A comparison of three soil tests for assessing Cd accumulation in wheat grain

Abstract

Three extractants, namely ethylenediamine tetraacetic acid (EDTA), CaCl₂, and Ca(NO₃)₂, were compared to assess the relationship between the amounts of cadmium (Cd) extracted from soil and the Cd concentration of wheat grain, with the view to using a soil test for predicting Cd concentrations in grain. The soils used came from 1 glasshouse experiment and 31 field sites sampled over 2 years, and they had received Cd only from historical applications of phosphatic fertilisers. The soils ranged from a heavy clay with a comparatively high carbon content to a sandy soil. The pH values ranged from 4.5 to 7.8. The relationship between Cd concentration in grain and CaCl₂- and Ca(NO₃)₂-extractable soil Cd was variable and for most cases r² value was <0.6. The use of pH alone to predict Cd concentration in wheat grain was significant (P < 0.05) for all soils used in the glasshouse except the soil with the highest clay content (Inman Valley). In the field experiments, the relationships between Cd concentration in grain and soil pH were significant (P < 0.05) but the r² values were low, ranging from 0.28 to 0.66.

The inclusion of pH and extractable soil Cd (CaCl₂- and Ca(NO₃)₂-extractable) to determine Cd concentration in grain only improved the relationship in one half of the cases in this study. This suggests that there may be little to be gained in prediction of Cd concentration in grain from the use of extractants compared with using soil pH. Soil pH is also an easier, cheaper, and quicker measurement than an extractable soil Cd measurement, particularly in soils with low Cd concentrations where the extraction procedure involves a concentration step. In all cases, grain Cd concentration and EDTA-extractable soil Cd were poorly correlated.