The fertilizer NH(4)FePO(4)center dot H2O (AIP) was synthesized under mild hydrothermal conditions to be applied on soils to prevent iron deficiencies. The effect of the addition of AIP on soil microbial activity was studied by calorimetry, determining both basal respiration and carbon mineralization by means of the addition of an external carbon source. Thermal analyses (TG and DSC) were also used to provide additional soil properties. The effect of different amounts of AIP on soil microbial activity was quantitatively analyzed by a mass and energy balance performed via the analysis of the power-time curves. These balances allowed determination of the impact of All? on soil more rapidly than conventional methodologies. The increase in the amount of added AIP leads to a less efficient metabolism, probably due microbial competition for the nitrogen source provided by the AIP and for the carbon source. (C) 2005 Elsevier B.V. All rights reserved.