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The objective of this study was to quantify the optimum rates of water-soluble phosphorus (P) under constant nitrogen and potassium on the growth of new guinea impatiens (Impatiens hawkeri Bull.) 'Paradise Violet' and vinca Catharanthus roseus 'Pacifi ca Red' in soilless media in a recirculating subirrigation system. The experiment was designed so that only phosphate varied between treatments while all other nutrients remained constant. The ammoniacal N to nitrate N ratio was varied to counter balance increases in phosphate. Sodium was used as a counter ion to phosphate at higher con- centrations of phosphate; sodium proved to be toxic at concentrations above 6 mM. In the new guinea impatiens experiment, there was a small increase in K due to the use of dibasic K phosphate to buffer pH. All growth parameters measured (height, leaf number, fl ower number, and shoot fresh and dry weight) showed signifi cant differences with in- creasing P rate. Depending on the growth parameter measured, quadratic-linear models revealed an optimum P rate of 0.1 to 0.96 mM for new guinea impatiens 'Paradise Violet' and 0.45 to 1.25 mM P for vinca 'Pacifi ca Red'. For dry shoot weight, a common measure of optimum plant growth, the optimum P rate was 0.75 mM P for new guinea impatiens 'Paradise Violet' and 0.67 mM P for vinca 'Pacifi ca Red'. For fl ower number, a common measure of fl oral quality, the optimal P rate was 0.96 mM P for new guinea impatiens 'Paradise Violet' and 1.25 mM P for vinca 'Pacifi ca Red'. Electrical conductivity (EC) of the growing media increased signifi cantly with increasing rate of P. At all rates, EC was signifi cantly greater in the top layer than in the bottom and middle layers. The pH of the growing medium did not vary in relation to P concentration. Pollution of surface water, particularly from phosphate accumulation in lakes, rivers, and coastal drainages has generated national attention and valid concerns for a dwindling fresh water supply. Eutrophication has been linked to phosphate water contamination, a key factor leading to fertility restrictions in several states (Carpenter et al., 1998; Gburek and Sharpley, 1998). Phosphorus leaching from soilless media in commercial container production is a prob- lem due to three factors. First, water-soluble fertilizers, continuous-liquid feed fertilizers, and controlled-release fertilizers often are used in excess due to their success in growth enhancement with intensively managed crops

Phosphorus Concentration Affects New Guinea Impatiens and Vinca in Recirculating Subirrigation