Water soldier is a plant growing all across lowlands in Poland, in eutrophic reservoirs of still and slowly flowing water. Because this macrophyte is used as a fertilizer or a component in fodder for cattle or pigs, the purpose of this paper was to determine the content of magnesium and calcium in water soldier depending on pollution of water with selected heavy metals. The study used water and plant samples collected from the natural environment and a hydroponic experiment (6 variants of water pollution with heavy metals and two terms of plants’ exposure). Concentrations of magnesium and calcium in the mineralized samples were measured in three cycles with the atomic absorption spectrometry method on a spectrometer Solaar S AA. Water soldiers (from the hydroponic experiment) contained 5.911 g Mg kg–1 d.w. and 16.32 g Ca kg–1 d.w. It was determined that both addition of a heavy metal and the exposure time had a statistically significant effect on concentrations of magnesium and calcium in water soldier, as well as on their concentration in water. Except the control and regardless the exposure time, most magnesium remained in those water soldier specimens that had been exposed to iron and cadmium. The smallest amounts of magnesium in a plant were observed when water had been enriched with ions of zinc and cooper. After 6 weeks of exposing the plants to the metals, the content of magnesium was 12% lower than after 3 weeks. On the other hand, the smallest amount of calcium in water soldier was recorded for specimens exposed to iron and cadmium, while the largest amount was found in those grown with an addition of zinc and in the control group. The drop in calcium content measured on the second term, compared to the first one, was 6.119 g kg–1 d.w. Pollution of water with heavy metals has a negative effect on development of water soldier and on accumulation of calcium and magnesium. The reduced content of calcium and magnesium in plants collected from waters polluted with heavy metals will affect the value of water soldier both as a fertilizer and a fodder additive.