Gypsum amendment of soils reduces phosphorus losses in an agricultural catchment

We estimated the changes in the losses of particulate and dissolved phosphorus (P) after treating 93 ha of agricultural fields with gypsum (4 t ha–1) in a 245 ha catchment in southern Finland. Runoff was monitored using online sensors and manual sampling during one high-flow period before and six periods after the gypsum amendment. Turbidity recorded by the sensors correlated with particulate P analysed in the laboratory, which enabled the evaluation of changes in particulate P from the online data. Using a covariance model, gypsum amendment was estimated to have reduced the loss of particulate P by 64%. The loss of dissolved reactive P appeared to decrease by one third, but was estimated with less precision. No such changes were found during the same period in a nearby ‘reference’ catchment, where gypsum was not used. Gypsum did not affect soil test values for P, K, Mg or Ca, but it did increase the ionic strength and soil test SO4. In clayey catchments discharging into the sea, gypsum may provide an efficient means to reduce P losses from field cultivation. The duration of the gypsum effect and impact of SO4 associated with gypsum amendment on the ecology of rivers and lakes has yet to be determined.