Phosphorus fraction that has been formed primarily by biological processes. Organic phosphorus is converted to orthophosphate only by oxidative destruction of the organic matter (APHA, 1989). Total organic P maybe quantified by subtracting dissolved hydrolyzable phosphorus and orthophosphate values from total phosphorus (EPA, 1979). Dissolved organic P maybe quantified by subtracting dissolved hydrolyzable P and OPO4 from TDP result.