A field experiment was established to determine whether excess boron (B) in a new, blended commercial fertiliser reduced growth of Sebago potatoes. Five rates of B as sodium borate (0, 2, 4, 8, and 12 kg B/ha) were mixed with superphosphate, potassium chloride, and ammonium sulfate and applied in bands, at planting, to an oxisol with a hot CaCl2-extractable B concentration of 0.7 mg/kg. At the common commercial rate of 1500 kg blended fertiliserba, 8 kg B/ha would be applied in the new blended fertiliser. Yield of grade 1 tubers was significantly (P39 and 26 mg B/kg for whole shoots and YFEL, respectively. The sufficiency range for B in Sebago potatoes was 33-39 mg/kg for whole shoots and 24-26 mg/kg in YFEL. The threshold B concentrations for yield reduction in potatoes due to toxicity recorded in this study are much lower than those previously reported for other cultivars.