Cadmium Levels in Soils and Crops in the United States

The concentrations of cadmium in agricultural soils depend upon the amounts present in the parent rocks from which the soils form, the amounts added in the form of fertilizers and soil amendments, the amounts deposited onto soils from the atmosphere, and the amounts removed by harvested crops and by leaching. On average, sedimentary rocks contain greater concentrations of Cd than either igneous or metamorphic rocks, and therefore, recent soils derived from sedimentary rock should contain greater concentrations of Cd than those derived from igneous or metamorphic rocks. Among the commercial fertilizers, phosphorus fertilizers contain somewhat elevated levels of Cd. Results show that the long-term use of phosphorus fertilizers will slightly increase the concentration of Cd in surface soils. Due to the combustion of fossil fuels and to the smelting and processing of ores, air in rural regions contains Cd. Results show that the amounts of Cd added to soils via aerial deposition in rural agricultural regions is of the order of a few grams per hectare per year and approximately equal to that added to soil from normal phosphorus fertilization operations. All crops contain Cd, and the harvested portion will serve to remove some Cd from soil. Amounts removed from soil in harvested crops, however, are quite small compared with the amounts present. Cadmium is reasonably immobile in soils, and the available data suggest that the amounts removed by leaching are also small compared with amounts present. The concentrations of Cd in non-contaminated agricultural soils in the United States range from about 0.1 to 1.0 mg/kg. Except for a limited number of soils derived from parent materials unusually high in Cd, organic soils (Histosols) tend to contain the highest total Cd concentrations, and highly weathered soils (Ultisols and Alfisols) the lowest. Other soil orders contain concentrations of Cd in amounts approximately equal to their geological abundance or from about 0.2 to 0.4 mg Cd/kg. The fact that highly weathered soils are somewhat depleted in Cd suggest thatgreaterquantitiesof Cdareremovedby cropsandleachingthanareadded through fertilization and atmospheric deposition. Surface soils commonly contain