CRITICAL PETIOLE NITRATE LEVELS IN POTATO

Critical nutrient concentrations (CNC), below which a yield reduction occurs, for potato (Solanum tuberosum L.) petiole nitrate (PN) have been reported for the Pacific Northwest growing region, but differences in method of determination or definition of this parameter have resulted in some discrepancies in values. The objectives of this research were to document seasonal PN levels with varying levels of applied N, to determine CNC at three growth dates [tuber initiation, TI; 21 d after TI(TI + 21); and 42 d after TI (TI + 42)], and to establish the relationship between yield responses to midseason N application and CNC. Experiments were conducted at two sites on a Typic Argiboroll and three sites on a Pachic Haploxeroll in western Montana. Treatments included six or eight N rates between 0 and 336 kg N ha-1 applied as NH4NO3 before hilling and a topdressing of 67 kg N ha-1 broadcast on subplots at TI + 21. Early season PN depended on applied N, with maximum values of 25 to 30 g NO3-N kg-1 at TI. Values declined with seasonal progression and became increasingly dependent on fertilizer N rates. The N topdressed at TI + 21 increased PN level 6 g kg-1 at TI + 42 compared to non-topdressed areas. Linear plateau regression of relative yield against PN revealed CNC values of 25 g kg-1 at TI, 14 g kg-1 at TI + 21, and 10 g kg-1 at TI + 42. Statistical analysis could not separate the TI + 21 and TI + 42 regressions, indicating a CNC of 13 g NO3-N kg-1 throughout the period of tuber bulking. Yield responses to topdressed N, however, were documented only when PN values were below 11 g NO3-N kg-1 at TI + 21. Conservative interpretation of critical nutrient concentration is justified.