Keynote: Phosphatic fertilisers, Cadmium.
Commercial fertilisers together with atmospheric deposition are considered the major input sources of heavy metals, especially Cadmium (Cd), to cultivated soils. Among heavy metals, Cd is easily transferred from soil to the food chain and it is also associated with various human diseases. Therefore, Cd in soil-plant system and its agricultural significance is mainly discussed. The concentrations of different heavy metals in soils as well as in phosphate rocks and commercial fertilisers are presented.
Transfer of Cd in the soil-plant system is affected by plant species, soil and climatic factors, sources and rates of fertiliser used and by its interaction with other metals.
Among the factors, soil pH was the most important and it showed a significant but inverse relationship with Cd concentration in different plant species.
The results show that the application of fertilisers over many decades increased the Cd levels in the upper layers (0-20 cm) of the soil. The differences between old and newly cultivated soils over a period of 40 years were calculated to be 33, 15 and 75 g/ha for south-eastern, south-western, and central Norway, respectively. Similarly to the total Cd, a significant difference in Cd extracted by 6 different extractants except that by Ammonium Nitrate between the newly- and old cultivated soils was observed. Both length of cultivation and plant species grown affected the extractable Cd.
Contrary to the soil Cd, plant Cd content of most of the species was higher in the newly cultivated soils compared to that of the old cultivated soils. Plant Cd correlated significantly with the Cd extracted by a number of extractants and the correlations were generally improved when soil pH was taken into the regression equation. Plant species absorbed Cd from different sources in proportion to the amount applied but the species varied greatly in their Cd concentration.
The atmospheric contribution to crops was in the range of 14-65%, being generally lowest in carrot and highest in oats.
The input/output balance indicated a general trend of accumulation of Cd in the soils studied and therefore, further additions of Cd through fertilisers or through atmospheric deposition need to be regulated for its sustainable management.
Regulatory control measures would be required to reduce further accumulation of Cd in the soil/plant system. Future research approach is discussed in terms of proper understanding of regional pollution problems, background levels of soils, soil chemical processes controlling transport and mobility of Cd, various sources of Cd inputs to soil-plant-animal system, soil factors affecting the phytoavailability of Cd, and relative contribution of soil and atmosphere to the Cd burden of food products.
Prof Bal Ram Singh, Department of Soil Sciences, Agricultural University of Norway, Ãƒâ€¦s, Norway.
28 pages, 3 figures, 18 tables, 38 refs.