Gypsum to Improve Soil Structure and to Reduce Phosphorus Loss

Keywords: gypsum, phosphogypsum, P losses, turbidity, particulate P, dissolved reactive P, electrical conductivity.

Phosphogypsum (CaSO₄ x 2H₂O), later in the text referred to as gypsum, is a by-product from the fertiliser industry. Studies of gypsum to improve soil structure and reduce phosphorus (P) losses on clayey soils in southern Finland are reviewed.

Clayey soil amended with gypsum (0, 2, 4 and 6 t/ha) showed small improvement in aggregate stability and infiltration capacity in wheat cropping by both direct drill or minimum tillage. Gypsum increased the ionic strength in soil, which increased adsorption of P on soil surfaces.

In a rainfall simulation study, soil monoliths were collected from two gypsum- (3 and 6 t/ha) and limestone- (CaCO₃ 4.3 t/ha) amended fields. Laboratory rainfall simulations were done 7, 19, and 31 months after the initial applications. Over the monitoring period, gypsum-amended soils exhibited substantial decreases in turbidity of 45%, in particulate phosphorus (PP) 70% and dissolved reactive phosphorus (DRP) 50% of the percolation waters compared to control samples. Limestone had no effect on field 1 and only a small effect on field 2.

In a catchment scale (2.45 km²) study, changes in loss of PP and DRP, after treating 93 ha of 101 hectares fields with gypsum (4.1 t/ha), were estimated. Using a covariance model, gypsum amendment was estimated to have reduced the loss of PP by 64%. The loss of DRP decreased by one-third. No such changes were found in the nearby reference catchment.

The modified ICECREAM model enables comparison between agri-environmental measures in the reduction of P losses. The model was able to simulate gypsum addition in three of the four monitored seasons with reasonable accuracy. According to simulations, gypsum reduced the total P losses by 44%.

Gypsum increased the ionic strength indicated by electrical conductivity (EC) in percolation waters and soil samples. It is suggested that the effect of remaining gypsum could roughly be estimated by measuring the EC in percolation water or soil solution. Still, in (2013) ongoing follow-up studies, gypsum has decreased the P runoff during a four-year period.

According to plot-scale field experiments, gypsum had no negative effects on field crops. In clayey catchments discharging into the sea, gypsum may provide an efficient measure to reducing P losses from cultivated fields. The impact of sulphate (SO₄) on the ecology of sulphate-poor lakes has yet to be determined. The sulphate provides sulphur supply for crops and thus do not necessarily leach from the profile.

Raimo Kauppila, Yara Suomi, Bertel Jungin aukio 9, FI-02600 Espoo, Finland.

Liisa Pietola, MTK (Central Union of Agricultural Producers and Forest Owners) PL 510, FI-00101 Helsinki , Finland.

19 pages, 2 plates, 6 figures, 3 tables, 36 references.