THE CHEMICAL FORMS OF ZN AND CU EXTRACTABLE BY MEHLICH 1, MEHLICH 3, AND AMMONIUM BICARBONATE-DTPA EXTRACTIONS

Abstract

Mehlich 1 (M1), Mehlich 3 (M3), and Ammonium Bicarbonate DTPA (AB-DTPA) have been used to determine the available micronutrients in soils. However, the forms of micronutrients extractable by these extractants are not known. The objectives of this study were: (i) to examine the correlation between the forms of Zn or Cu (readily soluble, organically bound, precipitated, and residual) in soils determined by a sequential extraction procedure and M1, M3, or AB-DTPA-extractable Zn or Cu and (ii) to directly determine the quantity of each form of Zn and Cu in soils that can be extracted by M1, M3, or AB-DTPA. The results show strong correlations between the organically bound or precipitated forms of Zn or Cu and the amounts extractable by any of the above three extractants. It appears that in sandy soils, Zn and Cu extractable by M1, M3, or AB-DTPA contain most of the readily soluble and the organically bound forms as well as a small fraction of the precipitated and residual forms. The rankings of these three extractants for extraction of organically bound Zn and Cu are M1 = M3 > AB-DTPA and M3 = AB-DTPA > M1, respectively. All of the above three extractants were equally effective in extracting the precipitated form of Zn or Cu. The efficacy of extraction of the residual form of Zn and Cu decreased in the order M3 > M1 = AB-DTPA.