Potassium (K) deficiency is a major constraint to crop productivity in Egyptian soils, particularly in coarse-textured soils. In pot experiments, the study evaluated impact of application of biochars at a rate of 3% (w/w) produced at 450 °C for 4 h, from four agricultural residues (sugarcane bagasse residues biochar (SBR), olive stone pomace biochar (OSP), orange fruit pomace biochar (OFP), and maize stover residues biochar (MSR)) on K availability, K dynamics, and specific soil physicochemical properties across four Egyptian soil types (sandy, loamy, clayey, and calcareous), in addition to its effects on wheat growth. Biochars varied in surface area (23.72–41.82 m² g⁻¹) and nutrient content, with MSR showing the highest plant available nutrients, while OSP exhibited the highest cation exchange capacity (56.78 cmol (+) kg⁻¹). Application of biochars increased soil water-holding capacity (WHC) by 17–35.5%, cation exchange capacity (CEC) by 18–163%, depending on soil type. Thermodynamic parameters of K were significantly improved; labile-K (KL) increased by 103.6% in sandy soil with MSR, 59.01% in loamy soil with OSP, 48.55% in clayey soil with SBR, 849% in calcareous soil with MSR. Activity ratio of K at equilibrium ( ) increased by 33.33% in sandy soil with MSR, 20.00% in clayey soil with SBR, 75.0% in calcareous soil with MSR, while in loamy soil decreased by − 16.66% in loamy soil with OSP. Potential buffering capacity of K (PBCK) increased with application of MSR by 96.99%, 119.89%, 45.90% in sandy, loamy and clayey soils, while PBCK increased by 421.39% with OSP application in calcareous soil. Gibbs free energy (ΔG) became more negative (up to − 5.565 kcal mol⁻¹), and Gabon selectivity coefficient (KG) increased by 82.5%. Wheat fresh and dry biomass increased by 25.9–84.6% and 16.9–63.8%, respectively. Uptake of N, P, and K on wheat tissues increased by 27.3–142.2%, depending on biochar type and soil. In general, biochar-amended treatments produced higher wheat biomass and nutrient uptake than the unamended controls, with MSR demonstrating the most consistent performance across different soil types, followed by OSP. These findings highlight the importance of matching biochar type with soil characteristics to optimize K availability, improve K-use efficiency, reduce reliance on mineral fertilizers, and support sustainable soil fertility management under arid and semi-arid conditions.