2008 Joint Annual Meeting (5-9 Oct. 2008): Bermudagrass Yield and Mehlich-3 Phosphorus Response to Phosphorus Fertilization.

The agronomic need for P fertilization of bermudagrass grown for forage has been overshadowed by environmental concerns. Our research objective was to evaluate the effect of P-fertilizer rate on bermudagrass yield, P uptake, and Mehlich-3 soil P across time. Triple superphosphorus was applied annually (2006 and 2007) at rates of 0, 22, 44, 66, 88, and 110 kg P ha^-1 to a Captina soil cropped to common bermudagrass. Forage was harvested three or four times annually. The initial Mehlich-3 extractable P (0-10 cm) was 116 mg P kg^-1. Individual harvest and season total yields always differed between years, but only third harvest yields were affected by P rate, averaged across years. Compared to the unfertilized control, application of >22 kg P ha^-1 yr^-1 increased third harvest yield by 13-22%. Harvested forage P removal increased linearly from 2.49 to 2.92 kg P Mg^-1 forage as P rate increased from 0 to 110 kg P ha^-1 yr^-1 and average P content was greater in 2006 than in 2007 (2.60 vs 2.84 kg P Mg^-1). The 2-yr net balance between P removal by harvested forage and P-fertilizer additions was negative for P rates <66 kg P ha^-1 yr^-1 and positive for greater P rates. After 2-years of fertilization and cropping, Mehlich-3 P decreased for P rates <44 kg P ha^-1 yr^-1, was unchanged for 44 and 66 kg P ha^-1 yr^-1, and increased for P rates >66 kg P ha^-1 yr^-1. Overall, the cummulative net balance showed that Mehlich-3 P changed by ±1 mg P kg^-1 per ±2 kg P ha^-1. Results indicate that managing forage for moderate to high yield on soil with slightly above optimum soil P (>50 mg P kg^-1) can slowly reduce Mehlich-3 P across time and late-season forage yields on such soils may respond positively, albeit nominally, to P fertilization.