Comparison of Kinetic Equations to Describe Release of Iron from Some Calcareous Soils

The release of Fe from soil solid phases into soil solution is a dynamic process that regulates the continuous supply of this element to growing plants. To ascertain the pattern of Fe release, the kinetics of Fe release from six soils by diethylenetriaminepentaacetic acid (DTPA) solution were investigated using soil samples taken from two different agroclimatic regions of Iran. All soils were calcareous with CaCO3 equivalent content in the range of 2.26 to 49.54%. The kinetic experiments were performed under a moisture condition within the range of field moisture content, i.e. %35 of saturation moisture. The release pattern was generally characterized by an initial fast reaction followed by a slower one. The slow release of Fe continued even after 960 hours in all soils. Different kinetic equations (zero-, first, and second order, parabolic diffusion, simplified Elovich, power function, shell progressive film and particle diffusion equations) were employed to describe the rate of release of soil Fe by DTPA solution. Comparisons of coefficients of determination (r2) and standard errors of the estimate (SE) indicated that the power function, parabolic diffusion, first order and shell progressive particle diffusion equations adequately described Fe release, whereas the zero-order, second-order, simplified Elovich and shell progressive film diffusion equations did not. However, the first order kinetic equation inadequately described the release data at short reaction times. Conformity of the shell progressive particle diffusion equation to the kinetic data may indicate that the release rate is controlled by diffusion of Fe and/or DTPA molecules through a reacted soil particle or aggregate rather than diffusion through the film surrounding soil particles.