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Fixation of Iron Applied as FeEDTA: Effect of Calcium Concentration and Soil Solid Phase¹

ABSTRACT

Iron fixation from FeEDTA was measured in three soils at various water contents and Ca concentrations as a function of time for periods up to 30 hours, using a batch technique. In the three soils under study the general pattern of the fixation reaction can be described as a first-order reaction if adsorption is assumed to be much faster than fixation reaction. The rate constant is markedly affected by Ca concentration: in Rehovot sand and Nazaret clay loam (0 and 61% CaCO₃, respectively) the reaction rate constant increases sharply as added Ca increases from 0 to 5 mM, followed by a moderate and more or less linear increase up to Ca concentration of 100mM. In Zophar sandy loam (69% CaCO₃), the reaction rate constant shows a maximum at Ca concentration of 50mM and then decreases.

In Rehovot sand and Nazaret clay loam the fixation process can be described by two independent first-order reactions taking place simultaneously on the solid phase surface and in the liquid phase. Zophar sandy loam does not fit this model. It is suggested that the solid surface area affects the fixation reaction in addition to the effects of pH and Ca concentration.

The effect of Ca on Fe-FeEDTA fixation is discussed in connection to average and local Ca concentrations. High Ca concentrations near the root surface, formed due to transpiration stream, constitute a "chemical barrier" for the movement of Fe-FeEDTA toward the root.

¹ Contribution from the Dep. of Soil and Water, The Faculty of Agric., Rehovot, The Hebrew Univ. of Jerusalem, Israel.

² Associate Professor and Graduate Student, respectively.

Received for publication June 13, 1977. Accepted for publication November 11, 1977.