Saturday, 15 July 2006

Agronomic Evaluation of Two Phosphate Rocks and Preliminary Testing of a Phosphate Rock Decision Support System.

L. I. Prochnow1, L. C. Tessaro2, S. H. Chien3, U. Singh3, S. A. Smalberger3, and M. M. Cunha4. (1) Dept of Soil and Plant Nutrition, ESALQ-Univ of Sao Paulo, P. O. Box 9, Piracicaba, 13418-900, Brazil, (2) Mato Grosso Research Foundation, Rua Pernanbuco, 1267, P. O. Box 79, Rondonopolis, 78705-040, Brazil, (3) IFDC, P. O. Box 2040, Muscle Shoals, AL 35662, (4) Intertrade Group, Rua dos Andradas, 1121 - 12o. andar, Porto Alegre, 90020-007, Brazil

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.

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