Accelerated eutrophication of surface water is often controlled by phosphorus (P) inputs in soluble (SP) and particulate (PP) forms. While most SP is immediately available for algal uptake, PP can provide a variable but long-term source of P for aquatic plant growth. Research has shown P extracted by 0.1 M sodium hydroxide (NaOH) from lake sediments to be correlated with P uptake by the algae Selenastrum capricornutum. This study presents a modification of this procedure, where amounts of P extracted from 20 mL of unfiltered runoff by 180 mL of 0.11 M NaOH in 17 h were correlated to algal growth (r² of 0.76 to 0.93), to allow routine measurement of potentially bioavailable P (BAP) in runoff. Bioavailable P loss in runoff from 20 agricultural watersheds in the Southern Plains was measured over a 5-year period. Losses were reduced by practices minimizing erosion and runoff, with 1.25 and 0.70 kg ha^-1 yr^-1 losses from conventional and no-till practices, respectively. However, as vegetative soil cover increased, BAP comprised a larger portion of total P (TP) loss (88% for no till and 19% for conventional till). As P bioavailability is a dynamic function of physical and chemical processes controlling erosion, particle-size enrichment, P desorption-dissolution, and plant residue breakdown, routine BAP measurement is recommended to more reliably evaluate the impact of agricultural management on the biological productivity of surface waters. This will also involve development of P bioavailability indices relating bioavailable soil P (NaOH- extractable, BIOP) to management. To use existing soil P test methods (for example, Bray-I, Mehlich-III, and Olsen), relationships between BIOP and soil test P were investigated for 201 soils. Although a large range in BIOP was found (1 to 712 mgP kg^-1), it was not consistently related to any P form. Thus, BIOP measurement is recommended in the development of indices facilitating identification of potential agronomic and environmental risks associated with current and proposed agricultural management.