Gypsum or calcium sulphate dihydrate (CaSO4 · 2 H2O) has been shown to considerably reduce the leaching of phosphorus from clay soils under laboratory conditions and in tests carried out at field and catchment levels. Therefore, gypsum treatment has recently been proposed as an effective means to reduce phosphorus loading on water bodies. The effect of gypsum is based on the sulphate and calcium it contains. Calcium,in particular improves soil structure by promoting the flocculation of soil particles. This promotes the formation and stability of soil aggregates, which makes the soil more resistant to erosion. This also reduces the loss of particulate phosphorus which flows with fine soil material in runoff waters into water bodies. In addition, dissolved phosphate phosphorus binds more tightly to the surface of soil particles and the risk of leaching decreases. However, phosphorus remains available to plants.
In some experiments, gypsum has been found to reduce the load of particulate phosphorus on waters by as much as 60%, but not all experiments have produced equally clear results. Gypsum can only be used in river catchments with no lakes where the river flows directly into the sea, as the sulphate in gypsum may increase the release of phosphorus from internal reserves at the bottom in lakes. In addition, it should be noted that gypsum does not remove phosphorus from fields and the treatment must be repeated at an interval of a few years. Therefore, to achieve a permanent reduction in loading, soil phosphorus levels in fields should still be reduced. A key problem in many fields is organic matter reduction, which increases nutrient leaching.
The aim of this measure is to support and promote a gypsum treatment project being launched (Finnish Environment Institute, University of Helsinki) to examine the use of gypsum in a considerably larger area of fields than previously. The long-term objective of the measure is to enable the extension of the use of gypsum to other suitable coastal catchments if the results of this experiment support it. The measure will examine the suitability of gypsum treatment for different types of fields and its effects on soil fertility, aquatic organisms in rivers, nutrient loading on water bodies and nutrient concentrations in water bodies. The project will also collect information on the feasibility of large-scale gypsum treatment in practice and on the experience, observations and attitudes of farmers relating to the new method. The project will last several years so that the effect of varying weather conditions and seasons can be taken into account. The gypsum treatment of fields could also be one of the methods in the measure EUTROPH 4, which seeks to improve the habitats of sensitive species in flowing bodies of water.
The measure will also examine the effects of the application of structure lime to fields. Structure lime is a lime product that contains at least 20% of reactive lime (quicklime CaO or slaked lime Ca(OH)2). Preliminary studies with structure lime have produced results very similar to those found with gypsum. Structure lime also reduces soil acidity, whereas gypsum only has a minor effect on soil pH.
Ministry of the Environment