However, phosphorus concentrations are normally low in acidic
waters due to the exchange acidity of sediments4
. The use of
gypsum may therefore reduce the productivity of acidic ponds.
It may be necessary to correct total alkalinity with lime and
increase phosphorus levels with fertilisation if treatment with
gypsum causes these parameters to fall below acceptable levels.
The calcium ions supplied by gypsum act as electrolytes in
the flocculation of colloidal particles. For example, gypsum
applied at rates of 100-500 mg/litre substantially reduced
turbidity in one study. However, similar reductions in turbidity
were achieved using 20-25 mg/litre of alum, which is a more
cost effective treatment7