Mitigating cadmium concentrations in spinach and onions by the application of silicon fertiliser to soil

Cadmium (Cd) is readily absorbed by plants and can sometimes accumulate to concentrations that exceed food standards. Few mitigations that can decrease plant Cd concentrations have been tested in New Zealand. We used a glasshouse experiment to measure whether applying silicon (Si) fertiliser to soil could decrease Cd concentrations in spinach (Spinacia olearacea) and onion (Allium cepa). The application of Si (1000 and 2500 kg ha-1) increased spinach dry matter yield and Si concentrations, but had no effect on Cd concentrations. In contrast, the application of Si had no significant effect on dry matter yield or Si concentrations in the onion crop, but there was a 45% decrease in Cd concentrations. It was found that the application of Si had no effect on Cd availability in the soil. The decrease in Cd concentrations in onions was likely related to Si restricting the movement of Cd from the root to the harvestable part of the plant. These findings indicate that Si may have some potential as a mitigation tool to decrease plant uptake of Cd, although further investigation is required to evaluate its effectiveness in soils with different properties and for other food crops, and in a field experiment.