Ontario’s Nutrient Calculator: Overview and Focus on Sensitivity Analysis

The long-term build-up of nutrients in agricultural fields has contributed to surface and ground water quality problems in many jurisdictions. The Ontario government has required the use of a decision support system, entitled NMAN, to develop nutrient management strategies for new or expanding livestock facilities. The NMAN software adds nutrient (N and P) sources (commercial fertilizer, manure, crop residual, grazing) and subtracts nutrients lost through crop removal and runoff to calculate the assimilative capacity on a field-by-field basis. NMAN is designed to help guide nutrient management at the farm-scale and to prevent the long-term build-up of nutrients in agricultural soils. However, success ultimately rests with the farmers’ willingness to use NMAN on a continuing basis for determining nutrient application rates. While several factors are shown to influence the use agricultural decision support systems, perceived usefulness or value is key. In an effort to inform users about the NMAN software, we provide a brief description and sensitivity analysis of selected input variables. The newest version, NMAN3, is more intuitive than earlier versions, and it is easier to navigate and to modify parameters. Using a sample nutrient management strategy, we determine the sensitivity of output parameters to changes in the input variables. We primarily focus on the Phosphorus (P) Index, which measures a potential P loss from agricultural fields. The results indicate that the some parameters used in NMAN3, i.e. slope percentage, slope length, etc., need to be accurately measured, and that agency officials should be aware of these parameters during site visits and/or inspections.