| To evaluate effects of reducing phosphorus (P) nutrition on plant growth, poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch 'Red Sails', 'Bright Red Sails', and 'Freedom Bright Red'), petunia (Petunia x hybrida 'Utra Red'), and geranium (Pelargonium x hortorum Bailey 'Ringo 2000') were grown in a glasshouse in soilless growth medium. Treatments for 'Red Sails' poinsettia and 'Ultra Red' petunia consisted of P from ordinary superphosphate (20% P2O5) (SP) incorporated in the medium at potting time and irrigated with 20N-0P-16.6K water-soluble-no-P fertilizer (WS0PF) or tap water. 'Red Sails' poinsettia received five SP treatments at 0.59, 1.78, 2.97, 4.15, or 5.34 kg·m-3, and 'Ultra Red' petunia received three SP treatments at 74, 297, or 519 g·m -3. Treatments for 'Bright Red Sails' and 'Freedom Bright Red' poinsettia and 'Ringo 2000' geranium consisted of irrigations with different combinations of 15N-2.18P-20.75K water-soluble-P fertilizer (WSPF), WS0PF, or tap water. Leachate was collected from each pot, the volume was measured, and the leachate analyzed for P. Nitrate nitrogen (NO3-N) in the leachate was measured to determine if NO3-N leaching was altered by plant growth effects caused by the reduced P. Symptoms of P deficiency were not detected in foliage or bracts of any poinsettia at any P level. Chlorosis and necrosis were observed on all plants of petunia at the lowest P level. Stunted growth (smaller leaf area) was found on geranium at the lowest P level. Plant size and bract size of each cultivar of poinsettia were visually acceptable regardless of the treatment type or level of P. Petunias were significantly smaller at the lowest treatment level but height did not increase above the intermediate P level. Height and days to flower of geranium were not affected significantly by P levels, but plant diameter and shoot dry weight were affected significantly. Phosphorus leaching increased with increasing P levels for all plants. In general, NO3-N leaching for all plants followed the overall pattern of total leachate volume. This research demonstrated that high quality poinsettia, petunia, and geranium plants could be grown using less P than is commonly used with the additional benefit of less P leaching. |
