Sulphur fertilizer and tillage effects on early season sulphur availability and N:S ratio in canola in western Canada

Field studies in Manitoba, Saskatchewan and Alberta over 3 yr evaluated immediate and residual effects of source, timing and placement of S fertilizers on sulphate-S supply to canola (Brassica napus L.). The treatments were compared under conventional tillage (CT) and in the establishment years of a reduced (RT) system. Sulphate-S in the soil was measured for two weeks after seeding, using PRS™-probes. Sulphur concentration and N:S ratio were measured in canola tissue at flowering. Tillage system had little effect on available sulphate, tissue S concentration, N:S ratio or response to S fertilization. Ammonium sulphate (AS) generally increased tissue S concentration and reduced N:S ratio with all placements, and the effects of surface-applied ammonium thiosulphate (ATS) were similar to the effects of AS. Tiger 90, a bentonite-elemental S formulation, generally produced lower S concentrations in canola tissue at flowering than did AS, and levels were similar with elemental S and Tiger 90. Both tissue S concentration and N:S ratio were good indicators of S sufficiency for canola. Sulphate carryover from AS increased tissue S concentration and decreased N:S ratio in the year following application. In contrast, increases in tissue S concentration and decreases in N:S ratio were lower with Tiger 90 than AS. The oxidation of the residual elemental S in Tiger 90 could not be relied upon to increase tissue S concentration above deficiency levels in the year following application where soil sulphate levels were low.