The first tropical 'metal farm': Some perspectives from field and pot experiments

Agromining is the chain of processes of phytoextraction of economically valuable elements by selected hyperaccumulator plants, and subsequent processing of biomass to produce targeted metals or commercial compounds of high value. Although substantial unrealized opportunities exist for developing economic nickel (Ni) agromining in the tropics, this technology has remained relatively unexplored. This study investigated the soil chemistry of a newly established tropical 'metal farm' and elucidated the performance of a prospective 'metal crop' species (Phyllanthus rufuschaneyi) to be used in a large-scale tropical Ni agromining program on ultramafic soils in Sabah (Malaysia). We found that a major portion of the site ( > 90%) had high total Ni concentrations ( > 2000 mu g g(-1)) in the soil (shallow Eutric Cambisol Magnesic). This study also recorded high phytoavailable soil Ni concentrations in the field site, which is a desired property of soils intended for Ni agromining. Moreover, the average soil pH of the field (pH 6.4) is ideal for maximum Ni uptake in the local candidate species. We recorded low concentrations of Ca, K and P, suggesting the need for a fertilizer regime in the farm. The extraordinary shoot Ni concentrations ( > 2 wt%), coupled with the high purity of the 'bio-ore' derived from Phyllanthus rufuschaneyi, confirm its high potential for economic Ni agromining. The success of our first field trial is critical to provide 'real-life' evidence of the value of large-scale tropical 'metal farming'. Research priorities include the need to intensify the search for candidate species, determine their agronomy, develop mass propagation methods, and to test technologies to process the biomass to recover valuable products.