Leaf chemical composition and nutrient removal by stems of Leucadendron cvv. Silvan Red and Safari Sunset

Abstract

Chemical analysis was used to determine the concentrations of 12 nutrients in youngest fully expanded leaves (YFEL) of Leucadendron cvv. Silvan Red and Safari Sunset at 2 sites in the Mount Lofty Ranges. Leaves were sampled every month for 3 years, commencing in July 1990. The leaf nutrient data were used to define seasonal nutrient trends, thereby identifying the most suitable time for leaf analysis; to determine the magnitude of the differences in leaf nutrient composition between Leucadendron cultivars, and between Leucadendron and Protea hybrids; to calculate total nutrient removal by harvested stems, which can be used to formulate maintenance fertiliser programs; and to determine the correlations between nutrients. The seasonal increase in concentrations of nitrogen (N), phosphorus (P), potassium (K), and sodium (Na) in YFEL corresponded with the spring growth flush, after which concentrations decreased with time, particularly during summer and autumn. Concentrations of copper (Cu) and zinc (Zn) were unstable during October-April and the seasonal trends were not consistent between sites or with other mobile nutrients (e.g. N, P, K). Concentrations of calcium (Ca), magnesium (Mg), manganese (Mn), iron (Fe), sulfur (S), and boron (B) at site 1 decreased early in the season, were lowest when vegetative flushing peaked, and tended to increase during autumn and winter. Seasonal variation in the main nutrients removed in marketable stems (i.e. N, Ca, K, Mg) was minimal during June-August. However, to assess the overall nutrient status of plantings, sampling in June is most suitable. Crop nutrient surveys conducted at this time, in conjunction with productivity and quality data, can be used to develop interpretation standards for leaf analysis. For all nutrients, the seasonal trends were similar for the 2 cultivars, but concentrations of Mn were consistently lower in YFEL of Silvan Red than Safari Sunset. In contrast to the small differences between cultivars, there were large differences in leaf nutrient composition between the Leucadendron cultivars and Protea 'Pink Ice'. For example, Mg, Na, and Mn concentrations were consistently lower, and N, K, Ca, and Fe higher, in YFEL of Pink Ice than in the Leucadendron cultivars. For these nutrients, different interpretation standards may be required for Leucadendron and Protea hybrids. The major nutrients removed in harvested stems were Na, N, Ca, K, and Mg. Based on nutrient uptake data alone, we suggest annual applications of N and Ca at 20-30 g/plant, and Mg and K at 10-15 g/plant, on acid sands. Significant (P<0.05) correlations were found between many nutrients. For example, N concentrations were positively correlated with P, K, Na, and Zn, and negatively correlated with Ca, Mg, and Fe concentrations. These significant relationships may indicate synergistic and antagonistic interactions between nutrients, which need to be considered when interpreting plant test data.