Floodplains particularly affected

The results of the study, about to be presented at the first symposium of ETH Zurich’s World Food System Center, show considerable variation in the amount of cadmium contamination of the soil (and uptake in cocoa beans) in Honduras. Professor Schulin’s collaborator on the project, postdoctoral researcher Anja Gramlich, discovered raised cadmium levels in floodplains and volcanic soils. Contamination was much lower in samples taken from other areas.

The total cadmium content averaged 0.3mg per kilogramme of soil – comparable with the level occurring naturally in Swiss soils. Soil is classified as contaminated when it contains 0.8mg or more of cadmium per kilogramme of soil.

If elevated levels of cadmium were available to plants in the soil parent material, there was also a high uptake in the cocoa beans. Unusually, the researchers found significant amounts of cadmium in the soils and beans of some remote hilly regions well away from polluting industry and intensive farming. Most plantations are only managed as agroforestry estates with minimum use of artificial fertilisers or pesticides, both of which can be sources of cadmium pollution.

The researchers were therefore able to rule out farming methods as being responsible for raising cadmium levels in the soil. “We have no concrete proof that the problem is man-made,” confirms Dr Gramlich. “The cadmium seems to stem almost exclusively from the parent bedrock.”

Soil parent material binds the cadmium

The situation is different in Bolivia, where researchers did find some places with higher cadmium levels in the soil, but not in the cocoa beans on the trees growing on this soil. Dr Gramlich’s explanation for this phenomenon is that the soil samples tested in Bolivia were less acidic than those tested in Honduras. The amount of clay and organic material contained in the soil also plays a role. “Both of these bind cadmium, thereby reducing its uptake by the cocoa tree.”

It is still unclear whether the variety of cocoa tree grown affects the cadmium uptake in the beans. Investigations, including genetic tests, are currently underway to establish this. Not until this research has been completed will it be possible to recommend certain types of cocoa tree for planting on soils with elevated cadmium levels, according to Dr Gramlich.

Is the soil parent material the problem?

Cadmium occurs naturally in the ground, but also enters the soil through intensive farming or atmospheric pollution. “Previous data from Latin America were not conclusive enough to accurately identify the factors responsible for the high level of cadmium in certain soils and beans,” stresses Professor Schulin. “We therefore first had to collect some solid data in order to identify the relationships between cadmium uptake in cocoa trees and the farming methods used or the underlying geological bedrock.”

Dr Gramlich has been busy collecting this data through field trials in cocoa plantations in Honduras and Bolivia. She has collected and analysed soil samples, along with parts of the cocoa tree such as roots, leaves and beans.

Back in the laboratory, the ETH researcher measured not only the cadmium content, but also the acidity of the soil samples (cadmium is more available to plant uptake in acid soil), as well as the quantity of organic material and phosphorus. In addition, she analysed the two trace elements zinc and iron in the plant samples. The recorded levels were then compared with the total cadmium content in the cocoa beans.

Contaminated cocoa leads to loss of earnings