Development of a new slow-release micronutrient fertilizer of iron-manganese is described. Studies include formulation, synthesis, kinetics, characterization, and testing. The compound is a partially polymerized, iron-manganese-magnesium polyphosphate, which is produced at 200 degrees C by reaction of iron, manganese, and magnesium oxides with phosphoric acid, followed by neutralization to pH 5.6. Polymerization is optimal at 21.2% with an Fe/Mn/Mg/P molar ratio of 1:0.51:1.15:7.33. Condensation kinetics show multistage processes with plateau formation at the end of each stage. The compound is crystalline with new XRD patterns indicative of a regular arrangement of polyphosphate chains. ESR spectra reveal Mn predominantly in the IV state. The product has ideal slow-release characteristics of low water solubility but high citrate and DTPA solubility, indicating high bio-availability of the micronutrients. Plant trials with chilli show a 45.6% increase in yield, at Fe 2 kg/ha-Mn 1 kg/ha as the slow-release fertilizer. The compound appears to be a promising, environmentally friendly alternative for Fe-Mn fertilization.