Coated triple superphosphate (TSP) fertilizer with slow-release and water retention properties was prepared using lignin extracted from olive pomace (OP) biomass and k-carrageenan biopolymer. The aim of this work was to develop coating formulations that serve as a barrier to phosphorus diffusion through TSP fertilizers. Different formulations of lignin and k-carrageenan with or without polyethylene glycol (PEG 2000) were prepared. The chemical composition and morphological, mechanical, and surface properties of different formulations were characterized and compared. The blending of lignin with carrageenan in the presence of PEG enhanced the tensile properties of the formulation. Indeed, the mechanical properties of different formulation composites are classified in the order of lignin-carrageenan-PEG > lignin-carrageenan > lignin. After 6 h in water, the lignin@TSP formulation released two times less phosphorus (P) than the uncoated TSP. Lignin-carrageenan@TSP and lignin-carrageenan-PEG@TSP released only 28.21% and 13.51%, respectively, of phosphorus after 6 h compared to 72% for uncoated TSP. Indeed, lignin-carrageenan-PEG@TSP absorbs more water than TSP. The addition of plasticizer did not significantly modify the mechanical properties of the formulation composites. In the future, these products will be tested to evaluate their impact on chemical and biological soil properties.