Potassium number densities were measured in 2001--2003 with a resonance lidar in Spitzbergen (78¿N). Typical number densities in the 85--100 km height range are 5--70 atoms/cm3 with a maximum around 92 km in summer. Comparison of the seasonal and height variation of K number densities with similar measurements at mid latitudes (54¿N) shows a remarkable reduction in the lower part of the layer during the summer months. This reduction strongly correlates with the appearance of ice particles detected as noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE) by the same lidar and by a VHF radar, respectively. In a total of 226 hours of simultaneous K and NLC observations, the upper edge of the NLC layer was always detected below the lower edge of the K layer (i.e., no overlap), even when both edges vary substantially with height and time. The ice particle effect on potassium seems to correlate with the ice particle size: 'large' particles (>10--20 nm, detectable as NLC) completely remove all available K atoms, whereas smaller particles (but still large enough to create PMSE) gradually reduce the number of K atoms. Our observations suggest that the loss of K atoms on ice is just in the right order of magnitude to compete with the major production and loss terms.