Spatial and temporal variation in soil Mn² was observed over a 12-month period at two field sites near Gerogery and Binalong in southern New South Wales (NSW), Australia. Three pot experiments were then conducted to emulate the range of soil Mn² concentrations observed in the field and to determine the effect of different concentrations on lucerne and subterranean clover seedling growth, as well as to determine the effect of heating a soil on pH and Mn² concentrations. Concentrations of soil Mn² in the surface 0.20 m varied at a given sampling date by up to 288% (2.5–9.7 µg/mL) and 183% (8.7–24.6 µg/mL) across the Gerogery and Binalong field sites, respectively. At both sites, the concentration of soil Mn² in a given plot also varied by up to 175% between sampling times. There was little consistency between sites for seasonal fluctuations of soil Mn² , although in both instances, peaks occurred during months in which newly sown lucerne plants might be emerging in southern NSW. Pot experiments revealed that high concentrations of soil Mn² reduced lucerne seedling survival by 35%, and on seedlings that did survive, reduced shoot growth by 19% and taproot length by 39%. Elevated concentrations of soil Mn² also reduced subterranean clover seedling survival by up to 55% and taproot length by 25%, although there were few effects on subterranean clover in treatments other than those imposing the highest soil Mn² concentrations. The third pot experiment demonstrated that elevated soil temperatures led to increased soil pH and increased soil Mn² concentrations, attributable to a decrease in biological oxidation of soil Mn² . This was in contrast to the commonly anticipated response of a decline in soil Mn² concentrations as soil pH increased.