FERTIGATION RATE, LEACHING FRACTION, AND GROWTH OF POTTED POINSETTIA

Single-pinched poinsettia (Euphorbia pulcherrima 'V-14 Glory') in 15-c m pots received constant fertigation with 50, 100, 200, and 300 mg . L (-1) nitrogen (N) from a 20N-4.4 phosphorus (P)-16.6 potassium (K) fer tilizer with a leaching fraction (LF) of 0, 0.2, or 0.4. Plants receiv ed 25 irrigations during the 13-week study. The shoot fresh and dry ma sses with 50, 100, and 300 mg . L(-1) N at the 0.4 LF were 30% larger than at the 0 LF. The 300 mg . L(-1) N fertigated plants had approxima tely 15% more leaf area and almost 122% more bract area than the 50 mg . L(-1) N fertigated plants. The leaf N concentration of plants ferti gated with 100, 200, and 300 mg . L(-1) N was near or in the normal ra nge of 4 to 6%, but was below the critical level of 3.5% with 50 mg . L(-1) N fertigation. In contrast, the leaf P concentration approached or exceeded the toxic lever of 0.9% with 100 to 300 mg . L(-1) N. The N fertigation of 100 to 200 mg . L(-1) is adequate for producing a qua lity poinsettia crop. Quality poinsettias can be grown at a 0 LF if qu ality irrigation water is available. With 11 mg . L(-1) P via fertigat ion, the leaf P concentration was in the acceptable range. The P conce ntration in the 20N-4.4P-16.6K complete fertilizer was excessive for p oinsettia and would contribute to unnecessary P leaching.