Saturday, 15 July 2006

Phosphorus Indices: Southern Similarities and Differences.

Sam E. Feagley, Texas A&M Univ, Soil and Crop Sciences, 2474 TAMU, College Station, TX 77843-2474, Deanna Osmond, North Carolina State Univ, Soil Science Dept, Raleigh, NC 27690, Miguel Cabrera, Univ of Georgia, 3111 Plant Science Building, Athens, GA 30602, Gene E. Hardee, USDA/NRCS, Charleston, SC 29401, Charles C. Mitchell, Auburn Univ, Dept Agronomy & Soils, 201 Funchess Hall, Auburn, AL 36849, R. Mylavarapu, Univ of Florida, PO Box 110510, Gainesville, FL 32611, Philip Moore, USDA-ARS, 1837 Overcrest St., Fayetteville, AR 72703-3030, James L. Oldam, Mississippi State Univ, Mississippi State, MS 39762, William Thom, Univ of Kentucky, Agronomy, Lexington, KY 40502, Forbes Walker, Univ of Tennessee, 1505 Dunbarton Dr., Maryville, TN 37803, Hailin Zhang, Oklahoma State Univ, Dept of Plant and Soil Sciences, Stillwater, OK 74078, and J. Stevens, New Mexico State Univ, 3005 North Prince Street, Clovis, NM 88101.

The US Department of Agriculture (USDA)-Natural Resources Conservation Service (NRCS) released an updated nutrient management policy in 1999. Previously, all Nutrient Management Plans (NMPs) were N based, leading to very high Soil Test P (STP) levels in fields where animal wastes were applied. This is due to the N:P ratio (1:0.4-2.0) in animal wastes not being balanced for plant uptake (approximately 4:1). The 1999 policy required animal waste NMPs to consider the potential risk of P loss from fields. Individual states were given three P strategies from which to choose: 1) set a STP limit over which P application cannot be made, 2) set environmental STP thresholds over which P applications cannot be made, or 3) develop an index to rank potential off-site losses of P. This presentation will be directed at the P Index. The original P Index has been modified slightly or greatly depending on the direction the individual states took. This paper will evaluate the P Index ratings of the Southern region US States including Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee and Texas. Due to dryer conditions in west Texas and New Mexico, these parts of the Southern region were not used in the comparisons. Each of the 12 states have differences in their P Indices with approximately 35 factors used across the region and no one state using all of the factors. Scenarios were developed for pasture, cropped uplands and drained cropped bottomland mineral and organic soils. Florida and North Carolina were the only two states that had organic soil comparisons. The variables that were changed were the ones most likely to affect the P Index ratings included STP (75 and 150 mg/kg Mehlich-3), manure application rates (2.2, 4.5, 9, 18 Mg/ha), presence or absence of riparian buffers and soil erosion rates (1.1, 2.2, 9, 18 Mg/ha) as affected by tillage. The variability of the states' P Index ratings ranged from Low to Very High for each scenario. The ratings for each scenario, potential reasons for the rating differences and how they affect nutrient management recommendations will be discussed.

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