Review: The behaviour and environmental impact of contaminants in fertilizers

The risks of contaminants accumulating in soils and crops due to inadvertent addition of impurities in agricultural fertilizers and soil amendments were assessed for Australian conditions. Elements considered of concern were arsenic (As), cadmium (Cd), fluorine (F), lead (Pb) and mercury (Hg). Consideration of background concentrations of these elements in Australian soils, inputs to soil in fertilizers and offtake in harvested crops indicates that Cd and F will accumulate in fertilized soils at a faster rate than As, Pb or Hg. The major factors affecting the accumulation of fertilizer-derived Cd, F, Hg and Pb in soils and their transfer to agricultural crops are reviewed in an Australian context where data are available. Cadmium is the element of most concern as its transfer from soils to the edible portions of agricultural food crops is significantly greater than for other elements. After consideration of the behaviour of F, Hg and Pb in the soil-plant system, we conclude that these elements pose negligible risk of accumulating to toxic concentrations in agricultural food crops. Proposed regulations governing maximum permitted concentrations (MPCs) of F in soils may need review and critical concentrations of F in agricultural soils need definition, given current F loadings to soil from fertilizers. Some agricultural produce currently exceeds Australian MPCs for Cd. However, the levels observed in crops and soils are in a range similar to those found internationally. While Cd concentrations in Australian phosphatic fertilizers have been historically high in comparison with fertilizers used in other countries, lower inputs of fertilizer per unit area and less atmospheric contamination of soils have resulted in similar or lower Cd loadings to agricultural land compared with Europe. In recent years the use of phosphatic fertilizers with lower Cd concentrations and the development of plant cultivars which restrict Cd uptake should assist in control of Cd accumulation by crops. However, acidification and salinization of soils in Australia poses a threat in terms of increasing Cd concentrations in agricultural produce. In comparison with other trace metals, Cd availability to plants appears to decline only slowly with time, if at all. More Cd is currently added to Australian soils than is removed in agricultural produce or by leaching. It is therefore important that the long-term behaviour of Cd in Australian soils be assessed, to determine if Cd concentrations in agricultural produce will slowly increase over time.