FACTORS INFLUENCING CD-CONTENT IN CROPS - RESULTS FROM SWEDISH FIELD INVESTIGATIONS

In this paper we review 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 fie lds subject to normal agricultural management where P-fertilizers and atmospheric deposition are the main Cd-sources. At present, these sour ces are roughly of equal size. Statistical analysis of the sampled mat erial shows that the Cd-content (extractable in boiling 2M HNO3) of th e soil (positive relation) and soil pH (negative relation) are the mai n soil factors influencing the uptake of Cd by plants from Swedish soi ls. In wheat, the soil Cd-level is the most important factor. Uptake i s also positively correlated with precipitation during the growing sea son. Budget calculations show that soil Cd-contents are still increasi ng. Furthermore, there are areas in Sweden with elevated soil Cd-level s where Cd-contents of 5-10% of the wheat lots produced are near or ab ove limit values (0.1 mg kg(-1)) for tolerable contents for cereals. L iming 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 importa nt and decisive factor determining its uptake and that there are areas where a further increase in soil Cd-levels should be avoided. To furt her efforts to decrease the input of Cd to soil we suggest that the de claration of the Cd-contents of all P-fertilizers be made mandatory. I t 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 c rops that require large doses of P-fertilizer or that naturally take u p large amounts of this metal. A further reduction in emissions of Cd to the atmosphere is also necessary to get a balance between the input and output of Cd with regard to agricultural soils.