Phosphorus (P) is the most limiting element in the Vertisols of southwestern Spain. There are few studies about P status, and crop responses to recommended P-fertilizer applications are erratic in these soils. The objectives of this work were (i) to investigate the P status and inorganic P fractions in these soils and (ii) to evaluate the relationships between soil inorganic P fractions, available P indices, and P uptake by plants. Total P, organic P, available P indices, and soil inorganic P fractions were studied in surface samples of 19 Vertisols of southwestern Spain. Total P uptake by Agrostis tennuis was determined after 11 weeks of plant growth in a greenhouse, and was used to measure plant available soil P. Total P ranged from 168 to 482 mg P kg−1; soil organic P ranged from 42 to 125 mg P kg−1 and averaged 28% of the total P. Soil inorganic P fractions were determined by a sequential-fractionation procedure (NH4Cl-P, NH4F-P, 0.1 M NaOH, CDB-P, 1 M NaOH-P, and H2SO4-P). Comparative rankings of the various inorganic P fraction quantities were H2SO4-P > NH4F-P > 0.1 M NaOH = 1 M NaOH-P > CDB-P > NH4Cl-P. Most of the inorganic P occurs in the least available form as Ca-P. The following soil test P extracted increasingly higher average amounts of P: Saunder (11.4 mg P kg−1), Truog (14.7 mg P kg−1), Olsen (17.8 mg P kg−1), Jackson (38.4 mg P kg−1) and Bray 2 (119.4 mg P kg−1). Jackson-P showed higher correlation with P uptake by the crop (r = 0.87) than did Olsen-P and Bray-P (r = 0.61 and 0.37, respectively), which are used to evaluate available P in the soils of the region studied. The results suggest that Jackson extractants removed easily soluble P as well as P from Fe and Al oxides. Moreover, organic P was an important fraction in supplying available P. A multiple regression model with Jackson-P, organic carbon, and cation-exchange capacity as independent variables explained 91% of the total variation in total plant P uptake. This implies that P availability and dynamics in these soils are influenced strongly by organic compounds and clay minerals.