Effects of Fertilizer Type and Rate on the Quality and Nutrient Concentrations of Three Species of Field-grown Shrubs in South Florida

Abstract

Three species of tropical shrubs, bush allamanda (Allamanda schottii), ixora (Ixora ‘Nora Grant’), and surinam cherry (Eugenia uniflora), were planted into a native sand soil and a calcareous fill soil in south Florida and were fertilized with a 24N–0P–9.2K (24–0–11) turf fertilizer or an 8N–0P–10K–6Mg plus micronutrients (8–0–12) palm fertilizer at rates of 10 or 20 g of nitrogen (N) per shrub four times per year. Two additional treatments using a 0–0–13.3K–6Mg plus micronutrients (0–0–16) palm fertilizer were applied at equivalent rates of potassium (K) (12.5 or 25 g/shrub of K) to that applied in the two 8–0–12 palm fertilizer treatments. Shrub size measurements, nutrient deficiency severity ratings, number of flowers, and shrub density ratings were determined at 6 months after planting (establishment period) and at 3 years after planting (maintenance phase). Data from these measured variables were subjected to principal component analysis to obtain a single measure of overall quality, namely, the scores for each plant on the first principal component. During the establishment period, ixora fertilized with the high rate of 8–0–12 had the highest quality on the sand soil, but there were no differences among treatments on the fill soil for this species or on either soil type for allamanda and surinam cherry. After 3 years of growth, ixora showed no differences in quality on either soil in response to the fertilizer treatments. On the sand soil, allamanda receiving the high rate of 24–0–11 or the low rate of 8–0–12 had significantly higher quality than unfertilized control plants, and the low rate of 8–0–12 produced the highest quality plants on the fill soil. Surinam cherry grown on sand soil had the highest qualities when fertilized with the high rates of either 24–0–11 or 8–0–12. In general, leaf nutrient concentrations were inversely correlated with overall shrub quality, with largest, highest quality plants having the lowest nutrient concentrations because of dilution effects. However, leaf manganese (Mn) concentrations were consistently within deficiency ranges for all species under most treatments, suggesting that Mn deficiency was stunting shrub growth on both soil types.