Weathering

A distinction is usually made between two types of weathering: physical and chemical weathering. Physical weathering manifests as the breakdown into smaller particles, whereas chemical weathering involves a more or less complete dissolution and a change in the chemical composition of minerals and rocks.

Maps of weathering for different elements

During chemical weathering of minerals and rocks, various elements are mobilised into plant-available forms. The estimated annual supply of calcium (Ca), potassium (K), magnesium (Mg), aluminium (Al), iron (Fe), sodium (Na), phosphorus (P), silicon (Si), as well as Ca+K+Mg through chemical weathering in Swedish forest soils is shown in the maps below. The methods used for these calculations are described in Olsson et al. (1993).

Physical and chemical weathering

Weathering comprises the physical and chemical changes (primarily disintegration) that rocks and minerals undergo under the influence of, among other factors, water and biological activity. A distinction is usually made between two types of weathering: physical and chemical weathering. Physical weathering manifests as the breakdown into smaller particles, whereas chemical weathering involves a more or less complete dissolution and changes in the chemical composition of minerals and rocks.

Physical weathering occurs through various forms of mechanical forces, such as frost action, rapid temperature fluctuations, or the fracturing effect of roots. The driving force behind chemical weathering is fluids with, among other properties, acidic characteristics. Sometimes biological weathering is distinguished as a third type of weathering. However, biological weathering can be regarded either as chemical weathering initiated by biological processes or as physical weathering caused, for example, by root action.

Physical and chemical weathering act in concert. Breakdown into smaller particles increases the surface area available for chemical attack, thereby intensifying chemical weathering. Chemical weathering, in turn, can create cavities in which ice and roots can exert mechanical pressure.

The chemical changes resulting from chemical weathering involve dissolution of minerals and rocks as well as the formation of secondary minerals. Chemical weathering therefore plays the most important role in the mobilisation of elements into plant-available forms, i.e. for the long-term nutrient supply of vegetation. Chemical weathering consumes hydrogen ions and is thus also a process that counteracts the accumulation of hydrogen ions and the resulting decrease in pH in soils and waters. When referring to the weathering of the mineral soil in connection with sustainable site productivity or sensitivity to acidification, it is therefore generally chemical weathering that is meant.

Weathering factors

The rate of chemical weathering depends primarily on mineral composition, particle size distribution, climate, and weathering-enhancing substances such as acids and organic matter. The most common soil minerals can be grouped according to increasing susceptibility to weathering:

With respect to mineral composition, rocks can be grouped according to their susceptibility to weathering as follows:

The weathering rate also depends on particle size. However, specific surface area is a more appropriate concept than particle size, since weathering—like most soil processes—occurs at particle surfaces. Specific surface area is defined as the total surface area of all particles in a given amount of soil.

The specific surface area typically amounts to approximately 50–300 m²/g mineral soil for clay particles, and less than 1 cm²/g for particles in the sand fraction. A small increase in clay content in absolute terms, from 2 to 4 per cent, nearly doubles the specific surface area. In the B horizon of a sandy–silty till, the specific surface area may amount to 5–10 m²/g. Most of the specific surface area in till is attributable to clay particles and organic matter. Organic matter has a very high specific surface area, >600 m²/g. In principle, weathering is proportional to the specific surface area.

Under natural conditions in forest soils, however, there is a risk of overestimating the effect of specific surface area. A large surface area implies small particles and small pores, which in turn impede water movement between particles. This is a disadvantage from a weathering perspective, as flowing water in the soil promotes weathering by supplying acids and removing weathering products.

Climate controls weathering through temperature as well as water surplus (the difference between precipitation and evapotranspiration, including plant transpiration) that percolates through the soil. Another important factor is the duration of soil freezing during the winter period.

Organic matter plays a key role in the chemical weathering of the mineral soil. This is due both to the acidic properties of organic matter and to its ability to complex metal ions, thereby promoting the removal of weathering products and lowering ion concentrations in the soil solution. At typical pH values in forest soils (approximately 4–5), weathering driven by organic acids is several times faster than weathering by inorganic, non-complexing acids such as sulphuric acid. This implies that a decrease in pH due to acid deposition under field conditions in forest soils is likely to have only a marginal effect on the rate of weathering.