Waste biomass consisting of rape and wheat straws and a waste from pea seed shelling was modified by addn. of films made of polyethylene-starch blends (content of a compatibilizer 5-7.5%) and composed in wet air at 3040 C for 180 days. The compost was studied for humic acids (extn. with NaOH soln.) and for total N. The addn. of the polymer film resulted in an increase in N content in humic acids from 55.1% up to 58.8%.