Solubility of Iron, Manganese and Magnesium Sulfates and Glucoheptonates in Two Alkaline Soils.

Sulfate and glucoheptonate forms of iron (Fe), manganese (Mn) and magnesium (Mg) are commonly applied to horticultural and agricultural crops.  Once these metals enter the soil solution, their solubility may be limited.  Glucoheptonate may prolong soil solubility but its influence in alkaline soils has not been documented.  The objective of this study was to determine the solubility of Fe, Mn and Mg sulfates and glucoheptonates in two alkaline soils.  A Tavares sand (Hyperthermic Uncoated Typic Quartzipsamments) and a Fuquay loamy sand (loamy, kaolinitic, thermic Arenic Plinthic Kandiudults) were incubated with soluble Fe, Mn or Mg applied as either sulfate or glucoheptonate.  At 1 h, 4 h, 1 day, 1, 2, and 3 weeks, soils were extracted with 0.01 M CaCl₂ and analyzed for Fe, Mn or Mg.  At 1 h, approximately 98% and 93% of applied Fe was insoluble in Tavares sand and Fuquay loamy sand, respectively.  The greatest differences between soils occurred with Mn solubility with 54% and 20% rendered insoluble in Tavares sand and Fuquay loamy sand, respectively, at 1 h.  Soluble Mg declined at 1 h by 10% with no further reductions throughout the 3 wk incubation.  Soil applications of Fe as sulfate or glucoheptonate should be avoided.  Applications of Mn sulfate or glucoheptonate may lead to increased soil solubility immediately following the application but may rapidly decline.  Whereas, Mg remains soluble for as much as 3 wks.  Glucoheptonate did not increase the solubility of Fe, Mn or Mg compared to sulfate in either soil.