EFFECT OF NUTRIENTS AND ELEMENTAL SULFUR PARTICLE-SIZE ON ELEMENTAL SULFUR OXIDATION AND THE GROWTH OF THIOBACILLUS THIOOXIDANS

Elemental sulfur (S) has many attractions as a fertiliser but it must be oxidised to sulfate before it is plant available. Two laboratory in cubation experiments with a high S sorbing basaltic soil (Haplohumult) from Walcha, New South Wales, are reported here. The first experiment was conducted to study the effect of P and other nutrients on the oxi dation of elemental S and the growth of Thiobacillus thiooxidans. The second experiment studied the effect of phosphorus (P) rate, elemental S particle size, and elemental S form on the oxidation of elemental S at different times. There were significant differences between treatm ents in the percentage and amount of elemental S oxidised, with the lo west oxidation occurring during the 6-week incubation in the -P treatm ent, which represented 1.8% of the applied S compared with 16.0% when all nutrients were supplied. There was a significant linear relationsh ip between T. thiooxidans population at the end of the incubation peri od and the amount of elemental S oxidised. The oxidation of elemental S was higher when fine (50-150 mu m) particle size elemental S was use d, compared with coarse (150-250 mu m) elemental S. There was no clear difference in oxidation rate between ground and recrystallised elemen tal S. The S oxidation rates recorded in these experiments were compar ed with those predicted by an S oxidation model and found to be in clo se agreement.