Kinetics of DTPA Extraction of Iron from Some Calcareous Soils of Iran

Iron is a micronutrient for plants and its deficiency is a common in calcareous soils of Iran. The kinetics of iron extraction by diethylenetriaminepentaceticacid (DTPA) from 12 sterile calcareous soils was investigated during a 11520 minute period. The zero, first, second and third order equations could not adequately describe iron extraction kinetics. Three equations of simplified Elovich, parabolic and two- constant rate well fitted to the experimental data but the best model for describing extraction data for all soils was the two- constant rate equation and a constant of this equation showed positive correlation just with cation exchange capacity (CEC). The minimum and maximum amount of extracted iron during 11520 minutes was 30.2 and 200. 4 mg Fe. kg-1 (with a mean of 63.8 mg Fe.kg-1), respectively. The amount of extractable iron for initial extraction times (ab constant) and the rate at which iron is extracted from the soil (related to the b constant) is quite variable and is not systematically related to any of the measured soil properties. Consequently a soil test for available iron that involved extraction for a single quite short time period may underestimate the amount of iron that is eventually release to soil solution and plants.