This paper describes the development of an environment-friendly, slow-release fertilizer of the micronutrient iron. The compound is water insoluble, and is based on a polymeric phosphate structure. Kinetics and solubility of products in the goethite [FeO(OH)]-H3PO4 and [FeO(OH)]-MgO-H3PO4 systems were studied at 170-300 °C. Polymerization patterns were complex. The presence of Mg as additive, improved product properties. The fertilizer was prepared by polymerization of goethite, magnesium oxide and phosphoric acid to an optimized chain length at 200 °C, followed by neutralization with magnesia. The fertilizer was soluble in citrate and DTPA. A significant increase in the yield of wheat and uptake of iron was observed at a dosage of 2 kg/ha Fe as the slow-release fertilizer. © 2009 Elsevier B.V. All rights reserved.