The Earth's surface is covered by a thin layer of soil – a complex and dynamic mixture of minerals, air, water, organic matter and countless living organisms. Soil is far from uniform. It occurs in many different forms around the world, each with its own history. Like a living organism, soil forms, develops, ages and breathes. New soil is constantly being created while existing soil is continually being eroded or lost. Within this thin yet vital layer, the atmosphere, water and bedrock interact in ways that sustain life. Soil is the foundation of life on Earth.

Soil develops through the combined influence of climate, living organisms, topography, parent material and time. Climate determines how rocks weather and break down into smaller particles such as sand, silt and rock fragments, providing the mineral component of soil. Temperature influences the chemical reactions that break down rock, while water transports dissolved minerals through the soil profile.

Living organisms also play a vital role in soil formation. Plant roots, soil-dwelling animals, bacteria and fungi all modify and mix the soil, contributing to its structure and fertility. Topography also affects soil development. The direction a slope faces influences the amount of sunlight and warmth it receives, while the movement and accumulation of water affect how soils form. The landscape also determines how vulnerable soil is to erosion and whether minerals and organic matter accumulate or are washed away.

The mineral component of soil originates from the underlying bedrock and geological deposits, or from sediments transported by water or wind. The properties of these source materials largely determine the characteristics of the minerals found in the soil. Time is another essential factor. Older soils that have developed over long periods differ from young soils that have only recently begun to form.

A good example can be seen where glaciers retreat. Close to the glacier, the land surface consists mainly of mineral material. As time passes, or with increasing distance from the glacier, organic matter gradually accumulates, particularly once vegetation becomes established. Over long periods, this process can give rise to deep, fertile soils.

Soil is made up of minerals, organic matter, water and air. The proportions of these components determine its physical and chemical properties, including texture, structure, pore space, chemistry and colour.

Soil consists of particles of different sizes. Its texture is determined by the size of these particles and the proportion of each size fraction, such as sand, silt, clay and organic matter. Sandy soils feel coarse when rubbed between the fingers and crumble easily. Fine-textured clay soils, by contrast, feel smooth and flour-like when dry, or sticky and easily moulded when wet.

Soil texture influences how easily water infiltrates the soil and how suitable it is for plant growth. The spaces between sand grains are much larger than those between fine clay particles. As a result, water drains more readily through sandy soils, whereas fine-textured soils with smaller pore spaces retain water more effectively.

The pore space within soil controls the movement of air and water, both of which are essential for life below ground. Soil structure describes how individual particles bind together to form larger aggregates. Organic matter from decomposing plants and animals, together with substances produced by soil organisms such as earthworms, fungi and bacteria, helps bind soil particles into stable aggregates.

Soil structure is an important physical property that influences the movement of water and nutrients, heat transfer, and the supply of air to plant roots and soil organisms. Well-formed, stable aggregates also make soils more resistant to erosion and compaction. Together, these properties have a major influence on plant growth. Well-structured soils provide plants with water, nutrients and oxygen while allowing roots to grow freely through the soil. Poor soil structure, on the other hand, restricts root growth and reduces both water infiltration and drainage.

The chemical properties of soil—including its pH, mineral composition, nutrient content and the amount of organic matter it contains—ultimately determine its fertility.

Much of Iceland's soil is classified as volcanic soil, or Andosol. These relatively young soils are formed from weathered volcanic materials such as ash, pumice and scoria. They are typically characterised by low bulk density, a high organic matter content, a pH of around 5–7, variable charge, high water-holding capacity, good water permeability, weak soil structure, strong phosphorus retention and the ability to exhibit thixotropy – a property that allows soil to become more fluid when disturbed and regain its structure when left undisturbed.

Volcanic soils are common in regions of the world with active volcanoes. However, European soil maps show that such soils are generally confined to localised areas, making Iceland unique within Europe. For thousands of years, Icelandic soils have been continually renewed by regular volcanic ash falls, which replenish them with fresh mineral material. Few countries have experienced such frequent and sustained ash deposition over such a long period.

Because Iceland is geologically young and was largely covered by glaciers during the last Ice Age, its soils are also relatively young, and in many places they are still actively forming. Few places in the world exhibit such a strong interaction between glaciers and volcanic activity. Glaciers grind bedrock into fine mineral particles, glacial rivers transport these sediments towards the sea, and volcanic eruptions regularly deposit fresh layers of ash across the landscape. Together, these processes continually renew Iceland's soils.

High capacity for carbon storage

Icelandic volcanic soils can store large amounts of organic carbon. Minerals such as allophane and imogolite form strong bonds with organic matter, slowing its decomposition. As a result, healthy, well-vegetated volcanic soils can act as highly effective long-term carbon stores.

Highly vulnerable to erosion

Despite their many favourable properties, Icelandic soils are also highly susceptible to erosion when the protective vegetation cover is damaged or removed. Fine volcanic ash is light and easily carried away by the wind. Where vegetation becomes sparse or disappears altogether, both wind and water can erode the soil rapidly. This helps explain the extensive soil erosion that has shaped large parts of the Icelandic landscape throughout history.

Excellent water retention and aeration

Volcanic soils have exceptionally high pore space. This allows them to retain large amounts of water while remaining well aerated, creating favourable conditions for plant growth where sufficient nutrients are available. However, because of centuries of land use and prolonged erosion, many Icelandic soils are deficient in essential nutrients, particularly nitrogen. Even so, the physical properties of volcanic soils mean that once vegetation becomes established, ecosystem recovery can proceed rapidly.

Volcanic ash layers record the history of the landscape

Unlike in most other countries, distinct volcanic ash layers (tephra) can be seen in many Icelandic soil profiles. These layers are an important part of the soil-forming process, but they are also invaluable for scientific research. Acting as natural time markers, tephra layers enable scientists to date soils and reconstruct past environmental changes with remarkable precision.