The effects of aluminium ions on the nucleation and crystal growth kinetics of potassium dihydrogen phosphate (KDP) are reported: primary nucleation is reduced by the presence of aluminium; crystal growth is reduced and sometimes completely inhibited; crystal habit is also strongly affected. ICPS and SIMS measurements of the aluminium content in the crystal are performed. Growth kinetics are determined: in the absence of impurities, a second order law is found. An new model of competitive integration of impurities and normal growth units into the KDP lattice leads to a qualitative and quantitative interpretation of the impurity effect.