Determination of optimal soil pH and nutrient concentrations for cultivated rooibos tea using the boundary line approach

Abstract

Optimal soil pH and nutrient conditions for rooibos cultivation in South Africa have not yet been established as limited field research has been conducted. Soil chemical (0–300 mm) and yield data were collected from 120 commercial rooibos plantations in the main (170 km wide) production region in the Western and Northern Cape Provinces. The optimum and 90%-yield sufficiency soil ranges were determined using the boundary line approach and quadratic regression. The optimum (sufficiency range) soil chemical properties were a pH (KCl) of 4.5 (4.2–4.7), electrical conductivity of 0.23 mS m⁻¹ (0.16–0.30 mS m⁻¹) and acid saturation of 27.5% (17.1–37.8%). The optimum (sufficiency range) soil available macro- and micronutrients (in mg kg⁻¹) were: 16.9 (7.4–26.4) for P, 20.8 (11.5–30.0) for K, 140.3 (95.9–184.7) for Ca, 50.0 (32.3–63.7) for Mg, 8.3 (4.9–11.7) for S, 34.6 (17.2–52.1) for Fe, 5.5 (2.2–8.8) for Mn, 0.30 (0.16–0.45) for Zn, 0.11 (0.07–0.15) for Cu and 0.11 (0.06–0.16) for B. The resultant optimal values for soil pH and available P, K and Mg corresponded with previous published reports on cultivated fynbos species, while published values were not available for the other parameters. The findings of this study are important for sustainable soil fertility management in commercial rooibos tea production in South Africa.