Acid sulphate soils are mineral soils with a high sulphur content, where sulphide has acidified into sulphate and therefore the pH has dropped to a very low level. Sulphide sulphur turns into sulphate on contact with air. All forms of land usage in which this can occur can cause acidification, both in the soil and in run-off water leaching from the area. Problems are also caused by metals solubilizing as a result of acidification, e.g. aluminium, iron and manganese.
Acid sulphate soils are typically found in coastal regions, but also inland in the vicinity of black shale sections in bedrock. The sulphurous mineral soil in coastal regions originates from the prehistoric seabed, which owing to the post-glacial land uplift may extend to areas as much as 100 m above the present sea level. However, by no means all the mineral soil in such areas is sulphurous.
Bedrock black shale is also of marine origin, but is considerably older. Black shale weathering and the minerals originating from it are found either directly on top of black shale bedrock or where the icecap deposited weathered sediment in the ‘fan’ of the rock. In Finland the last movements of the icecap are known fairly well, and use has been made of this in defining sulphur risk areas.
In peat production the biggest risk for the oxidation of sulphur in mineral soil and thus for acidification is at the preparation stage of a new production area, when sedimentation ponds extending down into the mineral soil and the deepest main ditches are constructed, and also towards the end of production, when the field ditches extend down to the mineral soil.
In sulphide risk areas, thorough planning, identification of possibly problematic soil and, if necessary, selecting a production method to minimize oxidation, can prevent the soil becoming oxidized and an acid load being created.
Vapo has mapped out its sulphide risk areas and takes the relevant risk factors into account in planning production areas and at the various stages of production.
Vapo participates in an ERDF-funded project, SuHE, on methods for forecasting and managing the acid load in sulphate soils, the aim of which is to find new methods to prevent acid load.