The presence of humic acid may change the way phosphate is absorbed or stabilized by soil and how it influences the growth of plants. The binding of phosphate with the humic substance requires bridging between phosphate ions and humic acid by metal ions, such as aluminum or iron ions. The bridging reaction can take place in peat and allophane soil under acidic conditions, altering the effectiveness of the phosphate in soil. Whether ternary phosphate-metal-HA complexes are actually formed has not yet been verified. This study considers variation in fluorescence intensity (FI) under various coagulation conditions to assess the mechanism by which humic acids and phosphate react with aluminum ions. A bond between the functional group of the humic acid and the electron-releasing group of the phosphate will enhance the florescent intensity of humic acids. Consequently the removal efficiency of humic acid, measured by florescence, declines as the phosphate concentration increases at low pH. This observation suggests that at low coagulation pH, the positively charged aluminum species can be used to bridge between the phosphate ion and the humic acid molecules. (C) 2003 Elsevier Inc. All rights reserved.