Phosphorus plays a crucial role in promoting plant growth and development. However, Pakistani soils face phosphorus deficiency due to their mineralogical characteristics and high pH levels, resulting in instant adsorption of applied phosphorus. Key factors affecting phosphorus accessibility in these soils include clay mineral composition, sesquioxide levels, pH, and free CaCO₃ presence. The current study aimed to develop phosphorus adsorption isotherms using Langmuir and Modified Freundlich equations and determine phosphorus application rates for maximum yield. The experiment comprised 11 treatments, i.e., T₁ (Control), with the 10 doses consisting of increasing levels of phosphorus concentration in CaCl₂ solution: T₂ (10 ppm), T₃ (20 ppm), T₄ (30 ppm), T₅ (40 ppm), T₆ (50 ppm), T₇ (60 ppm), T₈ (70 ppm), T₉ (80 ppm), T₁₀ (90 ppm), and T₁₁ (100 ppm). An analysis of collected samples for texture determination used the hydrometer method. After determining the soil textural class of both soils, phosphorus adsorptions evaluation for both textures utilized P concentration in calcium chloride. Phosphorus adsorption curves exposed a higher P fixation happening at less phosphorus contents, whereas high phosphorus quantities resulted in less fixation for clay loam and sandy loam soils.

Keywords: Phosphorus, isotherm, soil texture, adsorption, and Freundlich model

Key findings: The study concluded the Langmuir model performed efficiently even at lower P concentrations, while the modified Freundlich model was most suitable at medium to high P concentrations. Hence, the modified Freundlich model is the best option for calculating theoretical P doses for field applications.