  Among the four fundamental forces, only gravity does not couple to particle
spins according to the general theory of relativity. We test this principle by
searching for an anomalous scalar coupling between the neutron spin and the
Earth gravity on the ground. We develop an atomic gas comagnetometer to measure
the ratio of nuclear spin-precession frequencies between $^{129}$Xe and
$^{131}$Xe, and search for a change of this ratio to the precision of 10$^{-9}$
as the sensor is flipped in the Earth gravitational field. The null results of
this search set an upper limit on the coupling energy between the neutron spin
and the gravity on the ground at 5.3$\times$10$^{-22}$~eV (95\% confidence
level), resulting in a 17-fold improvement over the previous limit. The results
can also be used to constrain several other anomalous interactions. In
particular, the limit on the coupling strength of axion-mediated
monopole-dipole interactions at the range of the Earth radius is improved by a
factor of 17.
