Since 2001/2002 there has been a formidable expansion of annual crops in Uruguay which has determined changes in landuse, with a strong expansion of soybeans (Glycine max) under no-tillage. Improperly managed crop production systems canbe an important nonpoint source of pollution, accelerating eutrophication of surface waters resulting from nitrogen and phosphorus(P) inputs. Nutrient losses depend on many factors such as climate, soil characteristics and management practices. Thus, theobjective of this work was to estimate the annual P loss on a cropping system and to determine the main factors contributingto such losses. Runoff plots were located at the Experimental Station of the National Agricultural Research Institute in Colonia(INIA "La Estanzuela"), Uruguay (34º25´S, 58º0´W) during the period 2013-2014. Plots were under a Vertic Argiudoll soil with18.5 mg/kg P-Bray I on the first 7,5 cm, a 3% slope and available water content of 92.7 mm on the first 56 cm of soil depth. Therotation established was soybean - fallow/cover crop, non-fertilized for over 5 years. The amount of runoff water was measuredfor 23 rainfall events and analyzed for soluble P. During that period the soybean yield was similar to the national average yieldand the annual P loss was 0.5 kg/ha. Available water, precipitation, maximum rainfall intensity and runoff explained most of thevariation in P runoff losses. Rainfall events of high magnitude and intensity on soils with high available water were identified asthe events that produced higher water and P runoff. On the other hand, the model that best fitted P losses was just explained byrainfall and runoff (P = - 9.99 + 0.95*rainfall +21.56*runoff; R2=0.73). Overall, our results indicate that these soybean-covercrop systems under no-tillage, on soils with proper nutrient status and non-fertilized for a long time seem to be economicallyand environmentally adequate management systems