Cadmium (Cd) is a trace element that can accumulate to high levels in specific crops.  Cadmium has been linked to negative health effects, so it is desirable to reduce the Cd concentration of crops entering the human diet. Cadmium is present at varying concentrations as a contaminant in phosphate fertilizers.   Repeated applications of Cd in phosphate fertilizer may increase Cd content of soils, potentially increasing Cd content of crops.   Field studies were conducted at seven sites across the Canadian prairies, to determine the influence of monoammonium phosphate fertilizer on accumulation of Cd in crops.  At each location, three rates of phosphate fertilizer from three different sources varying in Cd concentration were applied in each of four years.  Rates of phosphate application were 0, 20, 40 and 80 kg P ha^-1 and concentrations of Cd in the fertilizer were 0.38 mg Cd kg^-1, 7.3 mg Cd kg^-1, and 211 mg Cd kg^-1. Treatments were applied to the same plots each year so that the cumulative effect of applications could be assessed over time. The sites were seeded annually between 2002 and 2005, following a durum wheat-flax-durum wheat–flax crop sequence.  Crop yield and the seed concentration of Cd and several other trace elements were determined each year.  Results from 5 of the sites from 2004 are shown in table 1.

Table 1:  Effect of source and rate (kg ha^-1) of phosphorus application on durum wheat seed Cd concentration (mg Cd kg^-1) in 2004, after three years of fertilizer application.

The grain Cd concentration in the unfertilized crop varied by as much as 100% from site to site (Table 1). However, the pattern of response to fertilizer application was similar in the various locations.  Crop Cd concentration increased with P application, even when Cd concentration in the fertilizer was low. Therefore, P fertilization can have an impact on Cd availability to crops apart from the addition of Cd in the fertilizer.  

When P application rates were low, within the range normally recommended for optimum crop yield, Cd concentration in the fertilizer had little effect on Cd concentration in the crop.  When P application rate exceeded the rate required to optimize crop yield, Cd accumulation in the crop increased with increasing Cd concentration in the fertilizer.  Cadmium concentration in the seed generally increased with the total amount of Cd applied. Therefore, improving P use efficiency to minimize application rates and reducing the Cd concentration of fertilizer can reduce Cd accumulation in soils and crops, both in the short- and long-term.