Solubility of ten iron fertilizers in eleven north American soils

Iron is a common component of granular fertilizers. Chelating agents are commonly used to increase Fe solubility and plant uptake. Numerous products are marketed as Fe chelates but few have been confirmed to increase Fe solubility in soils. The objective of this study was to determine the solubility of Fe from 10 common Fe fertilizers applied to 11 North American soils. Brookston, Cecil, Fuquay, Hallandale, Marietta, Mar-vyn, Nunn, Pinavetes, Stephenville, Troup, and Zook soils were incubated with soluble Fe applied as sulfate; glucoheptonate; polysaccharide; humate; oxide; citrate; 1,2 dicarboxyethyl D,L aspartic acid (IDHA); ethylene diamine tetra acetic acid (EDTA); diethylenetriamine pentaacetic acid (DTPA); and eth-ylenediamine-o-hydroxyphenylacetic acid (EDDHA). At 1 h, 1 d, 1 wk, 2 wk, and 3 wk, soils were extracted with 0.01 mol L^–1 CaCl₂ and analyzed for Fe. In each soil 1 h after application, greater than 95% of applied Fe was rendered insoluble from sulfate, glucoheptonate, polysaccharide, humate, and oxide. The chelates FeEDTA, FeDTPA, and FeEDDHA resulted in increased Fe solubility compared with untreated soil within each sampling time in most soils with Fe solubility at 21 d ranging from 5 to 40% of applied Fe. The natural organic chelate (NOC), FeIDHA, increased Fe solubility in 3 of the 11 soils for 1 d, but soluble Fe from FeIDHA declined to that of untreated soils thereafter. These results suggest that soil-applied Fe fertilizers should be limited to EDTA, DTPA, or EDDHA.