Five potassium fertilization rates ranging from nil to 80 kg/ha/cut were compared over 2 to 3 years in field trials on timothy leys at nine sites between 61 and 65 °N. The grass was cut twice a year and the contents of nitrogen, potassium, calcium and magnesium in yields were determined. The soils were tested at the beginning and at the end of the trials. On four peat soils the yields over two years without potassium dressings were 34 to 66 % of the respective yields with adequate potassium fertilization. On humus soil the relative yield without potassium was 81 % and on fine sand soil 76 %. On two finesand tills rich inorganic matter the responce of timothy to potassium was 5 %. No significant yield response was obtained on silty clay. In accordance to the depletion of available soil reserves, the differences between the potassium rates increased with time. In average on the six most responsive soils the relative yields without potassium fertilization for the first four successive cuts were 88, 75, 58 and 45 %. For maximum yields, 60 to 80 kg/ ha potassium per cut was required on the organogenic soils and on the finesand, 20 kg/ha was enough on the other three mineral soils. The potassium contents of plants increased greatly, and the contents of nitrogen, calcium and magnesium decreased with increasing potassium fertilization rate. The magnesium content of grass rose to an unusually high level with severe potassium deficiencies. At the end of the trials the soils were quite exhausted of potassium, the subsurface layers being most exhausted. The critical plant potassium content varied from under 2 % to over 3 %. As the large variation was coupled with plant nitrogen, plant K/N ratio was a better indicator for potassium status of ley than plant K. Yield was likely to begin degreasing when the K/N ratio decreased under 1.