In forestry, composts constitute a special kind of fertilizers which are used, primarily, for the fertilization of light soils. Their role consists, first of all, in the improvement of properties of these soils. One of the by-products used for compost production is bark, especially, pine bark which is characterized, among others, by low nitrogen content. For that reason, irrespective of its origin, nitrogen supplementation is necessary. It can be added in the form of mineral, for example urea or ammonium nitrate or organic nitrogen (chicken faeces, farmyard manure, etc.). Bearing in mind economic realities, frequently, plants and their mixtures are often used as sources of nitrogen. This study presents results of investigations regarding composting of pine bark (4 m3) supplemented with plant green material (serradella, buckwheat, field pea and vetch). The mass of the added mixture plant material amounted to 2.0 and 3.5 Mgźheap-1, respectively. One of the experimental treatments was bark alone or bark supplemented with urea. Significant differences were found in values of the examined parameters and the impact of each factor on the level and dynamics of these changes varied. The dry matter content was determined, primarily, by time (F0.01=849.9) and as the content of organic matter (F0.01=713.20) as well as the content of crude nitrogen (F0.01=146.61) by composition of the heap. It was found that losses of total nitrogen (mainly during the final stages of composting) were the greatest in composts supplemented with plant material.