The aim of this study was to evaluate the total content and water-soluble fractions of magnesium during the composting of plant materials with the addition of polyethylene film and maize starch. The object of the study was the biomass prepared from wheat straw, rape straw, fresh maize chips, waste from the cleaning of pea seeds and an addition of shredded film (8%) produced from maize starch and polyethylene. The total content of magnesium in the raw materials and composted biomass was determined after calcination in a furnace chamber at 450°C for 12 hours and mineralisation of the residue in a mixture of concentrated nitric and perchloric acid (3:2). Water-soluble forms of magnesium were separated after two hours of extraction of the material with water redistilled on a rotary mixer. Magnesium content in the solutions and extracts was determined by ICP-OES using a Perkin Elmer apparatus Optima 7300DV. On the basis of the study it was demonstrated that the rate of changes in the content of magnesium water-soluble fraction during composting was low. The content of water-soluble forms of magnesium decreased over the time of composting. The highest content of total forms of magnesium were determined in the control compost without any addition of film, while the smallest content of Mg was assayed in compost with the addition of a film produced from 65% polyethylene and 30% of maize starch.