Impact of Long-term Phosphate Application and Level of Fertilizer Cd on Crops and Soils

Public concern regarding the transfer of cadmium (Cd) from phosphorus (P) fertilizers to soils, and uptake by crops, has increased interest in evaluating the true impact of fertilization practices on food nutrient content. The objectives of the study are: 1) determine the cumulative impact of applying fertilizer P, varying in Cd concentration, on grain yield and micronutrient concentration, total soil Cd, phyto-available Cd, and grain Cd concentration, on a range of soil types; 2) determine the impact of soil characteristics on availability of native soil Cd and Cd added with P fertilizer, and; 3) determine the effectiveness of several soil testing methods in predicting availability of native and applied Cd across a range of soil types and environmental conditions.

Field studies were conducted at seven sites across the Prairie Provinces using P fertilizer rates of 0, 40, 80, and 160 lb P₂O₅/A and the three concentrations of Cd in the fertilizer (0.38 mg Cd/kg, 7.3 mg Cd/kg, 211 mg Cd/kg). Analysis was completed on the 2004 durum grain samples and Cd concentration at all locations were below proposed Codex limits in the absence of fertilization, and were generally below concentrations observed on these sites in the first year of the study. 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 field-grown 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. Application of high rates of high-Cd fertilizers increased Cd concentration above the 100 mg Cd/kg level, but not above 200 mg Cd/kg. The increase was highest at the Ellerslie and Melfort sites, which also showed the greatest proportional increase in grain yield with P application. Total Cd uptake was increased to an even greater degree at these locations, indicating that dilution was not occurring.

Soil samples were collected from selected treatments at a number of the field locations for more detailed analysis. Cadmium concentrations in the soil solution increased with both P fertilizer and Cd level in P. Increases in Cd concentrations were not proportional to the Cd input, indicating that the Cd concentrations were “buffered” in water extracts. Phosphate solubility is suspected to explain the higher Cd solubility in the lab experiment.

Cadmium concentration in the seed and Cd concentration in the soil solution generally increased with the total amount of Cd applied. However, at P fertilizer rates normally used in crop production in western Canada, the influence of Cd concentration in fertilizer is relatively low. Therefore, improving P use efficiency to minimize application rates as well as reducing the Cd concentration of fertilizer can reduce Cd accumulation in soils and crops, both in the short- and long-term.

Lab studies were initiated to evaluate the impact of additions of monoammonium phosphate (MAP) fertilizer, varying in Cd concentration of 0.38, 7.3, and 211.0 mg Cd/kg of fertilizer, on soil solution and plant uptake of Cd. The study found very little relationship between Cd added to soils and Cd found in plants. Even where the concentration of Cd added to soil varied to a large degree, only small increases in solution Cd were observed. Adding the equivalent of 15 years of fertilizer MAP to soils did not result in a drastic increase in soil solution Cd, even when very high Cd concentrations were used. MB-16