Abstract: A process for the preparation of water insoluble bio-release micronutrient fertilizers, containing iron and manganese with either zinc and/or copper, comprising (a) heating orthophosphoric acid at a temperature of 130° C.-150° C. with a mixture of (i) source of iron oxide, (ii) manganese dioxide or pyrolusite, and (iii) either or both compounds of zinc or copper selected from zinc ash or zinc oxide and cupric oxide, or cupric sulphate or cupric chloride and (iii) a basic compound such as oxide or carbonate of magnesium or calcium, to produce a multi-metallic polyphosphate; (b) neutralisation of the polyphosphate with a basic compound followed by drying and pulverisation to obtain a solid powdery material.