One can speak very broadly about three properties of the soil's parent material that are of decisive importance for soil fertility. These are the nutrient content of the rocks and minerals, their weatherability, and their water-holding capacity, which is influenced by the fine earth or clay content.
The bedrock is characterized by its constituent rocks. The rocks, in turn, are built up by specific combinations of rock-forming minerals. A rock can be dominated by several minerals or by one or two individual minerals. Limestone is an example of a rock consisting exclusively of one mineral, calcite, while gneiss can contain quartz, potassium feldspar, plagioclase, hornblende, muscovite, biotit, chlorite, and others. However, the majority of the rock-forming minerals are compounds of silicon and oxygen in combination with other elements, such as aluminum, potassium, sodium, calcium, and magnesium.
Rocks that have been formed from cooling magma through the crystallization of a sequence of minerals are usually classified according to their silica content. Magmatic rocks with a silica content (SiO2 content) exceeding 65% by weight are therefore referred to by geologists as acidic, while rocks with a silica content lower than 52% by weight are called basic. However, these terms have no correlation with the concept of pH in the soil, as pH is a measure of hydrogen ion activity.
Base Mineral Index (Basmineralindex)
A measure of the potential nutrient content in the soil can be obtained by determining the base mineral index, a rapid method for mineralogical soil assessment introduced in the early 20th century by the Swedish pioneer in soil research, Olof Tamm (1934). The base mineral index expresses the percentage of minerals with a density exceeding 2.68 g/cm³. In practice, this means the content of minerals containing magnesium, iron, and calcium, which have proven to be chemically easily weathered. This type of so-called mafic mineral occurs in rocks like gabbro, diorite, basalt, diabase, hyperite, porphyrite, and amphibolite. Collectively, forestry workers often refer to these rocks as greenstones, possibly based on the green color they assume during weathering. Greenstones in the true international sense, however, consist of amphibolite or altered basic rocks.
Quartz-rich rocks such as mica schist, leptite, gneiss, granite, and porphyries mainly contain minerals with a density lower than 2.68 g/cm³, and consequently, their base mineral index is very low.
Weatherability
The weatherability of a rock can refer to both its susceptibility to mechanical breakdown and chemical dissolution. Chemical weathering, which leads to the release of mineral nutrients, always begins on the surface and fracture systems of the mineral particles and then spreads further into the particle. The intensity of weathering will therefore depend on the surface area of the mineral particles. Rocks differ in this regard depending on their mineral composition and grain bonding.
A property of the rock that interacts with its weatherability is its tendency to form fine-grained soils. A widespread misconception is that the limestones in our sedimentary bedrock areas have broken down physically and given rise to the Quaternary till clays (moraine clays). In reality, it is likely that limestones, such as orthoceratite limestone, contain layers of clay of varying thickness. After the limestone has been leached out through chemical weathering over millions of years, it leaves behind a clay bed. This bed was then redistributed during repeated glaciations and incorporated into the rest of the rock material left behind as till. The fine grain size favors water-holding capacity and facilitates chemical weathering, which generally results in a good supply of plant nutrients in fine-grained soils.
Often, the fertility-promoting factors interact; that is, an easily weathered bedrock is frequently rich in nutrient-dense minerals and, at the same time, prone to giving rise to fine-grained soil types. Therefore, the bedrock can be categorized based on how good of a nutrient status it generates.
Examples of Areas in Sweden with Easily Weathered Rocks
Sedimentary rocks like limestone and shales give rise to fine-grained soils with a favorable plant nutrient content. Examples of limestone occurrences in the bedrock include areas adjacent to Lake Storsjön in Jämtland, the Örebro-Kumla plain west of Lake Hjälmaren in Närke, the Östgöta plain between Omberg-Motala and Lake Roxen in Östergötland, large parts of Öland, Gotland, and the southwestern part of Skåne. In an area of about 15,000 km² outside Gävle, the bedrock beneath the Bothnian Sea consists of limestones, shales, and sandstones. This area has influenced the properties of the soils, particularly in northeastern Uppland.
In most places in Sweden, there are locations where molten magma has intruded into or up through cracks in the otherwise solid basement rock, forming a locally distinct rock type. If this distinct rock consists of greenstones, for example, they usually have a major impact on the nutrient conditions of the soils formed adjacent to these local greenstone occurrences. Examples of this include the commonly occurring diabase dikes, the table mountains in Västergötland (including Billingen, Mösseberg, Kinnekulle, Halleberg, and Hunneberg), as well as hyperite crests in the landscape of Värmland and related hyperite-diabases in Småland. As another example of an extremely thick diabase dike, the Hällefors dike striking southeast of Lake Hjälmaren can be mentioned, which has a length of 40 km and a maximum width of 2 km.
Rock fragments and minerals from these distinct rocks can be found in the soil layers over a significantly larger area than the rock occurrence itself, thereby giving rise to highly fertile soils.
References
Loberg, B. 1999. Geologi. Material, processer och Sveriges berggrund.
Lindström, M., Lundqvist, J. & Lundqvist, Th. 1991. Sveriges geologi från urtid till nutid.
SNA-Sveriges Nationalatlas: Berg och jord. 1994 (red. C. Fredén).
SNV. 1994. Markens bördighet. SNV rapport 4337.
Tamm, O. 1934. En snabbmetod för mineralogisk jordartsgranskning. Sv. Skogsvårdsföreningens Tidskr. 32. sid 231-250.
Troedsson, T. & Nykvist, N. 1974. Marklära och markvård.