Moderate nitrogen rates keep queen’s flower thriving while curbing leaching losses

In the subtropical nurseries of southern Florida, growing a beautiful ornamental tree and protecting fragile water systems are two goals that often pull in opposite directions. A new study of queen’s flower (Lagerstroemia speciosa ‘Big Pink’), an increasingly popular landscaping tree, shows that the sweet spot lies in the middle: moderate fertilizer rates produced the best leaf growth and plant nitrogen status while releasing far less nitrate and potassium into container leachate than the highest inputs. The work, published in Frontiers in Horticulture, integrates root-zone chemistry monitoring with handheld optical sensing to offer a practical precision nitrogen strategy for container-grown ornamentals.

The research team, based at Florida International University and the USDA-ARS Subtropical Horticultural Research Station in Coral Gables, evaluated six fertilizer treatments on 30 two-year-old ‘Big Pink’ individuals drawn from the station’s germplasm collection. The plants, part of a regenerative process backing up national plant germplasm resources, were grown in 18.9-liter nursery pots filled with a substrate of pinebark, coir pith, perlite, and sand. All received an 8-4-8 N-P-K azalea fertilizer, but the treatments differed in total rate and timing: T0 received a single 25-gram application, while T1 through T5 received base rates of 15, 20, 25, 30, and 35 grams respectively, each paired with a 10-gram supplemental application in January and March. The plants were arranged in a completely randomized design with five replications per treatment and irrigated uniformly twice daily throughout the study.

Over eight months, from October 2023 to May 2024, the researchers tracked leaf count monthly alongside three plant-based optical indices: the SPAD chlorophyll meter reading, the atLEAF chlorophyll index, and the normalized difference vegetation index (NDVI) derived from a GreenSeeker handheld canopy sensor held 45 centimeters above the canopy. Simultaneously, they collected container leachate using the non-destructive pour-through method, applying one liter of deionized water and capturing 100-milliliter samples to quantify calcium, nitrate, potassium, and sodium concentrations with ion-selective sensors, along with pH, salinity, and electrical conductivity measured with a handheld multiparameter probe. Monthly repeated measurements were averaged within each plant across early, mid-, and late growth stages so that each individual served as a single experimental unit, avoiding pseudoreplication in the statistical analysis.

The leachate data told a clear story about nutrient mobility. Nitrate and potassium concentrations responded strongly and significantly to fertilizer rate at every growth stage, with the two highest treatments, T4 (30 grams plus 10 grams supplemental) and T5 (35 grams plus 10 grams supplemental), consistently producing the greatest concentrations. Because nitrate is highly soluble and mobile in container substrates, it leaches readily when fertilizer inputs exceed crop demand. Potassium, likewise highly soluble, showed the same pattern, with T5 generating the highest potassium concentrations across all stages. These findings indicate that the highest fertilizer rates increase nutrient losses through leachate with limited additional benefit to plant growth.

The highest treatment also altered root-zone chemistry in ways that signal salt accumulation. By the late growth stage, T5 had significantly lowered substrate pH compared with T0, raised salinity, and elevated electrical conductivity. The researchers attribute the pH decline to microbial nitrification: the ammonium supplied by granular fertilizer is oxidized to nitrate, releasing two protons per ammonium ion oxidized, so higher fertilizer rates generate a proportionally larger acidifying load in the substrate solution over the growing season. Elevated electrical conductivity reflects an increased concentration of dissolved salts in the root zone, confirming that excessive fertilizer inputs promote salt buildup that can impair root function over time.

Plant growth responses revealed a classic intermediate optimum. Leaf count was maximized under the moderate treatments T2 through T4, while both insufficient fertilization (T0 and T1) and the highest rate (T5) reduced growth. A particularly notable contrast emerged between T0 and T3, which both received the same 25-gram base rate but differed in whether the supplemental application was applied. T0 consistently produced lower leaf counts and chlorophyll index values than T3, suggesting that the timing and split application of nitrogen, rather than the total seasonal rate, resynchronized supply with the plant’s active growth demand during the spring flush. The leachate data support this interpretation: T0 showed the lowest nitrate and potassium concentrations across all stages, indicating that little residual fertilizer remained available in the root zone by the later stages. The authors emphasize that T0 should not be read as an unfertilized control but as the lowest-intensity regime, reflecting a split-application and timing effect.

