Keywords: Cadmium, Fertilisers, Soils, Plants.
The very toxic nature to man of traces of cadmium (Cd) and estimates that some Europeans may already have average daily intakes approaching 75% of the WHO recommended maximum intake is a cause for concern. Plant based foodstuffs are the largest source of dietary Cd and, therefore, the relative contribution of direct Cd deposition to vegetation and soil Cd to the acquisition of Cd by plants is important but as yet largely unresolved. Analyses of archived soils taken from Rothamsted experiments since the 1850s shows that atmospheric deposition of Cd to soil has been a significant source but, within Europe at least, atmospheric emissions have declined appreciably since the mid 1960s. This decline suggests that phosphate fertilisers may become a significant source of soil Cd and controls on the amount of Cd they contain are in place, or are proposed, in a number of European countries. There is some suggestion of differences in the immediate availability of Cd in phosphate fertilisers and atmospheric depositions. Larger amounts of Cd can be added to soils in sewage sludge and animal manures but they are applied to a smaller area of land than phosphate fertilisers. Although Cd may be firmly complexed with the organic matter, plant Cd concentrations should be monitored for plants grown on soils receiving large amounts of these amendments.
Total dietary intake of Cd is related both to the amount of a foodstuff consumed and the concentration of Cd in it. In the developed countries most people have sufficient food available to them so that decreasing dietary intake requires decreasing Cd concentrations. Factors controlling plant Cd include: direct aerial deposits if these can be adsorbed, the evidence is that aerial emissions are declining; dilution effects when Cd uptake increases less than dry matter accumulation, the yield potential of many crops has increased; plant control over translocation, especially to grains, there is some evidence that this may occur with cereals; the addition of phosphate fertilisers, they encourage root growth and this may increase Cd uptake, the Cd they contain may also be readily available to plants; the declining availability of Cd added to soil from different sources if it reacts with other soil constituents, one of the most important ways of decreasing Cd uptake by plants is to maintain soils above pH water 6.5.
Resolution of the concerns about Cd and human health requires the construction of Cd balances for a range of soils and cropping systems where all factors, especially aerial inputs, leaching losses and changing soil Cd availability with time are monitored. Such balances are also essential to determine the effectiveness of government, fertiliser industry and farming practices on decreasing the level of Cd in plant based foodstuffs.
A E Johnston, IACR Rothamsted, Harpenden, UK.
K C Jones, University of Lancaster, Lancaster, UK
39 pages, 6 figures, 9 tables, 76 refs.