Little attention has so far been given to the relationship of Olsen P to solution P at P levels in the soil solution within the range relevant to plant growth. In this work, we investigated the relationship between these two P forms in 24 noncalcareous and 25 calcareous Spanish soils, using the concentration of P in the 0.01 m CaCl2 extract (1:10 soil:solution ratio; 30 min shaking) as a proxy for the P concentration in the soil solution. Application of the Freundlich equation (y = axb) in linearized form provided a parsimonious, largely accurate description of the Olsen P/CaCl2-P relationship. The average value of exponent b, 0.53, suggests that Olsen P relates to the quantity of P in soil rather than to its intensity. The estimated Olsen P values at a CaCl2-P concentration of 0.01 and 0.03 mg/L (viz., Olsen P0.01 and Olsen P0.03, respectively) varied over wide ranges (0.6–33.9 and 2.0–38.9 mg/kg, respectively). Based on the results of a regression analysis, silicate clays, carbonates and, especially, Fe oxides contribute positively to Olsen P0.01 and Olsen P0.03. The fact that, as shown here, Olsen P at a specific solution P concentration can be predicted from easily measured soil properties can facilitate decision-making in managing fertilizer P.