Soilless substrates have little capacity to sorb PO4. One way to reduce PO4 leaching during production is to increase the substrate retention of PO4. Adsorption isotherms were created at 25 C for alumina (aluminum oxide); the 2:1 calcined clays arcillite (montmorillonite plus illite) and attapulgite.; and a medium of 70 peat: 30 perlite using solutions of KH2PO4 at rates of P ranging from 0 to 20000 μg·ml-1. Material sorbed at the rate resulting in maximum P adsorption was then desorbed 22 times. Sorbing concentrations necessary to establish an equilibrium P concentration of 10 μg·ml-1 in the substrate solution were estimated from these curves. Materials were-charged with P at these estimated rates and evaluated in a greenhouse study in which each material was tested at 10 and 30% by volume of a 70 peat: 30 perlite substrate used to produce Dendranthema × grandiflorum `Sunny Mandalay'. Phosphate, K, and pH were determined on unaltered soil solutions biweekly throughout the cropping cycle and foliar analyses were determined on tissue collected at mid- and end-crop. Isotherm and greenhouse data indicated that alumina, arcillite, and attapulgite effectively retained and slowly released K as well as PO4 over time. Alumina was most effective at retaining P, sorbing 16800 μg/cc compared to 3100 and 7800 μg P sorbed/cc for arcillite and attapulgite, respectively, when sorbed at P concentrations resulting in an equilibrium concentration of approximately 10 μg P/ml.