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ences were not significantly different in either soil. Bray-1 test levels were in the low range in the pre-fertilization control soils and the PCSCU treatment in native soil, but were in the medium or high ranges following 3 years of application of natural organic fertilizers. In the Pacific Northwest, turfgrass shows little or no response to added P in soils that test in the medium or high range (> 20 mg P/kg soil). To determine if the potential risk of soluble P loss had increased, oxalate extractions of Al, Fe, and P were run to determine if the fertilizer applications had affected P saturation (PSI) for each treatment and soil type. The results of these calculations showed no significant difference between PSI values for any of the fertilizer treatments on native soil after 3 years of fertilizer applications. However, on sand, both Organic 6-7-0 treatments had significantly higher PSI values than the other fertilizer treatments. The change in Bray-1 P was much greater than the change in PSI, reflecting that the soils had exceeded the upper threshold for plant response to P, but had not yet reached a level of concern for soluble P loss. The PSI of the fertilizers alone was 16.6 for the Organic 8-3-5 compared with 3.8 for the Organic 6-7-0 biosolids product. The PSI of Organic 8-3-5 is similar to that of chicken manure (PSI = 15) as reported by Elliot et al., while the PSI for Organic 6-7-0 was higher than reported for a range of biosolids products (PSI = 0.47 to 1.4). The Organic 6-7-0 applications had a greater influence on Bray-1 P and soil PSI than the Organic 8-3-5, despite having a greater P binding capacity, because nearly four times as much P was applied in the Organic 6-7-0 than in Organic 8-3-5. Organic 6-7-0 applications added six to nine times as much P each year as the synthetic control, resulting in a large excess of applied P when products were applied to meet N needs. We also calculated the relationship between the change in Bray-1 P applied for both natural organic fertilizers in both soils to compare the effectiveness of the fertilizers in raising soil test P. The change in Bray-1 P averaged 0.057 mg/kg for every kg/ha fertilizer P applied in the native soil, with no significant differences between the 8-3-5 and 6-7-0 fertilizers. In the sand/peat root zone mix the P effectiveness averaged 0.105 mg/kg Bray-1 P for every kg/ha fertilizer P applied, also with no differences between fertilizer sources. This suggests that the organic fertilizers had similar effects on soil test P per unit P applied, despite differences in the PSI of the two materials. Soil appeared to have a greater influence on P effectiveness than fertilizer, with the sand mix having a greater P effectiveness (less buffering) than the native soil. This is consistent with conclusions reached by Sneller and Laboski in agricultural soils fertilized with different types of manure. Because each experiment had only one synthetic P treatment, we could not calculate the P effectiveness of the synthetic P fertilizer in our soils. The sand/peat experiment can be considered a worst case for soil response to P application, because the coarse-textured soil is poorly buffered and P application rates were higher than those used for home lawns. When organic fertilizer with high P concentration and high PSI was applied to the sand/peat plots, significant increases in both Bray-1 P and soil PSI were observed after 3 years. Although it would take longer, similar changes would occur in the native soil, eventually increasing the risk of leaching and runoff loss of P. These results show the importance of evaluating fertilizer sources for the amount and availability of P. The soil test results show that www.stma.org Bray-1 P was higher when using P-rich organic fertilizer, compared with synthetic fertilizer containing P, because of the greater P application rate from the organic fertilizer when applied at rates to meet N needs. The greatest increase in Bray-1 P occurred in the sand-based fairway treatment. Changes in soil PSI were smaller, indicating only small changes in P saturation and the risk of P loss from the soil over the 3-year duration of this study. Some organic fertilizers could have sufficiently low P concentrations and PSI values that they could be used for years without risk of increasing P loss from soil, but that did not appear to be the case for the fertilizers used in this study. Our results suggest that use of high-P organic fertilizers to meet turf N needs would not likely lead to increased risk of P loss in the short run, but repeated use in the long run could increase future P loss risk. This information can provide guidance for legislation regarding turf fertilizer sources, fertilization practices, and water quality. n *Gwen K. Stahnke, PhD, was corresponding author for this research. She is with the Puyallup Research & Extension Center for Washington State University. Other authors include: E. D. Miltner, former associate professor, and C. G. Cogger, professor, Department of Crop and Soil Sciences, Washington State; R. A. Luchterhand, research technologist III, Institute of Biotechnology, Washington State; and R. E. Bembenek, Department of Entomology, Washington State. The article first appeared in the online publication Applied Turfgrass Science in March 2013. SportsTurf 13