Factors influencing Cd-content in crops. Results from Swedish field investigations

In this paper we review the results from 20 years of Swedish field studies focussed on evaluating the influence of soil Cd-content on Cd-levels in agricultural crops. These investigations have been performed on fields subject to normal agricultural management where P-fertilizers and atmospheric deposition are the main Cd-sources. At present, these sources are roughly of equal size. Statistical analysis of the sampled material shows that the Cd-content (extractable in boiling 2M HNO3) of the soil (positive relation) and soil pH (negative relation) are the main soil factors influencing the uptake of Cd by plants from Swedish soils. In wheat, the soil Cd-level is the most important factor. Uptake is also positively correlated with precipitation during the growing season. Budget calculations show that soil Cd-contents are still increasing. Furthermore, there are areas in Sweden ish with elevated soil Cd-levels where Cd-contents of 5-10% of the wheat lots produced are near or above limit values (0.1 mg kg-1) for tolerable contents for cereals. Liming has limited value as a measure for decreasing uptake since wheat soils generally have the near neutral pH value considered as optimal for most crops. We conclude that the level of Cd in soil is an important and decisive factor determining its uptake and that there are areas where a further increase in soil Cd-levels should be avoided. To further efforts to decrease the input of Cd to soil we suggest that the declaration of the Cd-contents of all P-fertilizers be made mandatory. It is also necessary to make "Cd-free" P-fertilizer available on the market. It could be used on soils high in plant-available Cd and for crops that require large doses of P-fertilizer or that naturally take up large amounts of this metal. A further reduction in emisssions of Cd to the atmosphere is also necessary to get a balance between the input and output of Cd with regard to agricultural soils.