The availability of K, essential for plant growth, from syenite (a silicate rock in which potassium feldspar is the dominant mineral; >90wt%), and phlogopite mica has been demonstrated using carefully designed plant growth pot experiments in which the only added source of K was the mineral of interest, with no loss of nutrients through drainage. Using pure quartz sand as a soil, both growth (increase in diameter) of leek plants and K-content of the plant material showed a dose-dependent positive response to the application (114–43000mgK/pot) of milled syenite with increases in plant diameter of 0.5–0.7mm/week, increasing with application rate. Phlogopite mica (114–6000mgK/pot) supported the highest observed increase in diameter (approx. 1mm/week) and plant K-content, both similar to that observed for a positive control (KCl). These experiments demonstrate that plants can obtain K for growth from milled syenite, in which feldspar is the dominant K-bearing mineral, and confirm previous observations that micas can be an effective source of K. Display omitted •Plant growth in artificial soil has derived K from potassium feldspar as well as mica.•Use of mica (phlogopite) as a source of K gives response similar to KCl control.•The observed growth response in both cases increases with increasing mineral dose.•K content in plant tissue increases with dose, and confirms a mineral source for K.•The results highlight the potential of feldspar, in syenite rock, as a source of K for tropical soils.