Influence of nitrogen form on yield and nitrate content of subirrigated early potato.

Potato is classified among the vegetables with low nitrate content but, in diet, it contributes most to the daily intake of nitrate, because of its high per capita consumption. Two trials were carried out in winter–spring and autumn–winter cycles using a trough bench subirrigation system. Potato seedlings were transplanted into pots containing peat, pumice and vermiculite in a 3:1:1 volume ratio. Both trials were carried out to compare three nutrient solutions having the same nitrogen concentration (6.4mM), but different ammonium:nitrate (NH4-N:NO3-N) percentage ratios (100:0, 50:50 and 0:100). In the winter–spring cycle, tubers were lower in weight and were more numerous than in the autumn–winter cycle. The tuber yield of ammonium-fed plants was lower than with the mixed form and 100% NO3-N, but only in the trial carried out in the winter–spring period. Nitrate-fed plants yielded a number of tubers almost 3-fold higher than ammonium-fed plants. The NO3 content of tubers harvested in spring in the presence of 100% NH4-N in the nutrient solution was a 25% of that in nitrate-fed plants (44 vs 169mgkg−1 of fresh mass); in tubers harvested in winter, with worse light conditions, nitrate content increased with increasing NO3-N in the nutrient solution (26, 109, and 225mgkg−1 of freshmass with NH4-N:NO3-N 100:0, 50:50 and 0:100, respectively). The substrate electrical conductivity increased with increasing ammonium concentration in the nutrient solution, and was higher in the upper layer of the substrate.