Optimizing NPKS Fertilization for Onion Yield and Soil Health in Dinajpur, Bangladesh

To optimize the combination of major nutrients (N, P, K, S) for maximum yield of onion (Allium cepa L.) and profitable return without degrading the soil fertility, a field experiment was executed during the winter season of 2023–2024 at the HSTU Central Farm in Sadar Upazila, Dinajpur, Bangladesh. The experiment was performed in a completely randomized design with ten treatments: T1 = control (no fertilization); T2 = 0-60-80-30 kg NPKS ha⁻¹; T3 = 50-60-80-30 kg NPKS ha⁻¹; T4 = 100-60-80-30 kg NPKS ha⁻¹; T5 = 100-0-80-30 kg NPKS ha⁻¹; T6 = 100-20-80-30 kg NPKS ha⁻¹; T7 = 100-40-80-30 kg NPKS ha⁻¹; T8 = 100-60-0-30 kg NPKS ha⁻¹; T9 = 100-60-40-30 kg NPKS ha⁻¹; and T10 = 100-60-80-0 kg NPKS ha⁻¹. The findings showed that the greatest plant height (63.0 cm) and leaf length (55.9 cm), as well as superior metrics for bulb development and yield—bulb diameter (18.3 cm), single bulb weight (59.3 g), fresh leaf weight (9.95 g), fresh bulb yield (10.71 t ha⁻¹), and biological yield (20.7 t ha⁻¹) were obtained with the treatment T8. The maximum harvest index occurred with T9, suggesting that the extra quantity of potassium (40 kg K ha⁻¹) in this treatment T9 promoted sulfur acquisition and increased the partition of biomass to the bulbs. Treatment T8 had the highest residual sulfur in post-harvest soil, indicating that lower rates of sulfur than the currently recommended dose may be enough for subsequent crops. On the other hand, reduced residual sulfur in treatment T9 suggested effective sulfur uptake, when potassium was applied with sulfur. Results suggests that balanced application of potassium with nitrogen and phosphorus plays crucial role in not only improving harvest index but also maximizing the overall nutrient uptake efficiency.