Secondary nucleation of potassium dihydrogen phosphate (KDP) is studied from electrical conductometry and turbidimetry experiments. Different possible mechanisms are investigated: initial breeding is observed even in the absence of stirring; production of nuclei by collisions with the reactor walls (contact nucleation) is studied in a liquid medium in which KDP is not soluble so that effects other than hydrodynamical/mechanical phenomena are eliminated. Similarities between secondary and heterogenous nucleation could indicate the existence of a mechanism of surface nucleation in addition to the other two effects. Turbidimetry is particularly useful for the quantitative study of the dynamics of secondary nucleation: in this work, it is applied to the study of the inhibition of nuclei production by dissolved aluminium ions. An estimation of the size of the secondary nuclei is calculated from turbidity data.