  We point out that the universality of free-fall can be tested by observing
surface-gravity changes of the Earth. The Earth's inner core is weakly coupled
to the rest part of the Earth by mainly gravitational forces. If there were a
violation of the universality of free-fall, because of their different chemical
compositions and/or of different mass fractions of binding energies, the inner
core and the rest part of the Earth would fall at different rates towards the
Sun and other sources of gravitational fields. The differential acceleration
could be observed as surface-gravity effects. By assuming a simple Earth model,
we discuss the expected surface-gravity effects of violations of the
universality and experiments to search for such effects by using
superconducting gravimeters. It is shown that the universality can be tested to
a level of 10$^{-9}$ using currently operating superconducting gravimeters.
Some improvements can be expected from combinations of global measurements and
applications of advanced data analyses.
