The effect of physiological age and planting date on the response of potatoes to applied nitrogen and on levels of residual nitrogen post-harvest

Experiments were conducted on sandy and deep silty soils to assess the effect of physiological age of seed tubers and planting date on the response of potatoes to nitrogen fertiliser. Nitrogen optima commonly had large standard errors which made interpretation of the data difficult. On the sandy soil with cv. Record, early planting increased yield in two years but had no effect on mean N optimum (303 kg ha(-1), NOpt). Mean NOpt was less on the silty soil where the cv. Pentland Squire was grown, and was reduced from 273 kg ha(-1) to 241 kg ha(-1) when planting was delayed by 4 weeks. Physiological age had no effect on NOpt. On the sandy soil, residual soil mineral nitrogen (RSMN) increased by 0.05 kg ha(-1) per kg N fertiliser applied; on the silty soil the increase was four times as large. RSMN was larger following the late planted crop on the silty soil in one year.