The aim of this study was to analyse the effect of different ways of mineral fertilisation (0, K, PK, NPK) on yielding, botanical composition and utility value of sward and biological values of twice mown meadows on peat-muck soil. The paper contains final results of 50-year-long cycle of studies in experiment established in 1957 in Experimental Station in Biebrza. Water conditions of soil underwent marked changes during the experiment, periods with deep soil drainage prevailed. In the years 1997-2001 capillary irrigation was applied and in the years 2001-2010 drainage was moderate. A lack of fertilisation in many years caused very strong meadow degradation leading to the disappearance of the turf. Hay yields were only about 1.0 t·ha-1. Fertilisation with potassium only slightly increased yields to 3.0 t·ha-1 at poor quality of the sward, facilitating the development of floristically rich communities with biologically valuable species characteristic for semi-natural wet, fresh and some bog meadows. Fertilisation with phosphorus and potassium (PK) increased the productive potential and good utility value of the sward. At deep soil drainage Bromus inermis Leyss was the dominant. Capillary irrigation increased the share of Poa pratensis L. s. str., Phalaris arundinacea L., Phleum pratense L. and Alopecurus pratensis L. NPK fertilisation did not significantly increase yields compared with PK fertilisation that enhanced mineralisation of organic matter.