Characteristics of forest soils

Soil horizons

Soils are organized in distinct horizons, which are horizontal layers that form through processes like weathering, organic matter accumulation, leaching, and mineral transformation. Typical horizons include the O horizon (organic layer), A horizon (topsoil), B horizon (subsoil), and C horizon (weathered parent material), each differing in color, structure, and composition.

Soil types

Forests in Europe grow on a diverse range of soil types. Soil types are defined by factors such as mineral composition, texture (proportions of sand, silt, and clay), organic matter content, structure, drainage, and chemical properties like pH and nutrient levels.

The different types of soils across Europe are:

Cambisols

Cambisols are common in temperate regions, have a loamy texture with moderate water-holding capacity and good nutrient availability, making them ideal for many forest types.

Podzols

Podzols dominate under coniferous forests in Northern Europe; these soils are sandy, acidic, and nutrient-poor, with low water retention and distinct leached horizons that reflect strong vertical movement of minerals.

Histosols

Typical for wetter colder regions, histosols are peat-rich, store large amounts of organic carbon and water but are highly acidic and low in minerals.

Luvisols

Luvisols are typically found under deciduous forests, have clay-enriched B horizons that hold water well but can become compacted if poorly managed.

Arenosols

Arenosols develop in drier pine forest areas, are sandy, drain quickly, and have low fertility and poor water retention.

Gleysols

Gleysols are found in waterlogged lowland forests, have a heavy texture and chemically reduced, often bluish-grey horizons due to long-term saturation.

INSERT GRAPH with title: Soil types vary across Europe, in big clusters or scattered, and determine the type of forest that grows on it.

Physical soil charcteristics

The soil’s ability to support plant growth, store and transmit water, and sustain soil life are determined by physical soil characteristics like soil texture (sand, silt, clay, loam, peat and coarse fragments), water-holding capacity and soil compaction. These factors determine how easily roots can penetrate the soil, how much water and air the soil can retain, and how resistant it is to erosion.

The interaction between climate, soil texture, and water-holding capacity plays a key role in determining forest type and productivity and carbon storage capacity—an important factor in mitigating climate change (see the climate page).

Soil texture

Soil texture describes the relative proportions of sand, silt, and clay particles in the soil, which influence its ability to retain water, support plant roots, and hold nutrients

Soil texture classes, qualities and European distribution

INSERT graph with title: Soil texture classes vary across Europe

INSERT graph with title: In southern Europe, the percentage of coarse fragments is higher than in northern Europe

Water-holding capacity

Water-holding capacity refers to the amount of water a soil can store. The soil texture often determines the ‘available water capacity’, which is the fraction that is accessible to plants. Silty soils strike a balance between drainage and moisture retention, making them ideal for most forest plants and trees. Sandy soils’s large particles form large gaps that allow water to drain quickly, so they retain less water and are less able to provide moisture to trees during dry periods. Clay soil has a high water-holding capacity due to its small particle size and tight structure, which creates many small pores that hold water tightly. However, a significant portion of the water is held so strongly in clay that plants can not extract it easily. In wet periods, drainage of clay soils can be very poor also hampering tree growth.

INSERT graph with title: In Northern Europe, available water capacity (AWC) is lower than in Central and Southern Europe

Bulk density

Bulk density (g/cm³) measures how tightly soil particles are packed together and is calculated as the ratio between dry soil mass and total soil volume. It is an important indicator of soil structure and root penetration and is used to determine soil porosity and moisture. The bulk density for disturbed soils, such as arable and permanent crops, ranges from 1.2 to 1.3 g/cm³. In contrast, forests (woodlands) exhibit lower bulk densities, ranging from 0.75 to 0.9 g/cm³.

INSERT graph with title: Forest do not appear where soil compaction is high
(left: soil bulk density in Europe, right: LUCAS points from which soil bulk density was derived for woodland)

Chemical soil characteristics

The chemical characteristics of forest soil affect growth and forest health. The main soil chemical properties include soil organic carbon, pH, Cation Exchange Capacity (CEC), total nitrogen (N), exchangeable potassium (K), available phosphorous (P) and the carbon-nitrogen (C:N) ratio.

Soil organic carbon content

Soil is the largest carbon pool in forests. Carbon accumulates in this pool through the decomposition of organic matter such as fallen leaves and branches, other wood, and roots. The soil organic carbon is higher in Northern European countries than in Central and Southern Europe. The cooler climate and wetter conditions slow down the decomposition of organic matter, allowing carbon-rich materials to accumulate in the soil. Additionally, extensive peatlands and wet forests in Northern Europe store large amounts of organic carbon due to waterlogged soils that limit oxygen and microbial activity. Together, these factors create ideal conditions for carbon accumulation and long-term storage in forest soils.

INSERT GRAPH with title: The soil organic carbon is higher in cooler regions in Northern Europe and mountainous areas.

Soil acidity

Soil acidity (expressed in the pH-CaCl₂ value), ranges from 0 to 14. When soil acidity is high, the pH-CaCl₂ value is low. Soils with values below 6.5 are classified as acidic, below 5.5 as very acidic. Values between 6.5 and 7.5 are classified as neutral, and over 7.7 as alkaline. Depending on the tree species, forest soils can acidify further, especially because of needles of coniferous trees that are naturally acidic. The average soil acidity of European forest soils amounts to 4.6 (pH-CaCl₂) (LUCAS 2015, SOEF 2020).

INSERT GRAPH with title: Title: Soil acidity is higher in North European forest soils, and lower in southern European forest soils.