Changes in nutrient availability in wetlands have been observed during the recent years, mostly due to human pressure. A shift from N limitation to P or K limitation causes changes in plant species composition, nutrient use efficiency, plant growth, interspecific competition and plant species performance. Several studies have shown that stoichiometry indices such as N:P and N:K ratios in plant biomass can be a good indicator of nutrient limitation. However, the implications of an N:K ratio for wetland vegetation have hardly been investigated. In order to estimate a critical N:K ratio that can indicate the type of nutrient limitations, a greenhouse experiment has been established. The response of two grass species: Holcus lanatus and Molinia caerulea, to the range of N and K supply was analysed for two years. The effect of six combinations of N:K supply ratios (from 0.5 to 225), combined with two levels of fertility in a factorial design, on aerial biomass production, nutrient concentrations and nutrient resorption efficiency was tested. The aerial biomass increased with an increasing N:K supply ratio during both vegetation seasons at the low level of supply. Significant differences were observed not only between species but also between the N:K ratios during the two years. In the first year, the optimal N:K supply ratio was 4.5 for Holcus lanatus and 225 for Molinia careluea at the high fertility level. In 2010, the optimal N:K supply ratio was similar for both grasses. At the high fertility level, the shoot biomass was the highest at an N:K supply ratio of 13.5; at the low level, shoot productivity reached the peak at a 225 N:K supply ratio. Moreover, both plant species showed the same pattern of aerial biomass production to N:K supply ratios at both fertility levels, but differences in the N:K biomass ratios make it impossible to determine a critical N:K ratio. The N:K nutrient supply ratio was a better indicator of plant performance than the N:K biomass ratio of the analysed species. The tested graminoids did not show a similar response to N:K supply ratios at the high and low levels of supply, indicating that nitrogen was the most important factor limiting the plant growth during the two years, and that these plant species were less sensitive to K shortage than to N deficiency.