Purpose <p>The main purpose of this study is to evaluate the impacts of tea waste biochar (TWB), farmyard manure (FYM), and NPK fertilizers on the physicochemical properties of soil and onion growth characteristics. The research question focuses on how these amendments affect soil fertility, moisture retention, and onion yield.</p> Method <p>A field study was conducted using TWB at a rate of 12 tons ha^-1, FYM at 12 tons ha^-1, and NPK fertilizers at a ratio of 100:70:70&#xa0;kg&#xa0;ha^-1. The pyrolysis process was used to derive TWB from used tea leaves, resulting in irregularly shaped particles with micropores of various sizes. Soil analysis was performed to assess the physicochemical properties.</p> Results <p>The analysis indicated that TWB exhibited a higher electrical conductivity (EC) of 1.47 dS/m^-1, compared to FYM (1.42 dS/m^-1) and NPK fertilizers (1.41 dS/m^-1). TWB significantly enhanced the soils water-holding capacity (0.55&#xa0;ml H₂O g⁻¹ dry soil), organic matter content (0.58%), nitrate nitrogen (NO₃-N, 2.02&#xa0;mg&#xa0;kg⁻¹), and available potassium (17.1&#xa0;mg&#xa0;kg⁻¹); although it did not significantly affect soil Olsen-P levels. The TWB treatment recorded the highest values for all measured parameters: bulb weight (6.3&#xa0;kg plot⁻¹), leaf length (32.3&#xa0;cm), total onion yield (268.3&#xa0;kg&#xa0;ha⁻¹), and bulb nitrogen (18.3&#xa0;g&#xa0;kg⁻¹) and potassium (19.1&#xa0;g&#xa0;kg⁻¹) content.</p> Conclusions <p>In conclusion, Tea Waste Biochar (TWB) serves as an effective organic amendment for enhancing soil fertility and promoting plant growth, outperforming traditional amendments like farmyard manure (FYM) and NPK fertilizers. TWB improves soil moisture retention due to its high porosity and surface area, which allow it to hold water in micropores and reduce evaporation. Its functional groups enhance water attraction, while TWB promotes stable soil aggregates that improve soil structure and reduce compaction.</p> Graphical abstract <p></p>