Effect of Fertilizer and Irrigation on Nitrate-Nitrogen and Total Nitrogen in Potato Tubers'

This study was conducted to determine the effect of N fertilizer and irrigation management on potato (Sotanum tuberosum L,) tuber N0,-N levels and the relationship to the potential health hazard created by high nitrate levels in food products. `Russet Burbank' potatoes grown using different N fertilizer rates, methods of application, and irrigation levels were analyzed for N0,-N concentration. The NO,-N concentration in the tubers on a wet weight basis varied from 36 to 131, 34 to 75, and 25 to 50 ppm in the 3 years of this study. The NO,-N concentration for each year of study was found to be directly related to the level of applied N fertilizer. The initial concentration and increase in N0,•N due to N fertilizer varied with the season. The addition of manure did not increase the N0,-N level above those to be expected from similar quantities of inorganic sources of N. Phosphorus fertilizer did not increase the N0,-N level. The N0,-N concentration in the tubers where more water was applied at each irrigation was less than on the lower level of applied water at each N rate. These data indicate that greater NO,-N levels in the tubers will result by increasing N fertilization rates. The levels of N0,•N obtained in this study were not expected to contribute substantially to the methemoglobinemia health hazard. Additional index words: Nitrate health hazard, Nitrogen fertilization, Leaf N0,-N. N ITRATE concentration is high (>67 ppm NO3N) in certain vegetable products such as beets (Beta vulgaris L.), spinach (Spinacia oleracea L.), and lettuce (Lactuca sativa L.) (3, 7, 8, 10, 12, 15, 16). There is considerable concern that use of these high nitrate containing vegetables could cause methemoglobinemia, especially in infants. Although nitrites are the toxic principle which may be formed prior to ingestion or during digestion and absorption of food, nitrates may be considered as the index or precursor to the amount of nitrite which may be formed. If foods contain high levels of nitrate, the potential hazard may be increased if conditions during storage or processing are conducive to conversion to nitrite (11). In spite of the appreciable nitrate content of some vegetables such as beets and spinach, no authenticated cases are known for nitrate poisoning of human adults. However, several cases of methemoglobinemia and one death have been reported recently when children, aged 2 to 10 months, were fed spinach purchased as a fresh vegetable and held under questionable storage conditions (1). In recent years, there has been a steady increase in the use of nitrogeh fertilizer to achieve maximum potato production. Also, an increasingly common practice is to add N to the irrigation water 1 Contribution from the Western Region, Agricultural Research Service, USDA; Idaho Agricultural Experiment Station cooperating. Received Aug. 13, 1973. 'Soil Scientist and Physcial Science Technician, respectively, Snake River Conservation Research Center, Rimberly, ID 83341. Mention of trade names or companies is for the benefit of the reader and does not imply endorsement by the USDA. throughout the season, The abundance and low cost of N fertilizer has encouraged the use of high fertilization rates in attempts to obtain maximum tuber yields. No reported cases could be found in the literature where high nitrate levels in potato (Solanum tuberosum L.) tubers were created by methods of application or rates of N fertilizer. However, Hlaysova, Tucek, and Turek (6) reported nitrate increases -in the tubers with increasing N fertilization rates. This paper reports the effect of irrigation, P, and N fertilizer on the N0 5-N concentration in whole `Russet Burbank' potato tubers grown in southern Idaho. MATERIALS AND METHODS Three experiments were conducted daring consecutive years on a Portneuf silt loam soil (Xerollic Calciorthid; coarse-silty, mixed, mesic) near Twin Falls, Idaho to evaluate the effects of N and irrigation management on production and quality of the tubers. This soil has a cemented hardpan commencing at the 40 to 45 cm depth that does not materially affect water movement but restricts root growth. The areas used in these experiments had not received fertilizer the year previous to these investigations and were considered deficient in both N and P but amply supplied with other nutrients. Experiment 1 was designed using four replications in a randomized block having split plots at two moisture levels Ot, and MO. Fertilizer treatments were five N rates (0, 45, 90, 180, and 360 kg N/ha) as NII,NO, and five P rates (0, 20, 40, 80, and 160 kg P/ha) as concentrated superphosphate in all combinations. An additional treatment of 180 kg N, 80 kg P, and 9,700 kg/ha of dry manure (200 kg N/ha) was also included. Experiment 2 was designed using four replications in a randomized block at one moisture level (M 1). Fertilizer treatments were six N rates (0, 45, 90, 180, 360, and 720 kg N/ha) as urea and ureaform (Dupont's Uramite) with a uniform application of 80 kg P/ha. In a separate study, approximately 38% of the N in ureaform was nitrified in 11 weeks. Experiment 3 was designed using four replications in randomized block at one moisture level (MO. Fertilizer treatments were five N rates (0, 90, 135, 180, and 360 kg N/ha) as urea in one, two and three applications with a uniform application of 80 kg P/ha and 112 kg K/ha (KSSO,). The urea N treatments were applied all preplant; 112 preplant and 1/2 on July 11; and 1/3 preplant, 1/3 on July 11, and 1/3 on August 8. The N applied during the season was broadcast followed by the application of 4 cm of water applied by sprinklers to move the N into the root zone, The preplant fertilizer application in all experiments was broadcast and worked into the upper 8-cm soil layer. Following preplant fertilizer application in early May, the potato seed pieces were planted 23 to 28 cm apart in 91-cm rows. The time and amount of irrigation were determined by the use of tensiometers placed throughout the experimental area. Irrigations were applied when the mean soil moisture stress reached 0.5 to 0.6 atm at a depth of 20-cm below the top of the hill for the M,, or when the soil moisture level was approximately 60% of the total available soil moisture capacity in the active root zone of the soil. Irrigation water was applied to alternate furrows at each irrigation for the M 1 treatment (76 cm water per season). For the M, treatment (124 cm water per season), every furrow received water at each irrigation, The irrigation date and duration were the same for the two moisture levels. Leaf samples (leaflets and petiole) were taken on all fertility and moisture treatments at one sampling date. The first mature