Knowledge on the phosphorus capacity factor (PCF) of soils of the state of Paraíba is important to improve the interpretation of the available phosphorus (P) content and the recommendation of phosphate fertilization in soils of the State. The objective of this study was to evaluate the parameters of P adsorption isotherms (Langmuir and Freundlich) by the techniques of non-linear regression and linearization, in soil samples of the state of Paraíba, and to correlate the values of these parameters with chemical and physical soil properties. Samples of 12 representative soils of the state of Paraíba were collected in the 0-30 cm layer (six soils with a higher and six with a lower weathering degree), with a wide variation of values of physical and chemical properties within each group of six soils. The hyperbolic model of the Langmuir isotherm was adjusted by the technique of non-linear regression (single region) and of linearization (first region and second region). Correlation analyses were performed with the values of isotherm parameters and soil properties that reflect the FCP. In all soils, the model adjusted by non-linear regression fitted the data best, in comparison with the same model adjusted by linearization. In the most weathered soil group, the values of maximum adsorption capacity of phosphate (CMAP) were highly correlated with the contents of remaining (equilibrium) phosphorus (r = -0.95**), free Fe oxyhydroxides (r = 0.90**) and water retention (r = 0.88**), but did not correlate with the clay content (r = 0.64ns). Within the group of less weathered soils, the CMAP values were correlated with the contents of equilibrium phosphorus (r = -0.99**), water retention (r = 0.93**) and clay content (r = 0.97**), but did not correlate with free Fe oxyhydroxides (r = 0.10ns).
phosphorus adsorption; Longmuir's isotherm; phosphate buffer power