The optical sensing results added a layer of diagnostic precision. SPAD and atLEAF readings reliably detected nitrogen deficiency under T0 from the mid-growth stage onward, with T0 consistently showing the lowest values compared with all fertilized treatments. Because leaf chlorophyll concentration is closely correlated with leaf nitrogen content when measurements are collected consistently, these handheld chlorophyll meters serve as well-established non-destructive indicators of plant nitrogen status. NDVI, by contrast, became a responsive indicator only at the late stage, once canopy development had increased sufficiently for the canopy sensor to detect differences. Correlation analysis confirmed strong agreement between SPAD and atLEAF across all stages, with the relationship strengthening from r = 0.36 early on to r = 0.72 and r = 0.73 at the mid and late stages respectively, supporting the atLEAF meter as a reliable and lower-cost alternative to the SPAD meter for assessing plant nitrogen status in nursery settings.

The correlation patterns among leachate variables also shifted over time, becoming progressively stronger and more interconnected as fertilization continued. Salinity and electrical conductivity showed an almost perfect positive relationship by the late stage (r = 0.98), confirming conductivity as a faithful proxy for total dissolved salts in the substrate solution. Nitrate and potassium remained positively correlated across all stages, likely reflecting their common fertilizer origin and high mobility. Meanwhile, pH showed significant negative correlations with nitrate, calcium, and salinity at the early stage, consistent with progressive root-zone acidification as nutrient availability increased through nitrification of fertilizer-derived ammonium.

The authors acknowledge that leaf count was the only direct growth parameter recorded, a deliberate choice for germplasm-collection material where destructive sampling is undesirable. They note that future studies should incorporate additional non-destructive measurements such as caliper-based height and canopy width to validate the optical indices independent of leaf number. They also caution that because the analysis involved multiple tests without correction for multiple comparisons, borderline results close to the significance threshold, such as late-stage calcium and late-stage pH, should be interpreted with care.

Despite producing leaf counts statistically comparable to the moderate treatments, T4 is not recommended as an optimal strategy because it consistently generated higher leachate nitrate and potassium concentrations than T2 and T3, indicating a disproportionate increase in nutrient loss risk relative to the growth benefit gained. In contrast, the moderate treatments T2 (20 grams plus 10 grams supplemental) and T3 (25 grams plus 10 grams supplemental) achieved growth and plant nitrogen status equivalent to T4 while releasing appreciably less nitrate and potassium to the leachate, making them the more nutrient-efficient choice. Under the subtropical conditions of South Florida, the findings suggest that moderate fertilizer rates with split supplemental applications are sufficient to support optimal growth of container-grown ‘Big Pink’ while minimizing nutrient losses, offering a data-driven precision nitrogen management strategy for subtropical ornamental nursery production and germplasm maintenance alike.

Subject of Research: Precision fertilizer management for germplasm maintenance of queen’s flower (Lagerstroemia speciosa)

Article Title: Precision fertilizer management for germplasm maintenance of queen’s flower (Lagerstroemia speciosa)

Article References: Farina, S. L., Poudel, I., Nageswara-Rao, M., & Khoddamzadeh, A. A. (2026). Precision fertilizer management for germplasm maintenance of queen’s flower (Lagerstroemia speciosa). Frontiers in Horticulture, 5, Article 1933427. https://doi.org/10.3389/fhort.2026.1933427

Image Credits: AI Generated

DOI: 10.3389/fhort.2026.1933427

Keywords: Precision, fertilizer, management, germplasm, maintenance, queen, flower, Lagerstroemia, speciosa, scientific research