An overuse of mineral fertilizers leads to the degradation of soils and decreases their humus content; thus, alternative sources need to be considered. The goal of this paper is to create an organomineral fertilizer based on co-composting of fish-farm sapropel and cattle manure and analyze the humification process, physicochemical properties, and biological activities of the created compost. Sapropel collected from Syrdarya region was mixed with cattle manure using different mass proportions (90:10-10:90) and was kept aerobically for 120 days, with moisture content being controlled (30-50%). It was established that the duration of the composting process and the ratio of the substrate have a significant impact on the humification of the compost and its mineral composition. The ideal ratio gave the highest level of maturity (humification percentage is equal to 44.12%, gaseous losses are only 1.72%) of the compost. Based on the data of the microbiological study, it can be concluded about the presence of active microbial flora. In the course of bioassay tests, extracts of compost significantly positively influenced the germination of cotton seeds and their development. It can be said that co-composting of sapropel with cattle manure is a promising and effective way of obtaining organic fertilizer.