Mike McLaughlin

CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia.

There is no doubt as to the necessity of applying fertilizers, and other soil amendments, to soils to ensure the sustainability of food production. Commodity exporting countries such as New Zealand and Australia are well aware of this basic mass balance principal of nutrient replacement. However, when adding fertilisers to soil, we often add many unwanted impurities - are they a threat to sustainability? Nitrogen, sulfur and potassium fertilizers are relatively free of impurities, at least in materials which are not by-products of urban or industrial activity (i.e. manufactured fertilisers). Unfortunately, manufactured phosphatic fertilisers often contain several impurities which can potentially adversely affect agricultural production, either through degrading food quality, or affecting the fertility of the soil. Similarly, waste materials, which have potential as fertilisers, often contain potentially toxic elements.

In manufactured fertilizers the main elements of concern are Cd and F, while in waste materials As, Cr, Cu, Hg, Ni, Pb and Zn may pose a threat to the soil resource. The critical pathways through which each element poses risk differ, so that research needs to target the appropriate pathway. Research on Cd availability in agricultural systems in Australia and New Zealand has provided a much better understanding of Cd behaviour in soil and has suggested several management practices to control this element's transfer through the food chain. The critical question is whether these practices alone are sufficient to guarantee sustainability in the longer-term, and a critical question in this regard is the relative long-term availability of Cd versus P in soil.

Fluoride is an element where less is known of the potential long-term effects, with the critical pathway being ingestion by grazing animals and possibly effects on the soil biota. Arsenic, Cr, Hg and Ni generally do not pose significant risks to agricultural soils in Australia and New Zealand. Copper and Zn are usually added to soil in significant amounts in urban and animal wastes used as fertilisers, yet the database for these elements on which we base regulations or guidelines to protect the soil resource is surprisingly small. In addition, the scientific rationale on which regulations are based is open to criticism for being simultaneously both over protective and under protective for different elements. Alternative approaches to controlling contaminants in agricultural soil will be presented and discussed, with the aim of answering the title question.

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