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Phosphate Rock and Superphosphate Combinations for Soybeans in a Cerrado Oxisol¹

The extent to which local low reactivity Patos de Minas phosphate rock (PR) and banded superphosphate (OSP) applications can replace high cost broadcast OSP applications was studied during a 2-year period on a Typic Acrustox at the Cerrado Research Center near Brasilia, Brazil. Highest soybean (Glycine max) yields were obtained in two consecutive crops with a single broadcast OSP application of 352 kg P/ha or by broadcasting 172 kg P/ha + banding 44 kg P/ha per crop, all as OSP. Combining a broadcast PR application of 88 kg P/ha + 44 kg P/ha as banded OSP per crop resulted in 81% of the maximum yield at 31% of the cost of applying 352 kg P/ha as OSP. Increasing the broadcast PR rate to 352 kg P/ha while maintaining the same banded OSP rate produced 89% of the maximum yield at 50% of the fertilizer cost. Other OSP combinations (44 broadcast + 44 annual band; 88 broadcast + 22 annual band; 88 broadcast + 44 annual band; and 178 broadcast without annual bands) also produced over 80% of maximum yield at 38 to 50% of the fertilizer cost.

Broadcast P applications were found necessary to obtain the highest soybean yields observed in this experiment. Phosphate rock alone was unable to supply sufficient P; it must be supplemented by annual banded OSP applications. About 4 kg P/ha as PR were required to obtain similar increases in available P as 1 kg P/ha as OSP. Phosphorus availability in the broadcast OSP treatment decreased with time, while that of broadcast PR remained stable. Response to bandapplied OSP, in combination with either broadcast source, declined with increasing levels of Bray 1 available P. This soil test helps estimate further needs of banded OSP applications. The Bray 1, Olsen, and resin extract ants produced the same critical ranges with either source, suggesting that the two rapid extractants could be used in Oxisol areas where PR use is being introduced. Critical ranges of available P for soybeans in this soil were 11 to 15 ppm Bray 1 P, 6 to 8 ppm Olsen P, and 8 to 12 ppm resin-extractable P.

Key Words: Phosphorus sources • Phosphorus placement • Soil tests • Critical levels • Glycine max • Tropical savannas

² Visiting assistant professor and professor, Dep. of Soil Science, North Carolina State Univ., Raleigh, NC, respectively.

Received for publication September 25, 1981.