Studies were carried out in the years 2006–2008 in a plot experiment at the Experimental Farm Biebrza on permanent productive meadow grown on peat-muck soil. The aim of the studies was to analyse the effect of different types and rates of fertilization (including cattle manure and liquid manure) on yielding, magnesium content in the sward, soil and ground water and on magnesium balance. Due to proceeding soil mineralization and nitrogen release, mineral P and K fertilization (30 and 60 kg·ha–1, respectively) was applied. Nitrogen fertilizer was applied at two levels: 60 and 90 kg·ha–1, the smaller dose was accompanied by P and K fertilization as above and the larger dose was applied together with 45 kg P·ha–1 and 90 kg K·ha–1. As in the mineral fertilization also fertilization by manure and liquid manure was used on two levels of manure in amounts respective to mineral fertilization. Obtained results indicated a substantial yield-forming potential of applied fertilizers ranging from 6 to 9 t DM·ha–1. About 50 kg·ha-1 Mg was brought in annually with manure, causing more positive balances. And about 15 kg·ha-1of Mg was brought in annually with liquid manure, causing improvement of balance in comparison with mineral fertilization. Fertilization with natural fertilizers increased the abundance in Mg of the upper layers of soil without increasing its concentration in ground water. Mg concentrations in water were determined by soil abundance.