Establishing Critical NPK Concentrations to Enhance Nutrient Use Efficiency and Optimize Yield of Chinese Cabbage (<em>Brassica rapa </em>L. subsp. <em>chinensis</em>)

Suboptimal nutrient management is a major limitation to Chinese cabbage (Brassica rapa L. subsp. chinensis) productivity in Tanzania, where yields average 5.1 t ha⁻¹ compared to the global potential of 80–100 t ha⁻¹. This is largely due to nutrient-depleted acidic soils, low fertilizer use, and reliance on blanket recommendations. This study aimed to establish critical concentrations of nitrogen (N), phosphorus (P), and potassium (K) for optimal yield and nutrient use efficiency of Chinese cabbage under Morogoro conditions. A two-season field experiment (Nov 2024–Jan 2025; Feb–May 2025) was conducted at Sokoine University of Agriculture in a randomized complete block design with three replications and 22 treatments. Each macronutrient was varied systematically while others were held constant. Soil analysis revealed strongly acidic conditions (pH 5.01), low available P (2.24 mg kg⁻¹), low total N (0.14%), and low cation exchange capacity (7.28 Cmol(+) kg⁻¹), emphasising the need for balanced fertilization. Growth and yield data were subjected to ANOVA and Tukey’s HSD test, while quadratic regression models were used to determine critical nutrient concentrations. Balanced NPK fertilization significantly enhanced growth and yield compared to the control (p &amp;lt; 0.05). Optimal N, P, and K rates were 220–300 kg N ha⁻¹, 90–120 kg P ha⁻¹, and 120–180 kg K ha⁻¹, producing 6–8-fold yield increases (40–55 t ha⁻¹) over unfertilized controls (≈6 t ha⁻¹). Nutrient uptake and tissue concentrations followed curvilinear responses, with diminishing returns beyond optimal rates. Excess N (≥450 kg ha⁻¹) resulted in luxury consumption and reduced nutrient use efficiency. Balanced NPK fertilization at identified optimal rates maximizes yield and nutrient use efficiency while minimizing environmental and economic risks. These results provide science-based recommendations that could substantially improve Chinese cabbage productivity in Tanzania.