Fig. 3 (a) Orthophosphate concentration (Cp) and (b) pH in solution of Ca–kaolinite + rock-phosphate suspensions (1:40) as a function of time. Inoculation took place at t = 24 h. An asterisk (*) represents a significant difference (t test comparison, P < 0.05) between biotic (E37) and abiotic (no E37) treatments at given time and initial pH treatment