The aim of this paper is to maintain soil fertility on an organic farm without livestock production by using alfalfa green biomass. The research was carried out on the farm of Mokrin PP company, by modeling and sizing of crop rotation with alfalfa (Medicago sativa L.) on the non-carbonate humoglay. To ensure a cost-effective technical solution, alfalfa seed production was organized. In the autumn of 2015 alfalfa sowing was carried out in a field of 5 ha. Green biomass of the first and third cuttings, as well as crop residue after harvesting of seeds in the second cutting, were mowed and chopped by harvester for low silage and stored in the silage-pit. After nine months, a mature alfalfa compost was obtained with optimum values of total nitrogen (5.04%), organic matter (42.56%), C/N, pH, humidity, and EC. Two-year alfalfa utilization is the recommended time in this research because to the following benefits: in crop rotation, alfalfa field is provided with nitrogen by symbiotic ni?trogen fixation and the alfalfa is cultivated every five years in the same field, while in the middle of that period the field is fertilized with compost produced on the farm. The amount of compost obtained by crop rotation (2016 - 48.80 t; 2017 - 62.30 t) is enough for about 20% of the arable area per year. Thus, the fields are fertilized every fourth year with 10 t ha-1 of compost. Thanks to alfalfa biomass and seed and also nitrogen fixation, maintaining soil fertility is resolved in a sustainable and natural way. [Projects of the Serbian Ministry of Education, Science and Technological Development, Grant no. III 46005 and Grant no. III 46006]

Soybean yield depends on the choice of cultivar, soil fertility, cultivation practices, and weather conditions in different years. Ploughing down crop residues increases the content of soil organic matter, and thereby positively affects soil fertility. The use of crop residues as an energy source has been promoted in recent years. It would be wrong to refer to this as a renewable energy source as the removal of crop residues from agricultural fields reduces and ultimately damages soil fertility, which in turn leads to reduced yield and a crop residue decrease in the future. Due to the reduced application of manure and organic fertilisers, it is necessary to return crop residues to the soil to preserve soil structure and prevent soil fertility decline. The effect of ploughing down crop residues of preceding crops on soybean yield has been the focus of studies for eleven years. Ploughing down maize crop residues resulted in the soybean yield increase by about 11.69%, i.e. the annual yield increase ranged from 2.89% to 15.94%.