Despite extensive research, the behaviour of the key nutrient element, phosphorus (P), in soil is not yet fully understood. This study focussed on the outstanding issue of the coadsorption of protons (H + ) and P by soils. We developed a congruent set of measures to determine the net H + :P co-adsorption ratio and tested it on goethite, for which a ratio of 1.6:1 had been estimated under CO 2 -free conditions for additions of NaH 2 PO 4 . Under our conditions, and using additions of KH 2 PO 4 , the net H + :P co-adsorption ratio was estimated to be 1.44:1, i.e., in passable agreement with the published value. Application of the protocol to acidic soils resulted in a net H + :P co-adsorption ratio of 1.92:1, and substitution of H 3 PO 4 for KH 2 PO 4 gave a ratio of 1.96:1. These ratios for soils differ significantly from that for goethite. The soils for which we estimated net H + :P co-adsorption ratios had a wide range of properties and two had received previous applications of P fertiliser (Ca(H 2 PO 4 ) 2 ), which does not appear to have affected the net H + :P co-adsorption ratios. The H + :P co-adsorption