Fertilizer Impacts on Cadmium Availability in Agricultural Soils and Crops

ABSTRACT

Ingestion in food is a major pathway of cadmium (Cd) exposure for humans. It is therefore desirable to ensure that Cd concentrations in crops that enter the human food chain do not increase to levels that may lead to health risks. Phosphorus fertilizers contain Cd as a contaminant at levels varying from trace amounts to as much as 300 mg Cd kg^–1 of dry product and therefore can be a major source of Cd input to agricultural systems. Fertilization can influence Cd accumulation in crops by direct Cd addition and by indirect effects on soil pH, ionic strength, Zn concentration, rhizosphere chemistry, microbial activity, and plant growth. Cadmium will accumulate in the soils from fertilizer applications if the amount of Cd added in fertilizer is greater than the amount of Cd removal, whether in harvested crop removal or other loss pathways such as leaching, erosion, or bioturbation. Assessment of the impact of fertilizer management practices on the risk of Cd toxicity to the soil ecosystem and the risk of movement of Cd into the human diet must consider both the direct influence of Cd addition as a fertilizer contaminant and the indirect effects of fertilizer application on Cd phytoavailability. Cadmium accumulation in soils and crops can be minimized by adoption of management practices that improve fertilizer-use efficiency while minimizing Cd input.

Key Words:

• phosphate
• bioavailability
• fertilizer
• cadmium
• human health
• risk quotient

ACKNOWLEDGMENT

The authors gratefully acknowledge the support of the NSERC MITHE Research Network. A full list of network sponsors is available at www.mithe-rn.org. Additional support was provided by International Plant Nutrition Institute, Western Co-operative Fertilizers, Agrium, National Agri-Environmental Health Analysis and Reporting Program and Agriculture and Agri-Food Canada's Matching Investment Initiative. The constructive comments of two anonymous referees are also gratefully acknowledged.

Notes

^aPhosphorus (P) application rate based on average rates for the crops reported in the region.

^bCrop off-take is based on crops grown in the region, their reported yield, and an estimate of the portion exported;

^cCd addition in fertilizer based on average Cd concentrations for P materials commonly in use;

^dQuotient based on sum of fertilizer inputs and background and using the CCME (2005) guidelines for agricultural soil (1.4 mg kg⁻¹);

^e As above, except an approximation of soildependent guidelines was used, using an adjustment based on soil clay content;

^fGrain/soil concentration ratios measured for hard red spring wheat were used to estimate grain Cd concentrations;

^gUsing the current international trade limitation of 0.2 mg kg⁻¹ in grain.

^aY indicates variable is dependent (varies among) elements;

^bY indicates variable is dependent (varies among) units of land considered (termed polygons);

^cY indicates variable is dependent (varies among) crop types or fertilizer materials;

^dsource indicates which variables were measured, were derived from various sources, were assumptions, came from Statistics Canada routine reports, or were from literature sources;

^edescription of variance includes N (normal distribution), CV (coefficient of variation), LN (log normal distribution), GSD (geometric standard deviation), and triangular (a type of frequency distribution).

^aRussia, Florida and Idaho P fertilizers contained 0.2, 7.8, and 186 μ Cd g⁻¹, respectively.