Midrib nitrate concentration as a means for determining nitrogen needs of broccoli

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Abstract
Plant analysis has been used to evaluate the nutritional status of many crops for diagnostic and corrective purposes. This study was initiated to establish critical N plant tissue levels using midrib NO3‐N concentration for broccoli (Brassica oleracea L., Botrytis group) during the growing season. Tissue samples for nitrate analysis were taken from broccoli plants over a period of three growing seasons beginning at the 4 to 6 leaf stage of growth and biweekly through pre‐harvest. The midrib from the most recently matured leaf was sampled for NO3‐N concentration determination. The relationship of NO3‐N concentration of broccoli midribs at various sampling dates to yield was evaluated by stepwise regression analysis. A high degree of correlation existed between these two factors (R=0.75). Nitrate‐N concentration in broccoli midribs indicated the N status of the plant. Minimum or critical levels of NO3‐N in broccoli midribs for sampling dates throughout the growing season were established for the desert regions of Arizona as follows: 4 to 6 leaves, 10,000 mg kg‐1; 10 to 12 leaves, 9000 mg kg‐1; first bud, 6000 mg kg‐1; head development, 3500 mg kg‐1; pre‐harvest, 2000 mg kg‐1.