Phosphorus indices are field-scale assessment tools that have potential for identifying areas most likely to contribute P to water resources and for focusing management practices to control these losses. They function by evaluating factors known to affect the extent of P in runoff and using these results as the basis for fertilizer and manure management planning. Phosphorus indices are more comprehensive than soil test P alone for identifying field P runoff risks because they consider both the amount of P available to runoff (source factors) and the potential for runoff and erosion to occur (transport factors). They are currently developed at the state level, with some states following a multiplicative matrix approach that leads to categorical risk rankings and other states using a semi-quantitative modeling approach that estimates a field's annual runoff unit area P loads. Limited validation data show good relationships between measured site-specific, field-scale P runoff losses and P index values. The semi-quantitative modeling approach may provide some advantages, since it allows quantitative consideration of site-specific factors affecting P runoff losses. Substantial local research data bases on the effects of site and management factors on the risk of P losses in runoff are needed to construct reliable P indices.
Minnesota Water Research Digital Library
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