Salep phosphate modified biopolymers with different phosphate contents were prepared via reacting salep (a multi-component polysaccharide obtained from dried tubers of certain natural terrestrial orchids) with a solution of primary and secondary sodium phosphates under basic conditions in a semidry process. Structural characterization of salep phosphates were carried out by P-31 NMR, FT-IR spectra and TGA curves. Determination of the phosphate content in samples was done by a standard spectrophotometric method. Then, novel (salep phosphate)-based hydrogels were synthesized by graft copolymerization of acrylic acid (AA) monomer onto salep phosphate backbones. Effect of the phosphate contents on swelling rate, equilibrium water absorbency in various pHs and potential of the hydrogels in release of KNO3 fertilizer were investigated. The results indicated that phosphorylation of salep could greatly improve equilibrium water absorbency, swelling rate and fertilizer release behavior of related hydrogels. (C) 2012 Elsevier Ltd. All rights reserved.