The increased application of inorganic fertilizers nowadays has led to ecological problems such as the eutrophication of water, greenhouse gas emissions, and the accumulation of toxic elements in the soil. A promising alternative to reduce high nutrient inputs are formulated fertilizers with a slow release function, which optimally supply the plants with nutrients during the entire vegetation period. The aim of our research is to form biodegradable slow-release fertilizers for the model plant blueberry. We hypothesize that negatively charged polymers slow down the release rate of the positive cations (e.g. NH₄⁺, K⁺, Fe^2+/3+, etc.) through electrostatic interactions. The initial research was started with the formulation of a biodegradable hydrogel containing ammonium cations and carboxymethyl cellulose (CMC). The structural analysis and the release kinetics of the cations from the hydrogels will be presented. The influence of negatively charged polymers on the release behavior of cations and the ability of biodegradation of hydrogels will be assessed.