Effects of Different Liming Materials on the Uptake of Cadmium and Mineral Nutrients in Cacao Plants

Considerable attention has been focused on the bioaccumulation of Cd in cacao beans due to trade restrictions associated with the Cd content in cacao-derived products. It is essential to develop countermeasures to reduce plant-available Cd. Even though liming is commonly considered as an accepted practice to increase the soil pH and hence reduce the uptake of Cd by plants, cacao farmers usually report mixed results upon field application, in some cases even reporting that liming can increase Cd content in cacao beans. Herein, we compared the effect of different liming materials on soil pH, plant growth, and Cd and mineral nutrient uptake by cacao plants. It was found that the continuous use of liming could have the unwanted effect of increasing Cd uptake even if the pH in the upper layers of the soil is increased significantly; this is dependent on (i) the type of soil and (ii) the type of liming material used and is attributed to the release of soil-bound Cd due to the increase of ion activity of alkaline earth cations in the soil and the competition for available specific adsorption and cation exchange sites. In general, mineral uptake was enhanced by the increase of soil pH, except for Mg and Ca, which were strongly affected by the type of liming material applied. These results highlight that not only are the pH levels important to reduce the Cd concentration in cacao beans but also it is important to consider the soil properties and the composition of the liming material being used.