Org.-mineral fertilizers were prepd. by mixing the optionally composted mushroom substrate with urea, single superphosphate, a K salt, NH4NO3, (NH4)(2)SO4, and/or sewage sludge and used on a test microfield (N doses 100 or 200 kg/ha) for growing oat. The straw crop (dry matter) and Cu content in the oat grain and straw were detd. The highest oat straw crop yield was found on the mushroom substrate, NH4NO3, single superphosphate and K salt-fertilized field. The Cu content did not depend on the fertilizers used.