Increasing Nitrogen Use Efficiency in Potato by Application of Slow Release N-Fertilizers

To examine the effect of slow-release nitrogen fertilizers and its comparison with urea, several experiments were conducted in Khorasan Province during two years. In the first year, the eight fertilizer treatments included two-stage application of 140 and 200 kg N. ha^-1 (equal to 70% and 100% of the recommended level) from urea source, one-stage application of 140 and 200  kg N. ha^-1 from urea form (before cultivation), one-stage application of 140 and 200  kg N. ha^-1  from methylene urea before cultivation, application of 100 kg N. ha^-1  from urea form before cultivation followed by 40 kg N. ha^-1 as top dressed from urea, and application of 100 kg N. ha^-1 from methylene urea (before cultivation) followed by 40 kg N. ha^-1 as top dressed from urea source. The experiment had a Randomize Complete Block Design and was replicated 3 times. In the second year, the effects of seven fertilizer treatments consisting of 150, 200, and 250 kg N.ha^-1 from urea (at three stages) and urea-form (at one stage) with a control plot (without nitrogen application). The treatments were conducted in two soils with different textures (loam and sandy loam) as a Randomized Complete Block Design, replicated three times. The results of the first year showed that application of nitrogen fertilizers had a significant effect (α=1%) on tuber yield, and nitrogen concentration in leaves. The maximum tuber yield (54.66 ton.ha^-1) was produced by applying 200 kg N.ha^-1 from methelen urea, which had a slight difference with the same amount of nitrogen from urea formaldehyde source. The economical evaluation of different treatments in the first year showed that, in spite of the maximum economic net profits obtained through application of 200 kg N.ha^-1 as urea formaldehyde and/or methelene urea, the maximum economic efficiency was obtained by the application of 140 kg N.ha^-1 from methelene urea source (equal to 70% of the control plot). The results of the second year experiment showed that, in both soils under study, the maximum tuber yield was obtained through the application of 250 kg N.ha^-1 (25% more than the nitrogen recommended level) from urea form source. The leaf and soil analysis of different treatments showed that there was no difference among the treatments concerning leaf N concentration and soil N at different stages of the crop growth. This indicates the ability of ureaform in slow releasing of nitrogen, since in all stages of sampling; the concentration of nitrogen in leaf and soil of the urea form treatments was even more than the corresponding values in the urea treatments. On the whole, it can be concluded that, in similar conditions, using slow release nitrogen fertilizers (urea form and methylene urea) in potato farming is economically feasible and increases nitrogen use efficiency.