Assessment of iron availability in soilless potting media

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Abstract
Ferrous sulphate added to potting media based on wood wastes is rapidly rendered insoluble in water. The extractability by DTPA of the added Fe declines over about 7 days to a steady value which is maintained for at least 10 months whether plants are growing in the media or not. The sawdusts and barks tested all required at least 100 mg/L Fe, added as ferrous sulphate, to provide an optimum amount of available Fe; peats needed less Fe and were quite variable in their requirements. In several pot trials, excellent correlations (R2 0.80-0.86) were obtained between quality or growth of shoots and a combination of medium pH and the concentration of Fe in unbuffered DTPA extractants of concentrations in the range 0.001 to 0.005M, or a saturation extract containing DTPA. Fe concentrations in DTPA/TEA/CaCl2, NaHCO3/DTPA, 0.5M ammonium acetate solutions and water were less precise indicators of Fe availability. At media pH values of 5.5-6.0, 0.002M DTPA (1:1.5 volume) extracts need to contain at least 20 mg/L Fe and preferably 30 mg/L Fe for adequate Fe supply if ferrous sulphate is the source of added Fe. Some species need at least 40 mg/L Fe or must be grown at a lower pH. These criteria do not hold if the main source of Fe is FeEDDHA.
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