Effects and economically feasible rates of nitrogen and phosphorus fertilizers on potato (Solanum tuberosum L.) production for rainy season

Potato is very important food and cash crop in Ethiopia. However, a factor like poor soil fertility is a critical challenge of its productivity. Nitrogen (N) and phosphorus (P) are first and second essential macro-elements and are limiting nutrient in potato production. Therefore, a field experiment was conducted to assess response of N and P fertilizers and select economically feasible fertilizer rate for a rainy season. Four levels of N (0, 55, 110, 165 kgha-1) and 4 levels of P (0, 45, 90, 135 kgha-1) were combined in 4×4 factorial arrangement in randomized complete block design with 3 replications. Raising the application of NP fertilizer levels to 165 and 135 kgha-1 delayed days to flowering (9.48 and 4.46 days), while maturity extends 14 and 10.18 at Dabark and Dabat, respectively. Application of 110 to 90 and 165 to 45 kgha-1 NP increased marketable tuber number by 122 and 119%, respectively. Similarly, 165 to 90 and 165 to 45 kgha-1 NP raised marketable tuber yield by 141.8 and 127.5%. However, both 165 to 90 and 165 to 45 kgha-1 NP had MRR below acceptable level (<100%). In contrary, application of 110 to 90 kg ha-1 NP resulted in 136.6 and 125.4% marketable tuber yield increment and MRR of 1993.72% for Dabark and 1376.5% for Dabat location. Therefore, application of 110 to 90 kgha-1 NP was proven to have high MRR and can be used for the studied areas.   Key words: Marginal rate of return, Solanum tuberosum, marketable tuber yield, interaction effect, main effect.