Tan, C. S. and Zhang, T. Q. 2011. Surface runoff and sub-surface drainage phosphorus losses under regular free drainage and controlled drainage with sub-irrigation systems in Southern Ontario. Can. J. Soil Sci. 91: 349-359. Soil phosphorus (P) loss and its partition in various pathways may differ depending on water management practices. A study was conducted using large field plots equipped with automatic flow volume measurement and sampling systems over a 5-yr period to determine the effectiveness of regular free drainage (RFD) and controlled drainage with sub-irrigation (CDS) for mitigating soil P losses of various forms [dissolved reactive P (DRP), dissolved un-reactive P (DURP), and particulate P (PP)] and to identify the relative roles of surface runoff and sub-surface tile drainage in soil P loss. For RFD, flow weighted-means (FWM) of DRP, DURP, PP and the total P (TP) concentrations over the 5-yr period were averaged at 0.057, 0.057, 0.627, and 0.741mg P L^-1 in surface runoff water and at 0.034, 0.053, 0.393, and 0.480 mg P L^-1 in tile drainage water, respectively. CDS increased FWM of DRP, DURP and the TP concentrations in surface runoff water and DRP concentration in tile drainage, but decreased the FWM of DURP, PP and the TP concentrations in tile drainage water. The CDS produced similar annual total dissolved P (TDP) loss, the sum of DRP and DURP, but reduced losses of PP by 15% and of TP by 12%, relative to RFD. The PP loss accounted for over 80% of TP loss for both CDS and RFD. Of the total soil P loss, from 3 to 5% was accounted for in surface runoff water, while from 95 to 97% was accounted for in tile drainage water, for RFD. For CDS, from 29 to 35% of the total soil P loss was in surface runoff water, while 65 to 71% was in tile drainage water. Subsurface tile drainage played a predominant role in soil P loss. CDS can be considered a beneficial management practice to reduce soil P loss under the similar climate and relatively flat field conditions in Southern Ontario.