Acquiring a high grain yield while maintaining the required grain protein content (GPC) level is vital for profitable wheat farming, but doing so is typically difficult as GPC is affected by climatic and soil-related factors and management. This study was conducted to investigate the applicability of N application at flowering time, with an emphasis on late sowing conditions, for adjusting GPC to expected levels without compromising the yield of hard and soft wheat in volcanic ash soils with high nitrogen supplying capacity. Two groups of field experiments were conducted between November 2010 and June 2013, with two hard wheat and two soft wheat cultivars under early, standard, and late sowing conditions with two split N regimes, wherein N was applied: as basal, at the stem elongation stage, and at flowering. Our results suggested that the N management strategy at flowering time for increasing GPC is suitable under optimum sowing conditions, and that the GPC of both hard and soft wheat sown at an optimum timing can be adjusted for fitting into the quality bonus window, by altering the fertilizer application rate at flowering time. This study contributes toward improving our understanding of the effects of split N fertilizer application at the stem elongation and flowering stages, and the effect of sowing time on the grain yield and GPC of both hard and soft wheat grown in volcanic ash soils.