Surface water quality is an important issue in a state where this resource provides a major economic engine. Phosphorus can be a major factor in lake eutrophication. Phosphorus (P) is the nutrient limiting algae growth in most fresh water systems. When P in runoff is allowed to enter surface waters the resultant algal bloom leads to depleted dissolved oxygen levels and the associated degradation in water quality. Phosphorus is also an important plant nutrient. Fertilizer, manure, and other organic P sources are land applied to support adequate plant growth. The challenge for people who make land use decisions is to balance economic and environmental risks.
The Minnesota Phosphorus Index can be used to determine the relative risk posed by farm fields on the landscape and how to reduce the risk. The P Index is a management tool for individual fields or landscapes that provides a relative (unitless) assessment of the risks to surface waters of P losses from erosion, rainfall runoff, and snowmelt runoff. It allows the user to evaluate management options that can reduce the risk. It is not intended to be used as a regulatory tool, nor to estimate changes in surface water quality measurements. The Minnesota P Index does not consider the sensitivity of the receiving waters nor the environmental costs of entry of P to surface waters. It also does not consider the cost of adoption of different practices to reduce P losses from specific fields.
Other states have developed similar tools. As with other states, Minnesota’s P Index addresses its unique climate, soils, landscapes, and land use practices to develop a risk assessment. For example, it assesses the risk associated with snowmelt P losses. This would have little value in Florida or Texas but in Minnesota snowmelt can be a major source of P entering surface waters.
Three pathways are considered in the Minnesota approach. The first is the transport of sediment-bound P associated with the eroded particles in rainfall runoff. The second is the transport of dissolved P by rainfall runoff. The third is the transport of dissolved P by snowmelt. Losses from these three pathways are added, giving a total P index (unitless) for the given site. This index value represents the relative long-term average risk of P losses for a given site and set of management practices
Evaluation of test scenarios revealed that there are multiple ways to lower the P Index. Best management options can be evaluated for a specific site using the Minnesota P Index. P losses can be lowered using some combination of reduced P application rates, improved methods of P application, or adoption of practices such as conservation tillage or buffer strips that reduce the risk of P transport across the landscape. Usually one pathway (erosion, rainfall runoff, or snowmelt runoff) is more important than other pathways, and management changes that address that pathway will be the most effective method for reducing the overall risk.