Agricultural biomass residues are being considered as alternative energy sources due to their perceived carbon neutrality and renewability. There exists little Ontario-specific information on residue nutrient variability and the effect of weathering on residue nutrient properties. It is known that high residue nutrient concentrations and moisture contents are significant impediments to residue combustion quality. Potassium concentration in biomass is important in terms of impact on combustion processes, and also in terms economic impact due to nutrient removal. Within and across field variation of yield, harvest index, and potassium concentration and removal were assessed for residues of winter wheat (Triticum aestivum). Whole plant samples were collected from twenty commercial fields across Ontario, Canada in 2009 and 2010. The straw component was analyzed immediately and following a weathering period. Corresponding soil samples from each sample location were analyzed for potassium concentration to identify correlations between plant tissue and soil nutrient concentrations. Potassium concentration was highly variable from field to field. Initial potassium concentrations at harvest ranged from 0.51 to 1.56% in 2009, and 0.35 to 1.49%, in 2010. This variability could not be explained by soil potassium level. The weathering period consistently reduced potassium concentrations. Results indicate that within field variation was lower than across field variation for wheat potassium concentrations.