Standard characterization of phosphate rock samples from the FAO/IAEA phosphate project

Phosphate rocks (PR) are phosphate-bearing minerals that vary widely in their inherent characteristics and consequently their agronomic potential. In the framework of a FAO/IAEA networked research project, the evaluation of the agronomic effectiveness of natural and modified PR products under a variety of soil climate and crop management conditions was carried out. The characterization of phosphate rocks is the first and essential step in evaluating their suitability for direct application. If several PR sources are utilized, standardized methods should be used for comparison purposes to determine their agronomic potential. This paper describes the standard characterization of phosphate rock products utilized in the project, in particular the mineralogical and crystallographic analyses, physical analyses, chemical composition and solubility in conventional reagents. A total of 28 phosphate rock samples from 15 countries were collected and analyzed in specialized laboratories. The data on mineralogy, chemical composition and solubility in conventional reagents are closely interrelated. An arbitrary classification of the reactivity of the PR samples was made based on the solubility indices in conventional reagents. On another hand, the results of the crystallographic parameters, calculated indices of absolute solubility, specific surface and porosity reflect the variability of the physical state and the sample pre-conditioning treatment of the analyzed products. A proper characterization of phosphate rock samples should provide the maximum of basic information that can be obtained in a cost-effective manner in normal chemical laboratories. Based on the results of this characterization, the following determinations are recommended: a description of the sample, major elemental (total P, Ca, Mg) composition, solubility in conventional reagents (neutral ammonium citrate, citric and formic acid) and particle size analysis. The classification of PR samples for direct application as fertilizers requires the definition of standards. It should be noted that the European standards are very strict with regard to solubility and particle size. The studies carried out under this project are part of the efforts to provide some guidelines for the provision of such standards in other environmental conditions, in particular tropical acid soils. Follow-up studies are required to evaluate the agronomic effectiveness of the PR, i.e. actual performance of the PR under study as affected by a specific set of soil, crop, climate and management conditions. In a final phase, it would be required to relate the results agronomic potential from the study with those of actual agronomic effectiveness and several other socioeconomic criteria for developing adequate Decision Support Systems for PR application. (author)