Resumo

PEREZ, M. J  e  SMYTH, T. J. Agronomic potential and agronomic effectiveness of three phosphate rocks with different mineralogical composition. Rev. Fac. Agron. [online]. 2005, vol.22, n.3, pp.214-228. ISSN 0378-7818.

The objectives of this study were: (a) to compare the agronomic potential (AP) of three phosphate rocks (PRs) of different mineralogical composition, measured in neutral ammonium citrate (NAC), 2% formic acid (FA), and 2% citric acid (CA) and (b) to estimate the relative agronomic effectiveness (RAE) of these PRs in a P-and Ca deficient Ultisol The experiment was conducted in a growth chamber at 30/26^oC day/night temperature, 12/12 hours light/dark period and 30% relative humidity. The experiment consisted of a factorial combination of two forage species (Brachiaria decumbens and Stylosanthes guianensis), two pH levels (original pH 4.9 and limed to pH 5.8 with MgCO₃), and five P treatments, arranged in a split plot design with three replications. Phosphorus treatments consisted of 50 mg.kg^-1 of soluble P in NAC from Monte Fresco (MFPR), Riecito (RPR) and North Carolina (NCPR) phosphate rocks, monocalcium phosphate [Ca(H₂PO₄) ₂] reactive grade (TSP) as a P soluble source, and a control without P. Shoot dry weight and P content in plant were determined, as well as pH, Olsen-P and exchangeable Ca in soil. Values of percentage soluble P in CA and FA placed the NCPR, RPR and MFPR as PRs of high (7.1% in CA and 8.7 in FA), medium-high (5.0% in CA and 5.2% in FA) and very low AP (0.71% in CA and 1.0% in FA) respectively. While, the percentage of soluble P in NAC place the NCPR and RPR as PRs of high AP (3.8 and 3.7% respectively) and MFPR as low AP (0.68%). The RAE for NCPR and RPR were higher with the legume (Stylosanthes) than with the grass (Brachiaria). There was not significant difference in RAE between RPR (83.5% with Brachiaria and 95.2 % with Stylosanthes) and NCPR (79.2% with Brachiaria and 91.7% with Stylosanthes) in unlimed soil. In the limed soil, the RAE for RPR decreased in 29.2% (from 83.5 to 59.1%) with Brachiaria and (from 95.2 to 82.3%) with Stylosanthes, while NCPR's RAE was not significantly affected by liming. These results indicate the need of considering the PRs's AP to calculate P dose, as well as soil acidity and crop efficiency to use P from PRs.

Palavras-chave : Agronomic potential; agronomic efficiency; phosphate rock; forage species; available P.