1. Bowen, H. J. M. 1979 Environmental Chemistry of the Elements. Academic Press, London, 333.
2. Chlopecka, A. 1992 Forms of trace metals from inorganic sources in soils and amounts found in spring barley. Water, Air and Soil Pollution, 66
3. Code of Practice for Agricultural Use of Sewage Sludge, Department of the Environment, Her Majesty’s Stationery Office, London, 1989.
4. Council Directive on the protection of the environment and in particular of the soil, when sewage sludge is used in agriculture. 1989 Official Journal of the European Communities, No L 181/6.
5. Dalenberg, W. and van Driel, W. 1990 Contribution of atmospheric deposition to heavy-metal concentration in field crops, Netherl. J. Agric. Sci. 38, 367–369.