A method of and a product formed by entrapping a bioactive, growth-promoting additive in a starch matrix to form a starch-based, superabsorbent polymer product for use in agricultural applications involves (1) graft polymerizing a monomer and a starch to form a starch graft copolymer including a starch matrix; (2) isolating the starch graft copolymer; (3) forming particles of starch graft copolymer; and (4) adding a bioactive, growth-promoting additive such that at least some of the bioactive, growth-promoting additive is entrapped by the starch matrix. Following placement of the starch-based SAP including a bioactive, growth-promoting additive in proximity to a plant, root, seed, or seedling, growth of the plant, root, seed, or seedling is promoted because availability of beneficial nutrients is increased.