Role of soil quality in declining rooibos (Aspalathus linearis) tea yields in the Clanwilliam area, South Africa

Abstract

Global demand for rooibos tea is increasing whereas yields are decreasing in the primary production area of Clanwilliam, South Africa. Commercial rooibos producers report that tea yields decline over time following the initial clearing of the natural fynbos veld. Therefore, the aim of this study was to investigate soil and plant quality in cultivated rooibos plantations of various ages (1–60 years) and adjacent, wild rooibos stands in pristine fynbos in the Clanwilliam area. Soil chemical and physical properties, plant total biomass, tea yields, foliar elemental contents, extent of root nitrogen (N) nodulation and mycorrhizal colonisation were assessed. The most prominent soil quality changes at the oldest cultivated sites compared with pristine fynbos soils were an increase in soil phosphorus (P) from 1.3–1.7 mg kg–1 to 4.0–17.0 mg kg–1, a general decline in total carbon from 0.14–0.29% to 0.09–0.10% and decline in total exchangeable basic cations from 0.64–0.78 cmolc kg–1 to 0.34–0.51 cmolc kg–1, and a concomitant increase in exchangeable aluminium (Al) from 0.10–0.30 cmolc kg–1 to 1.03–1.83 cmolc kg–1. Foliar N : P ratios notably declined at the cultivated sites (12 : 0 – 20 : 1) compared with pristine fynbos sites (27 : 1 – 33 : 1), indicating foliar P accumulation and lack of N. Soil P was strongly negatively correlated with rooibos root mycorrhizal colonisation. The decline in soil organic matter and basic cations, especially potassium (K), was most strongly correlated with the decline in rooibos shoot biomass yields at cultivated sites. These findings highlight the significant role of soil quality in declining yields of rooibos tea in the Clanwilliam area. Management practices should be implemented that increase soil organic matter and essential basic cations such as K, and soil P and exchangeable Al levels should be monitored.