By soil type is understood the upper part of the ground which is affected by organisms, water, wind, and climate, and thus, to some extent continuously is evolving. The evolution of soil types is also influenced by topography and qualities of the mother material, like for example mineral and grain size distribution of the detached deposits. Factors affecting soil type formation are denoted soil type forming factors.
The soil type can also be regarded as the part of the ground used by plant roots, and thus, constitutes the basis for plant growth. In Sweden, soil types generally have been formed in unconsolidated glacial, or post-glacial deposits. In areas not affected by glaciation, and where soil type forming processes are intense, the soil type may comprise the entire unconsolidated deposit formed through the influence of organisms, climate and water.
Thus, development of soil types implies transformation of original material, that is rock or an unconsolidated deposit through different soil processes. Examples of soil processes are chemical weathering of minerals, and precipitation of difficultly soluble secondary materials, like oxides of ion and aluminium. Other soil processes are accumulation of dead organic material at the soil surface and transport to deeper soil layers. Percolating water can bring solid particles and dissolved material that either get stuck at deeper layers or are transported to other areas like, for example, discharge areas or lakes. Soil organisms alter the spatial distribution of particles in ground. In this way soil animals may contribute to an aggregated soil structure with large pores.
Ground has of course been influenced in different ways, for example because climate and soil organisms differ within Sweden, as well as in other parts of the world. This has resulted in a large number of different soil types. Apart from what is the case with plants and animals there are (genetically) no clearly defined species, but soil types gradually grow into each other. The aim within soil science is, nevertheless, to systemise and bring soil types together in genetically related units. It is generally considered to be of great importance that these units be useful in practice. This is achieved by considering aspects from a user perspective, for example prerequisites for forest production. The classification must be related to certain plot dimensions. For this purpose, the soil science community have agreed upon a pit with 1 m side, otherwise soil type description and soil type classification cannot be manageable. The idea is to prevent local micro-variations, for example, depending on earthworm activity or decomposing coarse litter, like stems and branches, being described as different soils types.
Through the soil type processes different soil horizons are formed, between themselves with different qualities. Each soil type is characterised by a specific sequence of horizons, these are: organic layer (O), leached horizon (A and E), horizon with enrichment (B) and parent material (C).
Soil types according to FAO
A national system for classification of soil types is used in Sweden. This covers among other things different forms of podsols, brown forest soils, waterlogged soils and lithosoles. Other countries or regions of the world apply their locally developed systems for soil type classification. The system developed in the US, called "Soil Taxonomy" is so comprehensive and so general, that it can be applied worldwide, for example in the Nordic countries. The system comprises 11 main groups, for example, Spodosols, Inceptisols, Entisols and Histosols. FAO-Unesco has compiled an international soil type classification-Soil Map of the World, revised legend. This system comprises 28 main groups, for example, Podsols, Cambisols, Histosols, Gleysols, Leptosols, Regosols och Arenosols. These are, in turn, divided into subgroups. The FAO-Unesco's system is, in addition to the national system, applied within the Swedish Survey of Forest Soils.
Because soil types are the result of different soil processes and site conditions, it thereby is possible to derive information about soil quality, sensitivity, applicability and management requirements from soil types. Worldwide soil types are therefore regularly used as indicators of the use of soils and soil use planning. Because soil types constantly are exposed to changes, recurrent surveys may indicate the consequences of the use of soils and environmental influence on soil processes.