Speciation of Nickel and Chromium in Paddy Soils of Masinloc, Candelaria, and Sta. Cruz, Zambales, Philippines

Rice is a staple food in the Philippines. Its sufficiency and safety are of primary concern. Properties of paddy soils are crucial in rice production. Paddy soils may contain high concentration of heavy metals and this poses human health risk if they are available for plant uptake, translocated to the rice grains, and consumed. However, total concentration of heavy metals in soil does not tell about its availability in soils and plants. This study was conducted to determine the concentration and availability of Ni and Cr in paddy soils of Masinloc, Candelaria, and Sta. Cruz, Zambales by speciation using modified BCR sequential extraction procedure. Soil properties such as particle size distribution, pH, organic carbon, total nitrogen, available phosphorus, total free iron oxides, amorphous iron oxides, and mineralogical properties were also determined. The relationship between Ni and Cr and soil properties were determined to assess the influence of soil properties to Ni and Cr mobility and availability. Results revealed that Ni concentrationsin Masinloc, Candelaria, and Sta. Cruz were 818 mg/kg, 2308 mg/kg, and 438 mg/kg respectively while Cr concentrations were 438 mg/kg, 860 mg/kg, and 1770 mg/kg respectively. These are 14 to 57 times and 2 to 9 times higher than the maximum allowable concentration for Ni and Cr in agricultural soils. Nevertheless, they have very low availability for plant uptake due to 71% to 72% of Ni and 75% to 87% of Cr in soil are mostly bound to mineral structures (F4) and their concentration in the cation exchange sites and carbonates (F1) which are potentially available for plant uptake are only 4% to 6% for Ni and 1% to 3% for Cr. Thus, study sites have low Ni and Cr availability for plants and these soils can still be used for rice production. Further studies may relate the availability of Ni and Cr in soil with the uptake of rice plant as well as its rice variety