EFFECT OF BLENDED NPS AND UREA FERTILIZER RATES ON YIELD RELATED TRAITS AND YIELD OF POTATO (Solanum tuberosum L.), AT HARAMAYA, EASTERN ETHIOPIA

Potato is the second most important and co-staple crop in eastern Ethiopia. However, the average tuber yield is low as compared to the potential yield of the crop. The use of proper types and rates of fertilizer is one of the measures to increase productivity of the crop in the region. Therefore, a field experiment was conducted to evaluate the effect of combined application of blended NPS and Urea fertilizers on growth, yield components, tuber yield and tuber quality of potato at Rare research site, Haramaya University during 2019/2020 cropping season. The treatments consisted of factorial combinations of six rates of blended NPS fertilizer (0, 50,100, 150, 200, and 250 kg ha-1 ) and four levels of Urea fertilizer (0, 50, 100, and 150 kg ha-1 ). The experiment was laid down in a randomized complete block design with three replications using potato variety Bubu. The results of analysis of variance revealed that the combined application of NPS and Urea fertilizers significantly influenced phenology, growth, yield components, tuber yield and tuber quality related traits of potato plants. However, main stem number per hill was not significantly influence by the application of NPS and Urea fertilizer. Flowering and maturity of the crop was significantly delayed due to application of highest rates of NPS and Urea fertilizers, which significantly enhanced growth, yield components, tuber yield and tuber quality related traits. The highest total (47.04 t ha-1 ) and marketable tuber yields (46.05 t ha-1) were obtained due to combined application of 250 kg ha-1 NPS and 150 kg ha-1Urea,while the lowest respective values of 24.37 t ha-1 and 17.04 t ha-1 were recorded for plots maintained without application of the two fertilizers. Combined application of the fertilizers also produced tubers with the highest dry matter content, specific gravity and starch content with respective values of 27.94%, 1.099 gcm-3 and 17.67 g/100g . The results of partial budget analysis showed that application of 50 kg ha-1 NPS without Urea fertilizer had the highest marginal rate of return (1508.11%), but could not offer highest net benefit and highest tuber yield. However, application of 250 kg NPS and 150 kg Urea ha-1 had 112491.75 Birr net benefit with benefit cost ratio of 1.54 (112.5%) and 22810 kg ha-1 (98.15%) marketable tuber yield advantages over application of 50 kg ha-1 NPS fertilizer and with marginal rate of return (1214.16%) above the minimum acceptable level. Therefore, combined application of 250 kg NPS and 150 kg Urea ha-1 could be recommended for potato production in Haramaya area and in areas with similar agro ecological and soil conditions.