Soil Formation

Soil Formation in Ireland

Soil formation is a process that takes thousands of years, and is mainly influenced by parent material (bedrock), topography, climate, organisms and time. Over time, physical, chemical and biological weathering occur and respond to climate and topography to form soil. Each process will be discussed in further detail below.

Parent Material

The bedrock underlying a soil has a strong influence on the development of the soil itself, physical, chemical and biological weathering combine to break down rocks which contain minerals that influence the mineralogy of the soil. Types of physical (also known as mechanical) weathering include freeze-thaw action, expansion and contraction due to temperature, and wind erosion. An example of chemical weathering occurs when CO₂ in the air dissolves in rainwater and can form a weak carbonic acid which can break down rocks – the evidence of this process can be seen in the clints and grikes within the limestone pavements of The Burren, Co. Clare. Biological weathering occurs when organisms living on the bedrock break down materials chemically and physically. Chemically, plants produce CO₂ which can combine with rainwater at the land-surface to cause weathering, while plant roots can penetrate cracks in rocks increasing the area available for water and acid rain to permeate.

There are two main types of parent material in Ireland, solid geology and drift geology. Solid geology refers to material which has been weathered and overlies solid bedrock, while drift geology refers to deposits or loose sediments which were transported by ice (glaciation), water, wind or gravity, which may be different to the underlying geology due to the distance they have travelled.

Topography

Topography refers to the shape of the landscape itself, and includes aspects such as altitude (height above sea level), aspect (the direction it faces, e.g. north-facing or south-facing), and slope. These topographic factors determine the influence of radiation and rainfall on the landscape itself, for example if an area is south facing, it will receive more sunlight (this is why south-facing gardens are desirable). If an area has a high degree of slope, the precipitation it receives will run down the slope, whereas if an area has very little slope, the water is more likely to remain in place leading to marsh or bog-like conditions.

Climate

Temperature and precipitation are major influences on soil formation, as they determine the degree of weathering that can occur, and the type of vegetation that can take root in a soil. Soil moisture is the dominant climatic factor influencing soil formation in Ireland, as water contributes to freeze/thaw action and chemical weathering as outlined in the Parent Material section above. Water transports acids produced by living organisms which can breakdown parent material and, over time, turn it into clay, allowing for plants to grow. The influence of the jet stream means Ireland gets a lot of rain, and the influence of the gulf stream means we don’t get too cold in winter. Precipitation exceeds evaporation in Ireland, resulting in leaching of nutrients down the soil profile (if the soil texture allows). This means free-draining soils in Ireland often require artificial renewal of soil nutrients to replace those lost by leaching.

Organisms

One handful of soil has more living organisms than there are people on earth, there are billions of bacteria which breath air (aerobic), millions of fungal cells, nematodes (worm shaped organisms which feast on bacteria and other organisms) and arthropods (organisms with a defined exoskeleton) of which there can be up to 1,000 in just a single handful of soil. These organisms break down organic matter and allow it to accumulate by becoming part of it, as organisms die and are broken down they perpetuate a cycle which allows soil to build up over time. The growth of vegetation also serves to deepen the soil profile as roots further permeate the subsoil, with different vegetation producing different types of soil structure, for example grasslands have large root biomass which creates dark surface horizons, while forests have a shallower organic layer of leaf litter. Vegetation also influences the pH of soil, with coniferous vegetation likely to remove base nutrients as they grow, leaving more acidic soil beneath.

A report on the biodiversity within Irish soils which includes the discovery of multiple new species is available here.

Time

All of the factors above are influenced by time, all the processes take long periods of time for what could be recognised as ‘soil’ to form. The majority of Irish soils have been formed since the last ice age, around 15,000 years ago. These soils are considered quite young, as some soils in Africa and South America are estimated to be millions of years old.

Humans

The final factor which can influence soil formation is human activity. Disruption of the natural development of soils as a result of land-use change, cultivation, fertilizer use, urbanisation etc. can all affect the development of soils – can you think of how these processes might affect soil formation?

Note: this information is partly adapted from The Soils of Ireland.