Comparative Assessment of Physico-Chemical Characteristics and Selected Trace Metal Distribution in Soils from Three South African Tea Estates.

Soil health represents one of the critical soil attributes that exert profound influence on the sustainability and profitability of agricultural production systems as well as human health through the foods obtained from soil-based agricultural activities. High acidity and elevated concentrations of trace (heavy) metals are among inherent soil characteristics that could ultimately result in metal enrichment in tea leaves in tea growing soils. Such elevated metal contents in unprocessed and processed tea leaves most often originate from various sources that include soil, farmers’ agronomic practices and industrial processing; and may potentially constitute health risks. The total content and phyto-availability index of Zn, Cu, Mn and Ni in fifty-four soil samples obtained from three tea estates were assessed in relation to their physico-chemical properties. Results revealed significant differences in soil extractable Mn and Cu, as well as total trace metal concentrations across the Estates. The total metal concentrations in soil across the three tea Estates were in the order of Mn>Ni>Zn>Cu while metal concentration in the tea leaf samples from the Estates varied greatly, which were in the order Al>Mn>Fe>Zn>Cu. Organic carbon, total N and availability indices of P, K, Ca and Mg differed significantly across the three tea gardens Although the total Zn level in the soils was relatively higher than the range reported in USA soils, extractable Zn content from the gardens though statistically comparable but was below acceptable threshold level reflecting the none to low inorganic fertilizer use. The responses of plant available Zn, Cu & Mn to soil clay content, pH (KCl), ECEC and acid saturation are variable with Ca and Mg representing the most limiting cations in these soils. Among the soil parameters measured, strong and positive correlation was obtained extractable Mn with clay (r² = 0.61***) and Zn showed with soil pH (r² = 0.78***) while the highest correlation coefficient of 0.94 obtained between soil organic carbon and total N. The variation in metal concentration of tea leaves across the Estates reflect strongly the age of the tea plants and soil geochemistry.