Keywords: mineral fertiliser, animal manure, heavy metals, trace elements, long-term field experiment, Askov.

Spring barley (grain, straw), grass-clover (two cuts), winter wheat (grain, straw) and silage maize grown in the Askov long-term experiment with different levels (0, ½, 1, 1 ½, 2) of mineral fertiliser (NPK) and animal manure (AM) had concentrations of As, Pb, Cd and Hg below the EC maximum permissible levels for feeds. The content of As was barely detectable even in crops grown with double rate of NPK. This was also true for Hg and Ni in cereal grains. The Cd content increased linearly with NPK rate but only in crops with fresh dressings of NPK. Yet, cereal grains were low in Cd. Contents of Cd in crops grown on unmanured soil and soil with AM were similar and below that of crops grown with NPK. For a range of other elements including Cr and Ni, we found no general relationship between fertilisation rate and their concentration in the crops. Except for Cd, no systematic differences were found in the analysed metal contents between crops grown with NPK and AM. Crop contents of uranium and thallium were below the analytical detection limits regardless of nutrient source and addition rate. Thus in a farming context similar to that of the Askov experiment, the long-term application of standard rates of NPK and AM does not pose a threat in terms of feed quality. However, the long-term accumulation of heavy metals added with mineral fertilisers and animal manure is essentially irreversible and may threaten soil quality.

Bent T Christensen and Lars Elsgaard, Department of Agroecology, Aarhus University, Blichers Allé 20, DK-8830 Tjele, Denmark.

28 pages, 7 figures, 10 tables, 54 references.