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Reactions of DTPA Chelates of Iron, Zinc, Copper, and Manganese with Soils¹

ABSTRACT

The Fe, Zn, Cu, and Mn chelates and the Na salt of DTPA (diethylenetriaminepentaacetic acid) were reacted with suspensions of acid and alkaline soils. The suspensions were aerated and shaken continuously for periods of 2 hours to 30 days. The concentrations of soluble Fe, Zn, Cu, Mn, Ca, and Mg were measured by atomic absorption spectrophotometry, and the concentration of ¹⁴C-labeled DTPA was determined by a liquid scintillation technique.

Additions of Fe-DTPA were effective in maintaining soluble Fe in four soils ranging in pH from 5.8 to 7.3. Even at pH 7.9 more than 13% of added Fe remained soluble after 30 days of reaction in a calcareous soil. DTPA was an ineffective chelate for Zn and Cu in soils at pH 5.8 and 6.2, but became more effective with rising pH. In the alkaline soils more than 50% of added Zn and Cu remained soluble during 30 days of reaction. Essentially all Mn from Mn-DTPA additions was lost from solution within 3 days in all soils. Discrepencies between the concentrations of ¹⁴C-labeled DTPA and soluble micronutrient metals suggest that an appreciable fraction of ¹⁴C-labeled DTPA decomposed to unknown products having different chelating capabilities.

Contribution from the Dep. of Agronomy, Colorado State Univ., Fort Collins 80521. Published with the approval of the Director of the Colorado Agr. Exp. Sta. as Scientific Series Paper no. 1753. This work was supported in part by Geigy Agricultural Chemicals Division of Ciba-Geigy Corp., Ardsley, N.Y.

² Former Graduate Research Assistant and Professor of Soil Science, respectively, Colorado State Univ. The senior author is now Assistant Soil Chemist, Connecticut Agr. Exp. Sta., New Haven.

Received for publication February 9, 1972. Accepted for publication May 22, 1972.