Abstract The kinetics of changes affecting sporangial inoculum of Phytophthora infestans in a suppressive soil amended with various doses of CaCO 3 was studied over a 30 day period, using the buried membrane method, in an attempt to clarify the influence of soil pH on suppressiveness. Germination significantly increased and fungal colonization significantly decreased with increasing soil pH in the range of 3.73–7.61. No significant effect of soil pH was observed on bacterial colonization and on overall lysis of inoculum. Although failing to reach statistical significance, a consistently higher infectivity of inoculum exposed in soils with highest pH was found. The data suggest that pH affects the two main mechanisms conditioning soil suppressiveness to P. infestans. This action is probably mainly indirect, through the release of toxic ions or the favouring of acidophilic, lytic microorganisms.