Total element content in mineral soil

Geochemistry of the parent material

Differences in the mineralogy of soil mineral material types are caused by the material being broken off from rock types of varying mineralogical composition. This also implies that the geochemical composition, that is the content of different elements in the minerals of the soil, varies.

Silicon is one of the main ingredients in most minerals. For example, the common mineral quartz consists of pure silicon dioxide, SiO₂. A large concentration of silicon in the parent material indicates that the proportion of mineral not easily weathered is large.

Calcium is part of many minerals found in soils. In the plants its most important function is to stabilise the cell membranes. As calcium is a fairly common element in most soils, plants seldom lack this element.

Being part of the chlorophyll, magnesium is an important micro nutrient. Lack of magnesium may be seen on acid soils, where magnesium is competed out by aluminium. Maps show that there is a large variation in the content of calcium and magnesium in the parent material. In areas with sandstone kinds of rocks, like for example the Jotnic sandstone in the north-western part of the province of Dalarna, concentrations of calcium and magnesium are particularly small.

Geochemistry of parent material is one factor affecting soil development, the vegetation and the forest ecosystem, and that contributes to, as well as, limits the productivity of forests. It is also of great importance for the capacity of soils to neutralise acid compounds deposited by precipitation.

Analyses of total geochemical composition

The amounts of macro and trace elements shown in the maps are the total geochemical composition at 50 cm depth. The total geochemical composition is described with the weight fractions of the elements being part of the mineral soil. For the main elements (macro elements) estimations normally are based on weight fractions of oxides of the elements. Trace elements, on the other hand, are shown as pure elements, normally in ppm (parts per million).

The total chemical composition is estimated as follows:

• The sample is passed through a sieve to get rid of the texture fraction > 2 mm.

• Fifteen gram of the remaining sample is homogenised by grinding.

• The sample is ignited at 1000 degrees Celsius.

• The sample is completely fused (melted) in a lithium-borate fusion (melt).

• The melted sample is dissolved in HNO₃ (nitric acid).

• The solution is analysed by means of the ICP method.