Interpreting the Rate of Change in Nitrate-Nitrogen in Sugarbeet Petioles'

Nitrate-nitrogen in sugarbeet petioles is used to evaluate current N status of sugarbeet crops. Since the NO,-N changes rapidly during the season, better relationships are needed to interpret these data relative to sugarbeet N nutrition. Sugarbeets (Beta vulgaris, L.) were grown at four N fertilization rates and two irrigation levels to determine the root yield, sucrose percentage, sucrose yield, and N uptake in relation to the NO,-N concentration in the petioles. NO,-N in beet petioles increased to a peak concentration and then decreased exponentially during the two growing seasons on all treatments. The exponential decrease after the peak enables prediction of the NO,-N in the petioles during the remainder of the growing season. This rate of change approach can be used to predict N needs when adding supplemental N for sugarbeets and to characterize the N status of soil-crop systems. Additional index words: Petiole analysis, Nitrogen uptake. T HE nitrate-nitrogen content of sugarbeet petioles is an excellent indicator of the nitrogen nutrition status of sugarbeets (Beta vulgaris, L.). Previous investigators have primarily related a minimum value of NOrN status during the season to sucrose production. For example, Ulrich suggested that the critical low range for NOrN, based on water-extractable nitrate from petioles, is 1000 ppm (6, 7), and that concentrations below the critical level for any appreciable time before midseason may result in lower root and sucrose yield. Experience in the Twin Falls, Idaho, area indicates that petiole NO3-N should be approximately 1000 ppm and that the available N supply of the soil should be nearly depleted about 4 to 6 weeks before harvest, or about Aug. 20, to maximize yield, sucrose percentage, and purity. The rate of change in petiole NO 3-N reflects the net effect of rate of uptake and rate of assimilation of N. The rate of change between two sampling periods after petiole NO3-N begins to decrease has the paten1 Contribution from the Northwest Branch, Soil and Water Conservation Research Division, Agricultural Research Service, USDA; Idaho Agricultural Experiment Station cooperating. Received August 5, 1969. Research Soil Scientist, Research Agricultural Engineer, and Physical Science Technician, respectively, Snake River Conservation Research Center, Kimberly, Idaho 83341. tial of providing not only additional information on the current N nutritional status of sugarbeets, but also enables the prediction of the probable status during the balance of the growing season. Predicting petiole NO3-N concentration 4 to 6 weeks before harvest, for example, from petiole analyses earlier in the season, would enable the producer to apply additional N fertilizer, if needed, to maximize yield and sucrose production. This paper summarizes procedures that can be used to predict the NOrN concentration in sugarbeet petioles during the balance of the season based on samples taken during two midseason sampling periods,