Using the pressure dependence of the mean-square displacement, the compression effect on the melting temperature of matrix Al is studied by Lindeman's melting law. The melting curve obtained increases as a function of the pressure and is in good agreement with the observed tendency for Al. The contribution of the transverse-like acoustic modes to the mean-square displacement is important for Al and becomes more predominant under pressure. Then, considering the contributions from the band and local mode frequencies, the temperature- and pressure-dependent mean-square displacement is quantitatively calculated for Al-Si and Al-Ge solid solutions. The concentration dependence of the Debye temperature at high and low temperatures is estimated, and the pressure effect on the solidus curves is presented by applying Lindeman's melting law to the Al-Si and Al-Ge alloy systems.