The fine gypsum was discarded from the trial and was replaced with superphosphate and coarse rock gypsum (from Jurien Bay), which release their S more slowly.
When the plants were assessed in the year in which the fertilisers were applied (Figs 1-3), the growth and N:S responses were greatest with the superphosphate and Ironman gypsum pellets, followed by unpelletised by-product and coarse rock gypsum. The fine gypsum produced very little improvement in growth with increasing application, shown by a flat growth response curve and a lack of change in N:S ratio with increasing application of fine gypsum.
The Ironman pellets produced similar or greater yield and N:S response than the superphosphate, followed by unpelletised by-product and coarse rock gypsum. The fine gypsum produced very poor, flat response curves.
The Ironman gypsum pellets reached a plateau at 15% of the S leached, superphosphate at 40% leached, and the coarse rock gypsum at 70% leached.