The Effect of Nitrogen Fertilizer and Nitrogen Splitting on Yield and Nitrate Content of Potato Tubers

Potato (Solanum tuberosum L.) is an annual and autotetraploid plant of the Solanaceae family that uses its underground tuber. In 2019, among West Asian countries, Turkey and Azerbaijan had a higher level of potato cultivation than Iran.  Nitrogen consumption management in potato cultivation is one of the factors affecting the quantitative and qualitative characteristics of the tuber yield produced. Some researchers consider potato yield it consists of three components the number of stems per square meter, the number of tubers per stem, and the weight of tubers per plant. One of the problems with the overuse of nitrogen fertilizers is the accumulation of nitrate in the tubers. The amount of nitrate allowed depends on the diet of the person. Some studies show that from 93 mg per day of nitrate consumed per day for an adult, 33% to potatoes, 21% to green leafy vegetables, 15% to other vegetables, 8.5% to beverages, 4.2% to Meat products account for 2.1% to various grains, 1.6% to bread and 5.1% to other items.

The present study conducted in 2018-2019 in Kabootarabad Agricultural Research Station of Isfahan. The effect of N application and nitrogen splitting on the tuber yield and nitrate accumulation of potato tubers in this study investigated. For this purpose, the split plots tested in a randomized complete block design with three replications in which three nitrogen levels (90, 180 and 270 kg ha-1) of the main plots and two nitrogen splitting methods (three and four stages) formed the sub-plots. At harvest time, two meters of the middle rows of each plot used to estimate the yield. Tubers with less than 35 mm in diameter considered non-marketable yield. To measure tuber nitrate, the prepared samples placed in paper bags and sent to the laboratory of Soil and Water Research Department of Isfahan Agricultural Research Center. After washing, the samples grated and after drying in an oven (temperature 65 °C) and grinding in a shaker (shaker), the amount of nitrate determined by spectrophotometry. All soil and plant tests performed in the laboratory. SAS software used to analyze the data and Duncan's multiple range test (at 5% probability level) used to compare the means.

The effect of nitrogen interaction and its splitting on total yield and efficiency of nitrogen utilization at the level of one per cent and on the characteristics of marketable yield, non-marketable yield, tuber nitrate, number and weight of tubers at the level of five per cent was significant. With the consumption of 180 and 270 kg N ha-1 by the four-step splitting method, the non-marketable yield was equal to 11.2 and 14.43% of the total tuber yield, respectively. With the consumption of 270 kg N ha-1 and three stages of nitrogen splitting, the amount of nitrate in the tuber reached 197 mg kg-1 of fresh weight of the tuber. In both three- and four-stage nitrogen splitting methods, nitrogen application efficiency reduced to one third by consuming 270 kg N  ha-1 compared to 90 kg N ha-1 treatment. Therefore, the use of nitrogen fertilizers in excess of the recommended amounts can, in addition to economic losses, be associated with a decrease in fertilizer use efficiency and an increase in tuber nitrate beyond the allowable limit.

The results showed that nitrogen splitting is a correct management approach when it recommended based on the amount of nitrogen consumption (in this study 180 kg ha-1). The use of excess nitrogen, even in splitting, not only did not increase economic yield but in some treatments (three-stage splitting of 270 kg N ha-1) was associated with an increase in tuber nitrate beyond the allowable limit (170 mg kg-1 tuber fresh weight). In the amount of nitrogen application more than the recommended fertilizer, in both methods of nitrogen splitting, the efficiency of fertilizer application decreased. So that in the treatment of 270 kg N ha-1 compared to the treatment of 90 kg N ha-1, the efficiency of fertilizer application reached one third.