Ceramic tension lysimeters are often used to sample soil water; however, the chemical composition and adsorption capability of the ceramic cup material may introduce uncertainties in the P concentration of the leachate being sampled. The goal of this study was to test a methodology for evaluating ceramic tension lysimeters to sample soil water containing PO4–P. Our results indicated varying chemical composition of the three ceramic materials evaluated (referred to here as Ceramics A, B, and C) with Ceramic A having the greatest P, Al, Ca, Mg, and K concentrations. Phosphorus sorption for Ceramics A and C was best described by the Freundlich isotherm, while the P sorption of Ceramic B was best described by the Langmuir isotherm. Ceramic A accurately estimated solutions of 0.1, 0.5, 1.0, and 20 mg P kg−1, while Ceramics B and C accurately estimated only the 20 mg P kg−1 solution. Ceramics B and C underestimated the 0.1, 0.5, and 1.0 mg P kg−1 solutions by 23 to 89%. This underestimation of the P concentration was attributed to P adsorption due to the presence of Al, Fe, Ca, and Mg in the ceramics. These findings imply that before ceramic lysimeters are used to collect soil water samples, the effect of the ceramic samplers on the composition of the water sample should be assessed to explore the P concentration range appropriate for a given ceramic lysimeter.