The research was conducted to determine the effect of composting process of mainly municipal waste sub-group, which in the waste catalogue is marked with code 20 02 (wastes from gardens and parks), with various supplements (starch, edible oil and urea), on the content of total and water soluble forms of selected macro (Mg, Ca, Na, K and P) and microelements (Zn, Cu, Mn and Fe), which may provide a basis for an assessment of the fertiliser value of the composted biomass. By assumption, the level and composition of the supplements, in view of “chemically pure” practices, was supposed to positively modify the composting process. The biomass for composting was prepared from the following organic waste materials (20 02 01); fresh cuts of deciduous  trees (10.18% share in the biomass); sown cuts of deciduous trees (33.69% share in the biomass); grass (23.12% share in the biomass), waste (02 03 03) from ersatz coffee production (21.94% share in the biomass) and plant waste (02 03 82) from tobacco industry (10.96% share in the biomass). The experiment comprised the following variants in two replications: K1 – “heating” composts without any supplements (control); K2 – “heating compost + starch (food product); K3 “heating compost” + edible oil; K4 – “heating compost” + urea (chemically pure). The supplement of starch, edible oil or urea constituted 5% in relation to fresh mass of the heating compost. Under conditions of discussed experiment, the composting process of biodegradable materials led to an increase in total forms of analysed macro and microelements. The assessed content of water soluble forms of magnesium, calcium and phosphorus forms decreased, whereas the content of potassium and sodium increased in the control compost with a supplement of starch and edible oil in comparison with the concentrations determined in “heating” compost. The influence of supplements applied for composting on the concentrations of water soluble forms of the analysed microelements was diversified and conditioned by the level of organic substance loss and by the chemical properties of the element. Introduction of a 5% urea admixture to the composted biomass caused an increase in water soluble forms of all investigated elements, except magnesium content.