Resumo

PEREZ, M. J.  e  SMYTH, T. Jot. Comparación del efecto de dos especies forrajeras sobre el pH de la rizosfera y la disolución de rocas fosfóricas de diferente reactividad. Rev. Fac. Agron. [online]. 2005, vol.22, n.2, pp.143-156. ISSN 0378-7818.

Abstract Low soil pH and low concentrations of Ca and P in solution are the main soil factors controlling dissolution of phosphate rocks (PRs). Plant roots may affect PR solubility by extrusion of H⁺ or OH^- in the rhizosphere. The objective of this study was to compare the effects of a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species on rhizosphere acidification and solubilization of three phosphate rocks (PRs) of different reactivities 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. It consisted on a factorial combination of two forage species, two soil 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 on 50 mg P kg^-1 of soluble P in neutral ammonium citrate, from Monte Fresco PR (MFPR), Riecito PR (RPR), North Carolina PR (NCPR), monocalcium phosphate reactive grade [Ca (H₂PO₄) ₂] as a P soluble source (TSP), and a control without P. Pots were designed to isolate rhizosphere and non-rhizosphere soil. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. Rhizosphere acidification and larger PR dissolution by Stylosanthes was associated with a larger root surface area, greater Ca uptake, and dependence on N₂ fixation by the legume, which may result in an excess of cathions in plant and induce H⁺ extrusion from roots into the rhizosphere. Solubilization of RRs was influenced by the interaction of forage species and mineral composition of the PR. The MFPR has less apatite and higher content of CaCO₃ than RPR and NCPR, which corresponded to the high soil pH values (pH >7.0) and exchangeable Ca, and low dry matter production with MFPR treatments. Acidification of the rhizosphere by Stylosanthes is not sufficient to promote dissolution of the MFPR. Riecito PR performs better than NCPR as soil acidity increases.

Palavras-chave : rhizosphere; phosphate rocks; forage species available P..