Applying gypsum on agricultural fields to reduce phosphorus discharge to the Archipelago Sea, Finland
Usage of gypsum amendment as a mean to reduce the load of eutrophying phosphorus entering the Archipelago Sea through the river Savijoki (Finland).
The measure is ideal for the coastal areas, however, due to the sulphate content of gypsum, the measure is not suitable for lake catchment areas.
Technical conditions
According to the typology of surface waters, the Savijoki belongs to medium-sized rivers in regions with clay soils and its ecological state is only passable. The river is a tributary of the River Aurajoki, which discharges into the Archipelago Sea (the Baltic Sea) suffering from severe nutrient enrichment and eutrophication.
In autumn 2016, in total of 6 200 tonnes of gypsum were spread on 1 550 hectares of field, and the effects of this operation were monitored both at the river and the fields. The pilot was divided into three areas: two in the middle reaches of the river and one control area (without application of gypsum).
The gypsum used in this project is a pure gypsum from YARA's factory in Siilinjärvi. This gypsum is a by-product of the phosphorus acid industry and can be used for soil amendment. The gypsum is free of any heavy metals and safe to use in the fields.
Implementation
Pilot project on gypsum application examined the viability of a new, promising water protection measure. Gypsum has the potential to significantly lower the phosphorus loading originating from agriculture and ending up in the Baltic Sea. With four tonnes per hectare, gypsum improves the soil structure enabling phosphorus to remain in the soil, thus reducing both dissolved reactive phosphorus and particle phoshorus run-off and improving the quality of run-off water. The effects begin immediately after the dissolution of gypsum and last for several years.
The method is easy for farmers to apply, since gypsum can be spread using the same machinery as lime or manure. As a water protection measure it is quick and effective. Earlier studies suggest that gypsum can reduce the phosphorus load by half without requiring changes in farming practices or a decrease in cultivation area or yields. Gypsum application to fields is more cost-efficient at reducing phosphorus loading than any other water protection method currently in use in agriculture.
Result
In clayey catchments, turbidity of water describes the concentration of soil particles that are eroded from the fields. The lower turbidity values in the gypsum area thus reflect the fact that the fields amended with gypsum have been less sensitive to erosion than the untreated fields, as expected from earlier studies and theoretical considerations. Fewer particles in the water may be beneficial for the ecology of the river itself and the recipient sea. However, the main aim of the gypsum amendment was to reduce the transport of phosphorus from fields to eutrophic coastal waters of the Archipelago Sea.
The manual water quality monitoring performed in the Savijoki showed that there is a strong and linear correlation between turbidity and the concentration of particulate, soil-bound phosphorus in the water. Therefore, the results from the two runoff events strongly suggest that gypsum had, indeed, reduced the load of particulate phosphorus.
Phosphorus bound to eroded soil particles is not entirely available to algae, therefore, particulate phosphorus is not 100% eutrophying. In addition to particle, soil-bound phosphorus, gypsum amandement also decreases more harmful phosphorus type, dissolved reactive phosphorus (DRP) load, which is fully available for algae. However, the manual water sampling indicated that the concentrations of dissolved, readily available phosphorus have also been lower in the gypsum area than in the control area. Thus, gypsum appears to reduce both forms of phosphorus.