Results concerning the chemical composition of composts, indicating their possible utilization as fertilizers, encouraged the authors of the paper to conduct vegetative experiments aimed at determining the effect of compost supplement to the soil on oat yielding. Nutrient content in the composts from Krakow was, except for phosphorus concentrations, important for their fertilizer value. Heavy metal concentrations in the tested composts were much lower than established by regulations of the Law on fertilizers and fertilization. Fertilizer effect of the tested composts on oat yield was extended in time and revealed itself in the second and third year of the experiment. On the basis of total oat yields over three years it may be said that compost fertilizer effect was comparable or better than that of farmyard manure but worse in comparison with mineral salts effect.