A chemical kinetic model is presented for the formation and accumulation of secondary ferrimagnetic minerals (SFMs) in soil constructed using experimentally determined rate constants and validated against field data. The primary objective is to critically assess the significance of competing causal mechanisms and disputed environmental controls under temperate conditions. Four findings are important in relation to current application of soil magnetic susceptibility data. First, transformation of hydrous ferric oxide to magnetite should dominate SFM formation, controlled primarily by parent material ferrous silicate concentration and climate. Second, abiotic reactions should account for most of the SFM production; the most significant impact of high Fe²⁺ concentrations created by dissimilatory iron-reducing bacteria is enhanced export of iron from the soil in runoff. Third, the model predicts a correlation between hematite and magnetite concentrations, weakening field support for direct transformation of hydrous ferric oxide to maghemite. Fourth, magnetic susceptibility enhancement should increase strongly with weathering duration.