Effects of Nitrogen and the Various Forms of Nitrogen on Phalaenopsis Orchid—A Review

Growers realize the importance of nitrogen (N) on the vegetative growth of phalaenopsis orchids (hybrids of Phalaenopsis sp.), but often overlook its influence on reproductive growth. Low N may result in slow plant growth, pale-green leaves, abscission of lower leaves, and few flowers in phalaenopsis. Increasing N concentration up to 200 mg·L −1 promotes leaf growth and increases flower count. High N concentration promotes lateral branching on the flowering stalk, thereby greatly increasing the total flower count and elevating the commercial value. It is important that N be continually applied during the forcing period for best flowering performance, particularly for those that had undergone international shipping. For the vegetative phalaenopsis plants that are induced to flower without being shipped internationally, the N that is already in the plant before spiking provides 43% and the N being absorbed by roots after cooling provides 57% of the total N in the inflorescence at time of visible bud. When insufficiently fertilized or no fertilization is applied during the forcing period, more of the existing N in a plant is mobilized for inflorescence development. Phalaenopsis roots can take up all three forms of N [i.e., nitrate (NO 3 -N), ammonium (NH 4 -N), and urea] directly. In two studies, phalaenopsis plants were supplied with the same amount of total N but with varying NO 3 -N from 100%, 75%, 50%, 25%, to 0% (a common N concentration was achieved by the substitution of the respective balance with NH 4 -N). Plants were smaller when receiving 75% or 100% NH 4 -N with a tendency of decreasing top leaf width and whole-plant leaf spread as NO 3 -N decreased from 100% to 0%. Spiking was delayed and spiking rate decreased when plants were grown in sphagnum moss, but not a bark mix, and received more than 50% of the N in NH 4 -N. As the ratio between NO 3 -N and NH 4 -N increased, flowers became increasingly larger. The negative effects of low ratios of NO 3 -N to NH 4 -N were more severe in the second flowering cycle. When supplied with 50% or more NH 4 -N, the absorption of cations by phalaenopsis roots declined, with reduced concentrations of calcium and magnesium in plants, while symptoms of ammonium toxicity appeared, including growth retardation, chlorotic leaves, and necrotic roots. In conclusion, adequate N and its continual supply during both vegetative and reproductive stages are recommended for the best growth and flowering of phalaenopsis. Since