Fertilizer usage and cadmium in soils, crops and food

Abstract: Phosphate fertilizers were first implicated by Schroeder and Balassa in 1963 for increasing the Cd concentration in cultivated soils and crops. This suggestion has become a part of the accepted paradigm on soil toxicity. Consequently, stringent fertilizer control programs to monitor Cd have been launched. Attempts to link Cd toxicity and fertilizers to chronic diseases are common. A re-assessment of this "accepted" paradigm is timely, given the larger body of data available today. The data show that both the input and output of Cd per hectare from fertilizers are negligibly small compared to the total amount of Cd/hectare usually present in the soil itself. Calculations based on current agricultural practices are used to show that it will take about 18 centuries to double the ambient soil-cadmium level, and about 8 centuries to double the soil-fluoride level, even after neglecting leaching and other removal effects. Hence the concern of long-term agriculture should be the depletion of available phosphate fertilizers, rather than the contamination of the soil by trace metals or fluoride. This conclusion is confirmed by showing that the claimed correlations between fertilizer input and cadmium accumulation in crops are not robust. Alternative scenarios that explain the data are examined. Thus soil acidulation on fertilizer loading, and the effect of magnesium, zinc, and fluoride ions contained in fertilizers are considered using recent Cd$^{2+}$, Mg$^{2+}$ and F$^-$ ion-association theories. The protective role of ions like Zn, Se, Fe, etc., is emphasized, and the question of cadmium toxicity in the presence of other ions is considered. These help to clarify and rectify difficulties found in the standard point of view. This analysis does not modify the accepted views on Cd contamination by airborne delivery, smoking, and industrial activity, or P-contamination causing algal blooms.