Trace elements extracted by DTPA and Mehlich-3 from agricultural soils with and without compost additions

Risks of soil contamination when waste materials are used as fertilizers have been a matter of frequent concern. The effect of compost from municipal organic waste on trace element status was examined in short-term field trials at neighboring areas of A Coruna (northwest Spain). The study sites were characterized as medium textured soils, with a range of pHs, organic matter content, and cation exchange capacity. The objective of this work was to compare two extraction methods (DTPA and Mehlich-3) to determine micronutrient contents in soils with and without compost additions. DTPA and Mehlich-3 extractions were carried out, and then analyses for Fe, Mn, Cu, Zn, Ni, Cr, Pb, and Cd contents were performed by ICP-AES. Overall, DTPA was less efficient than Mehlich-3 for Fe and Mn extraction. Lead, Ni, and Cd were extracted more effectively by DTPA than by Mehlich-3. In general, the efficiency of the extractants was related to compost addition, a trend that was most apparent for Zn and Cu. Levels of trace metals extracted by DTPA and Mehlich-3 from soils without compost addition were already highly variable, because of traditional farmyard manure and slurry fertilization. Diagnostic criteria for allowable loading limits of heavy metals during compost application should take into account direct measurements of background levels and relevant soil properties such as soil acidity.