Using the Phosphorus Assessment Tool in the Field

The P index (PI) is a proposed tool for rating the potential for offsite losses of P from a field or other management unit. The eight site characteristics comprising the PI were evaluated on several best management practice (BMP) demonstration plots in the Dairy-McKay Hydrologic Unit Area (HUA) in western Oregon and on an entire subbasin in the Granger Drain HUA in eastern Washington. An estimate of site vulnerability is obtained by adding the weighted values of the site characteristics. Site vulnerability ranged from low to medium in the Dairy-McKay HUA. The medium values were due to P fertilizer application rates of > 66 lb P/acre and surface sidedress application. Site vulnerability ratings by soil type for fields in the Granger Drain HUA were concentrated in the medium and high categories. This was due to high values for site characteristics of irrigation erosion and surface runoff of irrigation water, high soil test P levels, and application of organic P from animal wastes. This study demonstrates that the PI can be used to rank soils as to their potential for P loss between sites. Validations studies are needed to determine if high vulnerability sites are significantly affecting surface waters. Research QuestionIncreased concern over the movement of P from the landscape into surface waters has brought about a need to assess the potential of a given landscape to affect P movement to surface waters. A field-level P index (PI) is needed to assess and to assist in the development of management practices to lessen the potential for P movement. The object of this study was to determine if the current rating system would provide a logical numerical separation of sites with differing characteristics. Literature SummaryAttempts have been made to develop models and procedures that would estimate the actual P loading from a watershed to a body of surface water. The PI reported here has been developed to assess a site's potential to harm surface water resources through P movement off the site. This PI does not attempt to estimate quantities of P moving from the site, but only the potential for movement. Study DescriptionThe proposed PI was used to determine the site vulnerability of production areas that were part of ongoing water quality research and education efforts. The PI was applied to a study area located in the Dairy-McKay Hydrologic Unit Area (HUA) in Tualatin River basin of western Oregon in an area characterized by moist, mild winters and dry summers with 39 to 47 in. annual precipitation. Two areas designated MR and SU were selected for the study. The PI was applied to plots established to investigate the use of red clover as a winter cover crop, and management practices needed to control runoff and erosion from strawberry fields. A second study area was in the Granger Drain HUA located in southern Yakima Valley of Washington. This irrigated agriculture area is characterized by hot dry summers and cold dry winters, with 7 to 9 in. annual precipitation. Producer fields were used in this study area. The PI was applied to farming units with slight modification to fit available data. Applied QuestionWas the phosphorus index able to differentiate among sites with differing vulnerability for transport of phosphorus from the landscape to surface waters? At the Dairy-McKay HUA, the total weighted rating values for the six site × treatment combinations ranged from 5 to 15 (Table 1). Based on these values, site vulnerability was found to be low to medium, with the largest difference between the two sites. At the Granger Drain HUA, the total weighted rating values ranged from 11 to 51, with the major portion of fields in the 15 to 32 range (Fig. 1). These total weighted ratings produced site vulnerabilities from medium to very high. The lack of low site vulnerability rating was due to the relative high rating totals for inherent soil characteristics. All of the sites in the very high vulnerability rating were associated with very high organic P additions and long term surface exposure of applied P. At the Dairy-McKay site, the PI was sensitive to management factors (soil test P, fertilizer and organic P application rates and methods of application, but less sensitive in differentiating sites based on erosion and runoff-related site characteristics. At the Granger Drain site, vulnerability could be attributed to both soil and management practices. The numerical differentiation of fields at the Granger Drain was adequate for determining which fields should have high priority consideration. Fig. 1Distribution of total weighted rating values for fields sampled in the Granger Drain Hydrologic Unit. RecommendationThis study demonstrates the usefulness of the PI in assessing sites for their potential to harm surface water quality. This assessment would be very useful in determining which sites should be first to receive management attention. Additional research is needed to document that high and very high vulnerability sites are providing significant amounts of P to the affected surface waters. Table 1Phosphorus index rating values and site vulnerability for site × crop/treatment in the Dairy-McKay HUA. SiteCrop/treatmentPIVulnerabilityMRclover/interseeded 5.0LowMRWinter wheat 6.0LowSUStrawberries/bare interrow15.0HighSUStrawberries/bare diked13.5MediumSUStrawberries/straw mulched13.5MediumSUStrawberries/straw mulch13.5Medium + winter cover