Kinetics of Manganese Release From Some Calcareous Soils of Iran

Kinetics of native Mn desorption by diethlenetriaminepentaacetic acid (DTPA) from 12 calcareous soils of Iran was studied in a laboratory experiment. Comparisons of coefficients of determination (r) and standard errors (SE) for seven different models indicated that the two best models for describing the desorption data in all soils were two-constant rate and parabolic – diffusion equations. The zero, first, second and third order equations could not describe Mn desorption properly and the coefficients of determination (r) decreased from zero to third order. Organic carbon and clay contents were the most pertinent soil characteristics that predicted the rate constants of the two constant and parabolic kinetic models. The amount of extractable Mn for long extraction times (the constant ˝a˝) increased with organic matter content. Constant ˝a˝ ranged from 0.585 to 11.24 (mgMn kg-1min-1) and ˝b˝ from o.313 to 0.554(mgMn kg-1min-1)-1. The rate at which Mn was extracted from each soil (related to the constant b) were quite variable and was not systematically related to any of the measured soil properties. Consequently a soil test for available Mn involving extraction for a quite short time period might underestimate the amount of Mn that was released in reality.