Two experiments were conducted to investigate the effect of K fertilizer rates on growth of New Guinea impatiens (
Impatiens HawkeriBull.), vinca ( Catharanthus roseus(L.) G. Don) and petunia ( Petunia hybridaHort. Vilm.-Andr.) in a recirculating subirrigation system. Based on a variety of growth parameters, a broad range of K concentrations allowed maximum growth, notably 1 to 6 mM for New Guinea impatiens `Ovation Salmon Pink Swirl', 2 m mfor New Guinea impatiens `Cameo' and `Illusion', 2 to 8 m mfor vinca `Pacifica Apricot', and 2 to 16 m mfor petunia `Trailing Wave Misty Lilac'. Thus, the lowest concentration that allowed maximum growth was 1 to 2 m mK. A third experiment compared the optimum K concentration and K balance of vinca grown with recirculating subirrigation versus top-watering. Based on a variety of growth parameters of vinca `Pacifica Red', the lowest concentration that allowed maximum growth was 2 m mK with recirculating subirrigation and 4 m mK with top-watering. The K balance demonstrated that subirrigated plants were twice as efficient in K use compared to the top-watered plants. Leachate loss was the major contributor to inefficiency in top-watered plants. Electrical conductivity (EC) of the growing medium remained below the recommended level of 1.2 dSm-1 in both irrigation methods at K concentrations of 16 m mand below in the bottom layer and 8 m mand below in the middle layer. In the top layer of the growing medium, EC was above the recommended level at all K concentrations tested in subirrigation at all concentrations, and in top-watering at 16 m mand above.