Effects of Gradient, Fertilizer Application and Planting Time on the Growth and Yield of Round Potatoes (Solanum tuberosum L.) in Andosols of Uporoto Highlands, Southern Tanzania

This study investigated the effects of topographical gradient, fertilizer application and planting time, which is days after planting, on the growth and yield of round potatoes (Solanum tuberosum L.) in the Uporoto Highlands of southern Tanzania. The objective was to generate site-specific agronomic evidence to enhance the productivity and profitability of smallholder potato cultivation, which is often constrained by varied terrain and declining soil fertility. Understanding the interactions between slope position, nutrient management, and the interval from days after planting is critical for optimizing yield and promoting sustainable production in highland agroecosystems. A field experiment was conducted on two different gradients (upper and lower) using a randomized complete block design (RCBD) with three replications. Three fertilizer treatments were applied: a control (no fertilizer), a farmer-based practice (TSP+), and an innovative fertilizer regime (TSP++), formulated based on soil nutrient analysis and administered at both planting and early vegetative stages. Four industrial fertilizers - Triple Super Phosphate (TSP), Muriate of Potash (MOP), Nitrobor, and Urea - were applied in defined ratios, and the ‘Obama’ cultivar was used as the test crop. Results indicated that both gradient and fertilizer treatments significantly influenced most growth and yield parameters. The lower gradient consistently exhibited superior vegetative growth and higher tuber yields, which can be attributed to greater soil moisture retention and improved nutrient availability. The TSP++ treatment produced the highest total tuber yield (50.9 ± 3.7 t ha⁻¹), marketable yield (46.3 ± 3.1 tons/ha), and biological yield (54.6 ± 4.2 tons/ha). Gradient, fertilizer and days after planting significantly affected most growth variables, whereas stem number was predominantly influenced by their three-way interaction (p = 0.031). The study emphasizes the importance of gradient-sensitive and soil-informed fertilizer management for optimizing potato productivity in highland environments. It is recommended that smallholder farmers adopt site-specific fertilizer strategies guided by soil analysis and implement moisture conservation practices, particularly on upper slopes, to enhance yield stability, improve resource-use efficiency, and increase overall profitability in Tanzania’s highlands and comparable agroecological regions. Key words: Round potatoes yield, soil fertility, upper and lower gradients