The use of fertilizers based on organic waste as a result of microbial biodegradation and biotransformation is becoming increasingly common practice in plant cultivation. This is to limit the chemicals used in agriculture and thus protect the environment and consumer health. The aim of this study was to evaluate a hitherto unexplored effect of hydrolysates obtained after biodegradation of chicken feather waste by a soil strain of the keratinolytic fungus Trichophyton ajelloi on germination, early growth, and development of plants, in particular with high sulfur requirements, and to verify, in model conditions, their impact on soil biological activity and the total pool of soil DNA. Undiluted and diluted keratin hydrolysate generally stimulated seed germination as well as root and shoot growth of the Lepidium sativum L. (garden cress) and Brassica napus L. var. napus (oilseed rape) on sandy soil (Cambisol I), loamy soil (Cambisol II), and Chernozem. In the model experiment, in the variants with and without oilseed rape on sandy soil and Chernozem, the introduction of keratin hydrolysate generally increased the total abundance of microorganisms. In sandy soil, feather hydrolysate mostly increased respiratory activity, dehydrogenase activity, and alkaline phosphatase activity by an average of approx. 38% and the total DNA pool by 15% and 23% in the variant with and without plants. The activity of protease and acid phosphatase increased by an average of 4% and 6% only in the variant without oilseed rape. Respiratory and enzymatic activity in Chernozem, in the variants with and without oilseed rape, tended to show a downward trend, despite periodically recorded increases. The total DNA pool has increased by 8% in the oilseed rape variant. Oilseed rape biomass was almost two-fold higher after adding feather hydrolysate to both soils, and its yield was higher by 98% in Chernozem than in sandy soil. The results have demonstrated that keratin hydrolysate can be used as a biofertilizer.