Humic acid is an important soil component which can improve chemical, biological, and physical properties of soils and nutrient supplies. In this study, we investigated lability of phosphorus (P) in the mobile humic acid (MHA) and calcium humate (CaHA) fractions of four soils by orthophosphate-releasing enzymatic hydrolysis and ultraviolet (UV) irradiation. Less than 25% of P in these fractions was present in the soluble orthophosphate form. Enzymatic incubation increased the soluble orthophosphate to 60%. Treatment by UV irradiation released 6-20% of humic-bound P. However, treatment by both UV irradiation and enzymatic hydrolysis increased soluble orthophosphate only to a maximum of 70% of P in humic factions, indicating that about 30% of humic-bound P was resistant to both enzymatic and UV degradation. As enzymatic incubation released hydrolyzable organic P and UV irradiation abiotically released P mainly by breaking-down orthophosphate-humic substance complexes, this work represents a method for evaluating lability (i.e. availability for plant uptake) of humic substance-bound P. This information has potential utility for managing P availability toward efficient crop utilization of that P.