We conducted measurements of the upper atmosphere potassium layer and its seasonal variations during 110 nights between June 1996 and June 1997 at K¿hlungsborn, Germany (54 ¿N, 12 ¿E). The observations were performed by a ground-based metal resonance lidar. We measured the following average properties of the nightly mean K layer: column density of 4.4¿107 cm-2; peak density of 47 cm-3; layer centroid height of 90.5 km and RMS layer width of 4.0 km. The nightly mean column and peak densities exhibit dominantly semiannual variations with maxima in summer and winter. The centroid height varies semiannually too, attaining highest altitudes during the equinoxes. The RMS layer width shows a strong annual variation, though, with a maximum width in winter. For the first time the seasonal variations of the K layer can be compared in detail with the variations observed previously in the Na and Fe layers. For all three metal layers their centroid heights are dominated by annual variations with quite similar phases. The column densities are dominated by a semiannual variation in the case of the K layer but are dominated by an annual variation for the Fe and Na layers. The layer RMS widths are dominated by annual variations in the case of the K and Fe layers. That is quite different to semiannual variations in the case of the Na layer. ¿ 1998 American Geophysical Union