NPSBZn fertilizer rate and variety effects on carrot yield, quality, and soil acidity in the central highlands of Ethiopia: a two-year field trial

Carrot (Daucus carota L.) is a valuable vegetable crop in Ethiopia, yet national average yields remain low due to soil nutrient deficiencies, unbalanced fertilizer use, and limited access to improved varieties. Most existing research is single-season, which limits the robustness of fertilizer recommendations across varying climatic conditions. Additionally, the recently introduced blended NPSBZn fertilizer lacks crop- and site-specific recommendations. To address these gaps, a two-year (2020–2021) field experiment was conducted in the Hadiya Zone, central Ethiopia. Two carrot varieties (Nantes and Haramaya I) were subjected to seven NPSBZn rates (0 to 150 kg ha⁻¹) in a randomized complete block design with three replications. Across both years, Nantes consistently outperformed Haramaya I in stand count, mean root weight, marketable yield, total soluble solids (TSS), and β-carotene content, while Haramaya I produced longer but often cracked roots. Increasing NPSBZn rates from 0 to 150 kg ha⁻¹ steadily improved all yield and quality parameters, with no plateau observed at the highest rate. Marketable yield reached 38.5 t ha⁻¹ in Nantes at 150 kg ha⁻¹, with marginal returns exceeding 1200%. However, soil pH declined from 5.94 to 5.56, indicating progressive acidification likely due to nitrification of ammoniacal nitrogen from urea and NPSBZn fertilizers. We conclude that NPSBZn fertilization at 150 kg ha⁻¹ combined with the Nantes variety is economically viable and agronomically superior in the short term, but integrated soil management including liming or organic amendments is required to mitigate long-term acidification and sustain soil health.