A Phosphate Rock Decision Suport System (PRDSS) software was developed in joint collaboration between FAO-IAEA and IFDC (Smalberger et al., 2005). The final goal is to offer farmers and agronomists a technical tool for decision regarding the agronomic effectiveness and economical feasibility of using phosphate rock (PR), as compared to water-soluble P fertilizers (WSP), based specially on crop, soil properties and PR solubility (neutral ammonium citrate - second extraction = NAC - 2nd), . Field trials were started in 2004 in selected agro-ecological areas of Latin America, Africa and Asia to test the prototype, as well serve as a refinement. This work summarizes data on yield for the first crop (soybean) of the Brazilian trial. The experimental site was located in an Oxisol in Rondonopolis. The soil originally contained a very low amount of available P, pH in water (soil/water ratio of 1:2.5) of 5.3, cation exchange capacity of 8.7 cmolc dm-3, 75% of clay and 30.4% of P fixing capacity (Fassbeder & Igue, 1967). Single superphosphate (SSP; 19.2% P2O5), Araxa PR (1.6% P2O5) and Gafsa PR (7.8% P2O5) were used as the standard source of WSP, local P source and standard source of PR, respectively. The quoted P2O5 was the NAC - 2nd. The rates of P2O5 applied were of 60, 120 and 240 kg ha-1. A control, with no P added, was also included. The P sources were all broadcasted and manually incorporated to 15 cm. Soybean was used as the testing crop. Soil samples from 0-15 cm were collected after harvesting the soybean and analyzed for available P (Raij and Quaggio, 1983). A combined multiple regression analysis using a dummy variable, as described by Chien et al. (1988), was performed for all P sources. Statistics and models showed that SSP = Gafsa PR > Araxa PR in terms of soybean grain yield. Calculation of the relative agronomic effectiveness (RAE) based on regression estimates for models adjusted to field data were of 100%, 51% and 97% for the SSP, Araxa PR and Gafsa PR, respectively. The numbers indicate that the Araxa PR was 51% as effective as the SSP in increasing soybean grain yield in the average of rates tested, whereas the Gafsa PR was 97% as effective as the SSP. The RAE obtained by the prototype was of 100% and 32% for the Gafsa and Araxa PR, respectively, which can be considered a good estimation. The results of P-resin showed that the amount of available P measured by this methodology was in the general order of Gafsa PR > SSP = Araxa PR. The relative index in this case was of 100%, 47% and 317%. These results suggest a better performance of the Gafsa PR in terms of residual effect, as opposed to SSP and Araxa PR. The P-resin results confirms information provided by other authors that highly reactive PR may have a better residual agronomic effectiveness, when compared to WSP sources, in soils of high clay content and P sorption capacity (Chien et al., 1989). Acknowledgments: The authors express their gratitude for the technical and/or financial support from FAO-IAEA, Intertrade and Mato Grosso Agronomic Research Foundation. The senior author express his gratitude to CNPq for the scholarship in research. References: 1) Chien, S. H., D. K. Friesen, and B. W. Hamilton. Effect of application method on availability of elemental sulfur in cropping sequences. Soil Sci. Soc. Am. J. v.52, p.165-169. 1988. 2) Chien, S.H.; Hammond, L.L. Agronomic effectiveness of partially acidulated phosphate rock as influenced by soil phosphorus-fixing capacity. Plant and Soil, v.120, p.159-164. 1989. 3) Fassbender, H.; Igue, Y. Comparación de métodos radiométricos y colorimétricos em estúdios sobre retención y transformación de fosfatos en el suelo. Turrialba, v.17, p.284-287, 1967. 4) Raij, B. van; Quaggio, J.A.; Métodos de análise de solo para fins de fertilidade. Campinas, Instituto Agronômico, 1983. 31p. (IAC Technical Bulletim, 81). 5) Smalberger, S.; Singh, U.; Chien, S.H.; Henao, J.; Wilkens, P. Development of a Phosphate Rock Decision Support System for Direct Application. Submitted to the Agronomy Journal in March 2005.