The quantification of sidedress N rate from in-season information on crop N nutritional status is extremely important in order to use only the necessary amount of nutrient to achieve maximum yields without significant N losses from soil. During three consecutive years (1996, 1997 and 1998) field N fertilizer experiments with irrigated potato (Solanum tuberosum L., cv. Désirée) were conducted in Bragança (NE Portugal). Eight preplant treatments (0, 50, 100, 200 and 300 kg urea-N/ha and poultry manure, farmyard manure and municipal solid waste in rates equivalent to 100 kg organic-N/ha) were arranged as main plots and five sidedress N rates (0, 25, 50, 100 and 200 kg urea-N/ha) included as subplots. Petiole nitrate concentration, determined by a laboratory method (PNLab, g NO3-N kg−1, dry weight basis) and by the portable RQflex reflectometer (PNRQflex, mg NO3 kg−1, from fresh tissues), and leaf N content (LeafN, g kg−1, dry weight basis) were used as N nutritional indices. From the five sidedress N rates applied over each of the preplant treatments, critical sidedress N rates were estimated for several different crop N nutritional status. This was achieved by establishing exponential asymptotic curves between sidedress N rates and tuber yields and solving the equations for 95% of maximum tuber yield. In a second step, multiple regression equations were established between the estimated critical sidedress N rates (SNrate, kg N/ha), as dependent variable, and each crop N nutritional indices (PNLab, PNRQflex or LeafN) and days after emergence (DAE). The equations obtained are:
SNrate = 182.7 − 4.146 × PNLab −1.87 × DAE
SNrate = 161.0 − 0.013 × PNRQflex −1.34 × DAE
SNrate = 562.2 − 8.416 × LeafN −3.59 × DAE
These equations provide quantitative sidedress N rates for any level of crop N nutritional indices and sampling dates from 10 to 48 days after emergence.
