Gypsum application lowers cadmium uptake in cacao in soils with high cation exchange capacity only: A soil chemical analysis

Soil liming to lower cadmium (Cd) bioavailability is challenged in perennial cacao orchards by the low penetration of lime in soils, not reaching deeper roots. Some studies suggest that gypsum (CaSO₄) could reduce Cd uptake by enhanced Ca²⁺:Cd²⁺ competition at the root surface. A pot experiment was conducted to identify soil conditions affecting gypsum efficacy in cacao plantlets (5‐month‐old CCN‐51). Plants grew for 115 days in soils sampled from six locations with variable cation exchange capacity (CEC, 6–50 cmolc.kg⁻¹) and soil solution Ca²⁺ (1.3–17 mM). Soils were either mixed or not mixed with a dose equivalent to 4.0 Mg gypsum ha⁻¹. Leaf Cd concentration in plants grown on gypsum applied soils was statistically lower compared to unamended soils in only one of the six soils. The reduction factor of leaf Cd concentrations due to gypsum (RF = control/amended Cd concentration ratio) ranged from 2.32 (high effect) to 0.76 (no effect), and it was positively correlated with the Ca²⁺ in the solution of the unamended soil (R² = 0.58) and with increasing CEC (R² = 0.52), that is, gypsum only reduced Cd bioavailability in high CEC soils, confirming a trend found in earlier publications. Soil solution analyses showed that gypsum application enhanced total dissolved Cd by forming CdSO₄⁰ complexes with, in some cases, even enhanced Cd²⁺ ion activities, likely related to Ca²⁺‐induced Cd²⁺ mobilisation. Data suggest that Ca²⁺ derived from gypsum has counteracting effects on Cd mobilisation and ion competition at the root surface. The former reactions are less pronounced in high CEC soils. More research is needed to unravel gypsum effects on high Ca²⁺ soils to corroborate our findings. HIGHLIGHTS: The effect of soil properties on the efficacy of gypsum to reduce Cd uptake in cacao was studied. Gypsum only reduced Cd uptake in cacao in the soil sample with highest CEC and highest Ca in solution. This finding implies a possible competition between Ca²⁺ and Cd²⁺ for root uptake in cacao.