GROWTH-RESPONSE, NUTRIENT LEACHING, AND MASS-BALANCE FOR POTTED POINSETTIA .2. PHOSPHORUS

Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch 'V-14 Glory') gr own as single-pinched plants and received constant fertigation of Hoag land solution with N at 210 mg.L-1 of 100% NO3-N or 60% NO3-N : 40% NH 4-N; P at 7.8 and 23 mg.L-1; and leaching fractions (LPs) of 0, 0.2, o r 0.4, The P at 23 mg.L-1 used in this study was about half the P conc entration typically provided from a 20N-4.4P-16.6K fertilizer at 200 m g.L-1 N fertigation, The total P applied via fertigation ranged from 5 1 mg at the 0 LF to 360 mg at the 0.4 LF, The leachate P concentration ranged from 0.2 to 46 mg.L-1, With P at 7.8 mg L-1, the percentage of total P recovered in the leachate was 6% to 7%, At 23 mg.L-1 P fertig ation, however, the total P recovered in the leachate with 60% NO3-N t reatment was 2-times greater than with 100% NO3-N treatment, This resu lt is attributed to a lower substrate pH, which resulted from NH4-N up take and nitrification processes with 60% NO3-N fertigation, The P con centration in the recently matured leaves with 7.8 mg L-1 P fertigatio n was in the normal range of 0.3% to 0.6%, Fertigation P can be reduce d by up to 80% and still be sufficient for producing quality poinsetti as. Reducing the fertigation P concentration is beneficial because it reduces P leaching, reduces fertilizer costs, and reduces luxury consu mption.