Manure Phosphorus and Surface Water Protection IV: Assessment of the Risk of Agricultural P Delivery

Tools for the assessment of the risk of P loss from a field generally consider source (site and management) and transport factors that contribute to P loss (Table 1).  Assessment tools are often in the form of P indexes, some of which are computerized for quicker calculation. The P indexes are used for planning and teaching.  The indexes can be used to understand the causes of risk and in assessing the effects of alternative management practices on the risk of P loss in runoff.  Increasingly, P indexes are used in the regulation of land application of P. Risk assessment can be done on a whole field basis or by management zones within fields.  A major part of P entering surface waters is lost from a relatively small part of the land area of the field or watershed, implying a need for management by zones within fields.

 Table 1

Site and management factors	Transport factors
Soil P levels	Runoff
P application practices, including time, rate and method of application	Erosion from rainfall, snowmelt and irrigation events
Field management practices, such as tillage practices and use of cover crops	Surface and subsurface drainage
	Percolation and underground movement of P to seepage areas
	Distance from P source to concentrated water flow or a water body
	Atmospheric deposition

Phosphorus indexes have been greatly improved since the concept for risk assessment of P loss form agricultural land was introduced by Lemunyon and Gilbert (1993). The early P indexes estimated the risk of P loss as the total of scores given to the various site/management and transport factors--that is, the effects of various source and transport factors were considered additive. As understanding of P loss improved, the importance of the interaction of site/management factors with transport factors was realized, and multiplicative versions replaced the earlier additive version.  With multiplicative versions, risk is assessed as the product of the sum of source (site/management) factors with the sum of the transport factors.  Some P indexes attempt to estimate actual P loss.

In a recent comparison of five P indexes across a range of scenarios, it was found that there is much discrepancy in the importance given to different factors in the assessment of the potential for P loss.  Some P indexes need to be updated with more careful consideration given to research results published in recent years. Some P indexes give inadequate weight to erosion, while others appear to give too much weight to P application and soil test P.  In one study conducted in Nebraska, the variation in erosion and runoff volume, respectively, accounted for 78% and 10% of the variation in total P loss.  In contrast, soil test P, applied P, source P, and tillage accounted for very little and nonsignificant variation in total P loss.  The factors accounted for less of the variation in dissolved P loss; runoff and tillage method accounted for 12% and 20% of the variation in dissolved P lost, while the relationships of dissolved P loss with other factors were not significant.

For further study of the above P index concepts, two P indexes are briefly introduced here. The Nebraska 1998 P Index is an additive index representative of earlier P indexes.  The Iowa P Index (Iowa NRCS, 2004) uses a multiplicative approach to combine source and transport factors within three components based on three major P transport mechanisms:  erosion, runoff and subsurface drainage.