The material for the study was the biomass of four perennial species of energy plants (sugar miscanthus, giant miscanthus, Virginia mallow, coppiced willow) from an experiment in which the growth, development and yielding of the species were compared under the agroecological conditions of the Experimental Farm of the University of Agriculture, Lublin, at Felin. With relation to the Virginia mallow samples for the tests were also taken from an experiment involving two levels of nitrogen (100 and 200 kg ha-1 N) and phosphorus fertilization (90 and 120 kg ha-1 P2O5), as well as potassium fertilization in the form of KCl or K2SO4 (100 kg ha-1 K2O). At the Regional Chemistry-Agriculture Station in Warsaw the plant material samples were used to determine the content of crude ash, N, P, K, Mg, Ca, Na, S and Cl. Among the species tested, sugar miscanthus had the highest content of crude ash, while giant miscanthus was characterized by a low content of Cl, Ca and K. Biomass of Virginia mallow was notable for its highest content of K, Ca and Cl, and the lowest content of N and P. Compared to the other species, the two willow clones tested (1047, 1054) were characterized by the lowest content of crude ash and the highest content of nitrogen and phosphorus. The levels of N and P fertilization did not have any signifi-cant effect on the content of the determined elements, while the application of potassium sulphate, in comparison to potassium chloride, caused a decrease in the content of N, Mg, Ca and Cl in the bio-mass of Virginia mallow.