Soil‑forming processes

The soil‑forming factors that govern these processes are:

• the geological substrate (parent material)
• climate
• topography
• hydrology
• organisms
• time (age)

The geological substrate, or parent material, influences soil formation and soil properties through its mineral composition, particle‑size distribution (soil texture), and the thickness of the soil cover.

The mineralogical composition is determined by the composition of the rock from which the material originates. Most mineral soils have been transported during their formation by ice, water, or wind. They may therefore have a composition that differs from the bedrock at the site where they were deposited. In Sweden, till (moraine) dominates; it is a mixture of different particle‑size classes. This soil type is generally coarse‑textured but, due to the young age of the soils and their low degree of weathering, relatively rich in easily weathered minerals.

Climate influences soil processes

Climate affects soil processes by regulating the production of organic material and by influencing various chemical, biological, and physical processes in the soil. Seasonal variation in temperature, precipitation, and frost is of great importance. Important processes influenced by climate include weathering, leaching, and the input and decomposition of organic material. Sweden has high precipitation relative to evaporation, which leads to leaching. Frost, through ground frost, has a beneficial effect on soil structure. The climate factor can also include atmospheric deposition of acidic substances or other pollutants with precipitation. Such deposition affects nutrient availability and soil acidity.

Groundwater influences local climate and organisms

The position of the groundwater table and the mobility of water are determined by topography, which also influences the local climate and organisms. The availability of oxygen in the soil is strongly linked to soil water content. A high groundwater table can lead to oxygen deficiency, which in turn strongly affects soil formation through reduced decomposition of organic material, peat formation, and increased solubility of certain metals.

Soil structure

Organisms influence the soil through the input and transformation of organic material and by affecting the arrangement of soil particles—soil structure. This occurs through the action of roots and soil organisms. Organic matter is added to the soil as litter. Through decomposition, humus is formed. The mineralisation process releases nutrients, allowing them to re‑enter the cycle between soil and plants.

Humans are also an important factor; through land use they have had a profound impact on the properties of many soils. Harvesting of biomass entails losses of plant nutrients and biological acidification.

Sweden’s young soils

Time, as a soil‑forming factor, refers to the fact that soils develop gradually. In Sweden, soils are very young, having all formed after the last glaciation between about 8,000 and 12,000 years ago. By comparison, soils in tropical regions may be several million years old.

Changes themselves can also influence further soil development. One example is the formation of dense layers in the soil through precipitation of iron compounds or other substances. These layers affect water movement and root distribution and thus subsequent soil development.

Soil – the basis for plant production

Soil can also be regarded as the part of the ground that is utilised by plant roots and thus forms the basis for plant production. In Sweden, soils have generally developed in loose glacial or post‑glacial deposits. In other climatic regions outside glacial influence and with intensive soil processes, the soil may encompass the entire unconsolidated deposit formed through the influence of organisms, climate, and water.

Important soil processes

The formation of a soil thus involves various processes that transform the original material, i.e. bedrock or an unconsolidated deposit. Examples of soil processes include chemical weathering of minerals, as well as precipitation of sparingly soluble secondary minerals, such as iron and aluminium oxides.

Other soil processes include the accumulation of dead organic material at the soil surface and its downward transport to deeper layers. Percolating water can carry solid particles and dissolved substances, which may either be retained at greater depths in the soil or transported to other areas, such as discharge zones or lakes.

Soil organisms alter the spatial distribution of particles within the soil. In this way, soil fauna can contribute to an aggregated soil structure with large pores.

Soil variants – a result of climate and organisms

oils have naturally been influenced in different ways, among other reasons because climate and organisms vary across regions, both within Sweden and globally. As a result, a very large number of soil variants have developed. Unlike in floristics and faunistics, no clearly (genetically) delimited species or types arise; instead, soils gradually transition into one another.

Within soil science, however, attempts are made to systematise and group soils into genetically related units. The intention is also that these units should have practical value by giving particular consideration to properties that are important from a user perspective, such as production conditions. The concept of soil must also be related to a specific surface area—namely a pit with 1‑m sides. Otherwise, soil description and classification would become practically unmanageable. The aim is to prevent local micro‑variations, for example due to earthworm activity or a decaying twig, from being described as separate soils.

Soil as an indicator for land use

Because soils are the result of different soil processes and site conditions, soils provide information about soil properties, sensitivity, suitability, and management requirements. Soils are therefore routinely used worldwide as indicators in land use and land‑use planning. As soils are constantly subject to change, repeated mapping can show the consequences of land use and environmental impacts on soil processes.