Effects of Soil Amendments, Rootstock–Scion Combinations and Zeolite on Cadmium Reduction in Cocoa

Cadmium (Cd) accumulation in cocoa poses a regulatory challenge for cacao producers in regions with naturally elevated soil Cd, such as Indonesia. This study evaluated the potential of soil-based and plant-based solutions to reduce Cd uptake in cacao. The efficacy of soil amendments was tested with two experiments: (1) a 12-week soil incubation tested lime, biochar, and lime–biochar mixtures at five rates on sandy clay loam and (2) a field trial evaluating zeolite applied at three rates (300, 600, and 900 kg ha−1) with heat or alkali pretreatments. A third experiment evaluated the potential of four cacao genotypes and their rootstock–scion interactions to mitigate Cd uptake over the course of a 12-month nursery trial in Cd-augmented soil. In the incubation study, some lime and biochar treatments produced numerically lower soil Cd concentrations than the control (0.25 mg kg−1), with final means as low as 0.15 mg kg−1, but these differences were not statistically significant in this experiment. Application of zeolite in the field significantly reduced leaf and bean Cd levels (leaf: 0.35–0.50 mg kg−1; bean: 0.25–0.75 mg kg−1) compared to the control (p < 0.01). In the nursery experiment, average increases in leaf Cd concentrations from 6 to 12 months after spiking were lowest in rootstock MCC01 (1.46 mg kg−1; p < 0.001) compared to higher increases in MCC02 (4.16 mg kg−1) and Sulawesi 1 (3.53 mg kg−1), indicating reduced Cd uptake by MCC01 across scions, while scion and interaction effects were not significant. Targeted soil amendments and specific rootstock–scion combinations are promising strategies to reduce Cd concentrations in cacao systems.