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Growers lack practical decision aides that accurately predict nitrogen (N) credits for organic sources to adjust fertilizer rates. The simulation model, DSSAT, was used to predict N supply in relationship to N demand in irrigated potatoes (Solanum tuberosum). Tuber yield and soil inorganic N levels were substantially higher in the simulations than in field experiment observations, indicating the need for model improvement. DSSAT was successful at predicting relative mineralization rates and potato N uptake for different organic and inorganic N source combinations. Interestingly, both simulation and field experiment observations indicated that combining a high quality organic manure at 5000 lb/acre (5604.2 kg·ha-1), total applied N 250 lb/acre (280.2 kg·ha-1), and a fertilizer source of N 160 lb/acre (179.3 kg·ha-1) markedly increased yields and lowered leaching potential. Simulated tuber yield for the combined treatment was 660 cwt/acre (74.0 t·ha-1) with 48 lb/acre (53.8 kg·ha-1) inorganic-N in the profile at harvest, whereas the highest simulated N fertilizer response was to 235 lb/acre (263.4 kg.·ha-1), which produced 610 cwt/acre (68.4 t·ha-1) with 77 lb/acre (86.3 kg·ha-1) inorganic-N in the profile at harvest. The synchrony of N release and uptake for combined manure and fertilizer treatments may explain the efficient N uptake observed. Common soil types and weather scenarios in Michigan were simulated and indigenous soil N mineralization was predicted to be 6 lb/acre (6.7 kg·ha-1) inorganic-N in the topsoil at planting, similar to observed levels. The increasing aeration associated with a sandy versus a sandy loam soil only slightly increased the predicted rate of mineralization from organic inputs. Simulated soil inorganic N levels with different organic inputs was modestly increased in a warm spring [4.5 °F (2.50 °C) over normal temperatures] compared to a cool spring (-4.5 °F less than normal temperatures). For Michigan irrigated potato systems, DSSAT simulations indicate that the most important factor determining inorganic N supply will be the quality and quantity of organic inputs, not environmental conditions.