Evaluation of Soil Amendments in Reducing Plant-Available Cadmium in Three Cacao Growing Soils.

Excessive plant-available cadmium (Cd) in soils resulted in elevated concentration of Cd in cacao beans in southern Ecuador. In this study, the capacity of two soil amendments in reducing plant-available Cd in contaminated cacao-growing soils was evaluated. Soils with varying soil properties and Cd levels were collected from surface layer (0-15 cm) in cacao-growing regions of southern Ecuador. Soils were spiked with 0 (background level) and 5 mg kg^-1 (contaminated), with soil moisture adjusted to 70% of water holding capacity (WHC) and left for equilibrium for two days. Then, zeolite or vermicompost (from harvested residues) were incorporated at the rates of 0, 0.5, or 2%. Treated soils were placed in Ziploc bags (12.5x15 cm) and incubated at room temperature for 28 days. Subsamples were taken at the intervals of 0, 3, 7, 14, 21 and 28 d and analyzed for plant-available Cd using 0.01 M CaCl₂ (CaCl) and Mehlich 3 (M3) methods. At both rates, zeolite failed to reduce CaCl₂- or M3-extractable Cd in all the soils. In contrast, vermicompost at 2% significantly reduced CaCl-extractable Cd in all the soils (P < 0.01), but reduced M3-extractable Cd only in one Cd-spiked soil (P < 0.05). After 7 d of incubation, CaCl-extractable Cd decreased by 47- 67%, for the three soils.  Additionally, vermicompost at 2% significantly increased soil pH for all the soils (P < 0.01). On average, this amendment increased soil pH by 2.77 and 3.21 units for the non-spiked and spiked soils, respectively. A negative correlation between soil pH (Cd-spiked) and CaCl-extractable Cd was observed for all the three soils. Apparently, vermicompost is a promising amendment for lowering plant-available Cd in cacao-growing soils; however, further research is needed to confirm this conclusion.