Fig. 6. Solution ³¹P nuclear magnetic resonance (NMR) spectra of the mobile humic acids (MHA) and calcium humates (CaHA) from the maize field and the control (no fertilizer) treatment at the International Rice Research Institute (IRRI) Long-Term Fertility Experiment (LTFE) site. A line broadening of 10 Hz was applied. No peaks were found outside the depicted regions.