  With the GREAT receiver at the Stratospheric Observatory for Infrared
Astronomy (SOFIA), nine massive molecular clumps have been observed in the
ammonia $3_{2+}- 2_{2-}$ line at 1.8~THz in a search for signatures of infall.
The sources were selected from the ATLASGAL submillimeter dust continuum survey
of our Galaxy. Clumps with high masses covering a range of evolutionary stages
based on their infrared properties were chosen. The ammonia line was detected
in all sources, leading to five new detections and one confirmation of a
previous detection of redshifted absorption in front of their strong THz
continuum as a probe of infall in the clumps. These detections include two
clumps embedded in infrared dark clouds. The measured velocity shifts of the
absorptions compared to optically thin \CSEO\ (3--2) emission are
0.3--2.8~km/s, corresponding to fractions of 3\%\ to 30\% of the free-fall
velocities of the clumps. The ammonia infall signature is compared with
complementary data of different transitions of HCN, HNC, CS, and HCO$^+$, which
are often used to probe infall via their blue-skewed line profiles. The best
agreement with the ammonia results is found for the HCO$^+$ (4--3) transitions,
but the latter is still strongly blended with emission from associated
outflows. This outflow signature is far less prominent in the THz ammonia
lines, which confirms it as a powerful probe of infall in molecular clumps.
Infall rates in the range from 0.3 to 16~$10^{-3}\,M_\odot/$yr were derived
with a tentative correlation with the virial parameters of the clumps. The new
observations show that infall on clump scales is ubiquitous through a wide
range of evolutionary stages, from $L/M$ covering about ten to several
hundreds.
