Predicting nitrogen availability in irrigated potato systems

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 degreesF (2.50 degreesC) over normal temperatures] compared to a cool spring (-4.5 degreesF 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.