Cation imbalance and heavy metal content of seven Indonesian soils as affected by elemental compositions of parent rocks

Elemental composition of various fresh parent rocks has received little attention in relation to cation imbalance and potential for high concentrations of heavy metals in soils. The objective of the study was to assess elemental composition of fresh parent rocks and its effect on cation imbalance and natural heavy metal occurrence in soils. Seven fresh rocks consisting of basalt, gabbro, basaltic andesite, volcanic tuff, volcanic ash, mica schist, and serpentinite and their corresponding overlying soil profiles were sampled for various chemical analyses. The elemental composition of finely ground individual fresh rock was determined by X-ray fluorescence (XRF). Exchangeable cations and heavy metals of soils were extracted by NH₄OAc and by a mixture of concentrated HNO₃ and HClO₄, respectively. Results showed that major elements of fresh rocks, referred to as a source of ‘potential exchangeable cations’, were different among the rock types. The order of parent rocks in decreasing potential cations was gabbro>basaltic andesite>basalt>volcanic ash>volcanic tuff>serpentinite>mica schist for Ca, serpentinite≫gabbro>basalt>basaltic andesite>mica schist~volcanic ash>volcanic tuff for Mg, volcanic tuff>mica schist>volcanic ash>basaltic andesite>basalt>serpentinite>gabbro for K, and volcanic tuff>basaltic andesite>basalt>volcanic ash>gabbro>mica schist>serpentinite for Na. Based on base cation saturation ratio (BCSR) of Ca:Mg:K=6.5:1:0.5, none of the seven soils overlying different fresh rocks met the “ideal soil” suggesting cation imbalance. The critical cation deficiency status (CCDS) showed all soils (except MA5) were insufficient in K accompanied either by insufficient or high content of either Ca, Mg, or Na. The heavy metals of Pb, Cd, and Hg were not detected in all fresh rocks, while Cr and Ni were present in different amounts. In soils overlying fresh rocks, the order of magnitude of natural Cr agreed well with its order in fresh rocks, while Ni disagreed. This indicated that Cr in soils was mainly dictated by Cr content in parent rocks, while Ni depended on soil forming processes in addition to parent rocks. Cr and Ni have exceeded threshold limits in soils developed from serpentinite and gabbro, respectively, while soils developed from mica schist, volcanic tuff, basaltic andesite and volcanic ash parent rocks were below the threshold limit, viz., Cr and Ni contents were less than 11ppm each. Implications for the study are the risk of Cr and Ni toxicity that should be taken into account when selecting crops to be grown (e.g. non-food crops) on soils derived from serpentinite and gabbro respectively, to avoid heavy metal risk for human health. The content of heavy metals in soils derived from various rocks in the present study may be used as baseline data if no information is available under tropical conditions.