Determination of foliar nutrient sufficiency ranges in cultivated rooibos tea using the boundary-line approach

Despite the global importance of rooibos tea (Aspalathus linearis (Burm.f.) R.Dahlgren) as a health-promoting crop, foliar nutrient sufficiency ranges have not yet been determined for commercially cultivated rooibos. Determination of these ranges is critical for assessing plant nutrient status and maximising productivity. The aim of this study was to determine optimal rooibos foliar macro- and micronutrient ranges using the boundary-line approach (BLA) method across the main rooibos production areas in South Africa. Foliar nutrient and yield data were collected from 120 different commercial rooibos plantations in the six main rooibos producing areas of the country: Nardouwsberg/ Agterpakhuys, Clanwilliam, Gifberg/ Vanrhynsdorp, Nieuwoudtville, Citrusdal and Sandveld. The BLA-derived optimum (late winter) rooibos foliar macronutrient and 90%-yield sufficiency range concentrations (%) were: 1.55 (1.30–1.80) for N, 0.12 (0.09–0.14) for P, 0.54 (0.34–0.75) for K, 0.20 (0.14–0.26) for Ca, 0.22 (0.19–0.25) for Mg, and 0.10 (0.08–0.11) for S. Optimum micronutrient contents and ranges (mg kg−1) were: 85.8 (57.6–114.1) for Fe, 58.2 (26.9–113.8) for Mn, 16.6 (12.7–20.5) for Zn, 2.8 (1.3–4.2) for Cu, and 26.9 (21.8–32.0) for B. Maintaining foliar nutrient concentrations within these ranges will support maximal yields (1 200 kg ha−1). The foliar nutrient guidelines are of importance for enhancing sustainable rooibos production in South Africa.