure and fertility.

Components of Soil

• Parent Rock (46%):
  • Also known as mineral matter, this is the largest part of soil.
  • It consists of broken-down material from rocks, including stones, sand, clay, and silt.
  • This material is important for providing nutrients to plants.
• Water (25%):
  • Water is crucial as it helps dissolve minerals and nutrients in the soil.
  • Plants absorb these dissolved nutrients through their roots.
• Air (25%):
  • Found in the spaces between soil particles, air is vital for plant growth.
  • It supplies oxygen and nitrogen, which plants need to survive.
  • Air also allows living organisms within the soil to breathe.
• Humus (3-4%):
  • Humus is a dark, jelly-like substance rich in nutrients.
  • It forms from the decayed remains of plants and animals, a process known as humification.
  • Humus helps bind soil particles together, improving soil structure and fertility.
• Living Organisms (Less than 1%):
  • Includes creatures like earthworms, insects, bacteria, and fungi. This includes some micro-organisms that are too small to be seen without a microscope.
  • These organisms break down organic matter, mix the soil, and improve its fertility.

Factors which influence the development of Soil

Several factors contribute to the formation and characteristics of soil in a given area. Each factor affects how soil develops and its fertility.

• Parent Rock:
  • The type of rock in an area directly influences soil formation. Rocks like granite break down slowly, leading to soil that takes longer to develop. In contrast, limestone weathers more quickly, forming soil that is rich in nutrients.
• Climate:
  • Climate affects the rate of weathering and erosion, which are key processes in soil formation. In cold climates, processes like freeze-thaw action break down rocks slowly, leading to thinner soils.
• Relief:
  • Relief refers to the shape of the land, whether it is flat or sloped (upland or lowland). Upland areas tend to be wetter, which can lead to waterlogged soils that are less fertile.
  • Lowland areas usually have better drainage, resulting in deeper, more fertile soils.
• Time:
  • Soil formation is a slow process, taking about 400 years for just 1 cm of soil to develop. The length of time available for soil formation affects the depth and fertility of the soil. The longer the time, the more developed and fertile the soil becomes.
• Human Activity:
  • Human activities such as farming, ploughing, irrigation, and the use of fertilisers can improve soil fertility by adding nutrients.
  • However, activities like deforestation and overgrazing can lead to soil erosion, stripping the soil of its nutrients and making it less fertile.

Soil Profile

A soil profile is a vertical cross-section that shows the different layers, or horizons, of soil from the surface down to the parent rock. Each layer has distinct characteristics:

• O Horizon:
  • This is the topmost layer, also known as the organic layer.
  • It consists of plant litter such as leaves, branches, and other organic material.
  • As this material decomposes, it forms humus, which enriches the soil with nutrients.
• A Horizon:
  • Known as the topsoil, this layer is rich in humus and is where most plant roots grow.
  • It contains the highest concentration of organic material and is home to many living organisms, like earthworms and insects.
• B Horizon:
  • This is the subsoil, located beneath the A horizon.
  • It is lighter in colour because it contains less humus.
  • The B horizon often consists of minerals that have been leached (washed down) from the topsoil, and it includes partially weathered parent rock.
• C Horizon:
  • The lowest layer, called the parent rock or C horizon.
  • It contains large rock particles and solid bedrock.
  • This layer is mostly unaffected by soil-forming processes and serves as the source of the material for the upper layers as it gradually breaks down.

Case Studies: Brown Soils and Podzols

Brown Soils

Brown soils are among the most common and fertile soil types in Ireland. They have several key characteristics:

• Colour: Dark brown, indicating rich organic content.
• Profile:
  • The O horizon has a thick layer of leaf litter, usually from deciduous trees.
  • The A horizon (topsoil) is dark and rich in humus.
  • The B horizon contains minerals and nutrients, with less organic matter.
  • The C horizon consists of weathered parent rock.
• Formation:
  • Found in areas that were once covered by deciduous forests.
  • Common in the Midlands, south, and east of Ireland, where the climate is moderate.
  • Humification of the abundant leaf litter contributes to a fertile topsoil.
• Soil Processes:
  • Moderate rainfall leads to minimal leaching, allowing nutrients to remain in the topsoil.
  • Well-drained, with good soil structure.
• Uses:
  • Very fertile and suitable for arable and pastoral farming.
  • Ideal for crops and grazing due to its well-drained nature and rich nutrient content.

Podzols

Podzols are less fertile and more acidic soils, commonly found in cooler, wetter areas of Ireland:

• Colour: Grey, often due to leaching. Leaching in soil is when nutrients and minerals are washed downwards through the soil by rainfall or irrigation water.
• Profile:
  • The O horizon has a thin layer of leaf litter, typically from coniferous trees.
  • The A horizon is grey and heavily leached, meaning nutrients are washed down to lower layers.
  • The B horizon contains accumulated minerals like iron and aluminum, forming a hardpan layer.
  • The C horizon is made up of impermeable bedrock.
• Formation:
  • Found in areas that were once covered by coniferous forests.
  • Common in cold, wet upland areas such as parts of Wexford, Cork, and Galway.
  • Limited organic material and heavy rainfall lead to excessive leaching.
• Soil Processes:
  • High rainfall causes leaching, where nutrients are washed out of the topsoil.
  • The leaching process forms a hardpan in the B horizon, making the soil less permeable and prone to waterlogging.
• Uses:
  • Infertile and slightly acidic, so it is generally unsuitable for farming.