In this chapter, a detailed preparation of fertilizer systems from agrofood by-products using ionic gelation is presented. Yerba mate powder is proposed as an example for matrix phase, associated with a biopolymer, calcium alginate, responsible for the formation of the gel capsules. Urea and monopotassium phosphate (NPK) are presented as plant nutritional supplements. This approach enables the encapsulation of different minerals of agronomic interest, employing a wide variety of matrices, providing specific fertilizer agents to the final system.