Soilless container media have almost no capacity to retain PO4 or K. The nutrient retention of two calcined clays, attapulgite and arcillite, and brick chips, precharged with PO4 and K, was investigated. These could serve as an alternative slow-release fertilizer when incorporated into a soilless medium as a component of the mix. Sorption curves were developed at 25 °C for attapulgite of two particle sizes (0.8 to 1.6 mm and 1.6 to 3.2 mm), arcillite (1.1 to 3.2 mm), screened pieces of brick (1.0 to 3.6 mm), and a medium of 7 sphagnum peat: 3 perlite (v/v) using solutions of KH2PO4 (P at 0 to 20,000 mg.L-1). Curves indicated that PO4 and K sorption were similar for both particle sizes of attapulgite, so only the larger size [1.6 to 3.2 mm (8 to 16 mesh)] was used in greenhouse studies. Materials were evaluated in greenhouse studies by growing 'Sunny Mandalay' chrysanthemum [Dendranthema ×grandiflora Kitam. (syn. Chrysanthemum morifolium Ramat.)]. The precharged materials were tested at 10%, 20%, and 30% by volume of a peat: perlite root medium. Phosphate, K, and pH were determined on unaltered medium solutions collected throughout the cropping cycle and foliar analyses were determined on tissue collected at midcrop and end of the crop. Data indicated that precharged calcined clays retained and released PO4, and to some degree K, over time. Precharged clays did not provide K at levels which met plant needs during the latter half of the cropping cycle, but it was released and used at appreciable levels during the first month of crop production. Growth of plants receiving PO4 solely from precharged attapulgite and arcillite at 20% of the medium volume was not significantly different from that of a commercial control when the leaching fraction was maintained at 0.2. However, release of PO4 from the brick chips was not enough to match plant demand. Phosphate lost through leaching from the precharged clays was reduced by about two-thirds compared to control plants fertilized with P at 46.5 mg.L-1 from water-soluble fertilizer at each watering.