Eficiência agronômica de fosfatos de rocha em solo com elevado teor de cálcio trocável

Abstract

The vast majority of studies done on direct application of rock phosphates in Brazil have been performed in soils with pH less than 5.5 and low exchangeable Ca content. Studies under soil conditions different from these are not only rare, but have not been carried out because these soils may be restrictive to dissolution of these sources, depending on the species grown and the geological nature of the phosphate applied. A completely randomized block split-plot experimental design was set up, with plots subdivided into strips, and four replications, in order to study the agronomic effectiveness of Bayóvar reactive rock phosphate and Itafós rock phosphate in a Cambissolo Háplico Ta eutrófico vertissólico (Inceptisol) with pH 6.0 and high Ca content. The two rock phosphates and the reference source, triple superphosphate (TSP), were broadcast at the rate of 200 kg of P2O5 ha-1 in the main plots and incorporated by disk plowing. A main plot without amendment of P was also included in the study. In the subplots, P was applied as maintenance fertilization at annual rates of 0, 60, and 120 kg of P2O5 ha-1 through TSP in the plant rows. Maize was grown in the rainy seasons of 2011 and 2012. Maize yield and P leaf content indicated less need for maintenance application of P in the Bayóvar plots. The Mehlich-1 and mixed ion exchange resin method removed more soil P from rock phosphate plots than from the plot that received TSP. When applied alone, without maintenance applications, agronomic efficiency was 72.08 and 82.31 % for Bayóvar, and 43.85 and 47.47 % for Itafós in 2011 and 2012, respectively. The most profitable treatments were TSP alone and at the annual P maintenance application rate of 60 kg ha-1 of P2O5; and Bayóvar with annual P application at rates of 60 and 120 kg ha-1 of P2O5.