Nitrogen Fertilizer Rate Affects Yield and Tuber Quality of Drip-Irrigated Tablestock Potatoes (Solanum tuberosum L.) Grown under Subtropical Conditions

The response to nitrogen (N) rate for two potato (Solanum tuberosum L.) cultivars grown in a sub-tropical climate was evaluated during two spring seasons. Early-maturing tablestock potato cultivars, 'Fabula' and 'Red LaSoda', were grown using surface-drip irrigation. N fertilizer was applied through irrigation drip tape at 0, 112, 224, and 336 kg ha(-1) of N. There was no difference of aboveground and tuber biomass accumulation between N fertilizer rates from 112 to 336 kg ha(-1) of N during both growing seasons. Seasonal differences were observed in N recovery efficiency; in Season 1, apparent N recovery (ANR) was significantly higher for N rates of 112 and 224 kg ha(-1) N for 'Fabula' and 112 kg ha(-1) of N for 'Red LaSoda', respectively. In Season 2, there was no significant difference between ANR values among N rates between 112 and 336 kg ha(-1) of N, for both cultivars. There was a significant interaction of N rate and growing season on total and marketable yields. In Season 1, average total and marketable yields for both cultivars increased from 9.8 Mg ha(-1) with no N applied, to 25.8 Mg ha(-1) with 224 kg ha(-1) of N; no significant differences were observed at higher N rates. In Season 2, high early season rainfall likely depleted available soil N, resulting in decreased N recovery efficiency of plants and significantly lower yield compared to the previous season. N-fertilizer rates above 224 kg ha(-1) of N did not increase total or marketable yields in both seasons. High rainfall events at the beginning of the Season 2 likely caused N stress and reduced tuber yields by 54% compared to the previous season. Results also highlighted the dominant effect of environmental conditions, particularly rainfall, on tuber yield and harvest quality response to N fertigation rate.