L. P. Canellas, J. A. A. Espíndola, J. G. M. Guerra, M. G. Teixeira, A. C. X. Velloso, and V. M. Rumjanek, “Phosphorus analysis in soil under herbaceous perennial leguminous cover by nuclear magnetic spectroscopy”, PAB, vol. 39, no. 6, pp. 589–596, Jun. 2004.