A study was conducted to determine the response of potato (Solanum tuberosumL.) to different rates of nitrogen (N) applied as urea (0, 69, 138 and 207 kg/ha) and phosphorus (P) (0, 20, 40 and 60 kg/ ha) fertilization on Vertisols of Debre Berhan in the highlands of central Ethiopia. Application of 207 kg N/ha delayed days to flowering and physiological maturity by four and nine days, respectively compared to the control. Similarly, it increased plant height by 24 cm, above ground biomass by 224.5%, underground biomass by 108%, marketable tuber yield by 175%, total tuber yield by 119%, marketable tuber number by 95.6%, total tuber number by 34% and average tuber weight by 82% over the control. On the contrary, nitrogen fertilization significantly reduced tuber specific gravity and dry matter content without affecting stem number, unmarketable tuber yield and number, and harvest index. Application of 60 kg P/ha significantly increased days to flowering by two days, plant height by 10.5 cm, aboveground and underground biomass by 32 and 28% respectively, marketable tuber yield by 60%, and marketable tuber number by 43%. Other parameters such as days to physiological maturity, stem number, total tuber yield and number, unmarketable tuber yield and number, average tuber weight, specific gravity, dry matter content and harvest index were not significantly influenced by phosphorus fertilization. Total tuber yield was positively correlated with total tuber number (r = 0.60***), marketable tuber number (r = 0.87***) and average tuber weight (r = 0.81***) indicating that tuber yield increase in response to the fertilization was due to the increase both in tuber number and weight. It was observed that application of 138 kg N and 20 kg P/ha is required for optimum productivity of Gorebiella variety on the vertisols of Debere Berhan in the central highlands of Ethiopia under rain fed conditions.
Key words: Debre Berhan, fertilizer, potato, quality, vertisols, yield.
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