Comparison of Land Grant University Soil Test Recommendations for Nitrogen, Phosphorus, and Potassium.
Mark McFarland, 348A Heep Center MS 2474, Texas A&M University - Soil & Crop Sciences, Texas A&M University, Department of Soil & Crop Sciences, College Station, TX 77843-2474, Deanna Osmond, NCSU-Soil Science Dept, PO Box 7619, Raleigh, NC 27695, Richard Koenig, Washington State Univ, PO Box 646420, Pullman, WA 99164-6420, and Daniel Devlin, Kansas State Univ, 2014 Throckmorton, Manhattan, KS 66506.
Variations in fertilizer recommendations across state boundaries for a given crop based on soil testing have the potential to affect cost share programs for agricultural producers. Land Grant University (LGU) nutrient management personnel, in cooperation with USDS-NRCS regional personnel, compared soil test recommendations for adjacent states in the Western (Washington, Idaho, Oregon), Central (Kansas, Nebraska, Oklahoma), and Eastern (North Carolina, Virginia, Tennessee) U.S. A common spreadsheet was used to compare N, P and K recommendations for expected yield goals of three major crops across soil test ranges from very low to very high. Recommendations for selected crops also were compared for scenario soil samples representative of shared watersheds. In general, fertilizer recommendations in adjoining states within a region were very similar across the range of soil test levels for the major crops and cropping systems evaluated. Variations in fertilizer N, P and K recommendations based on soil test and/or yield goal, soil type, organic matter content, or nutrient index (e.g. P-Index) typically ranged from 0 to 14%. Selected cases of more substantial percentage variation (33 - 150%) in N recommendations were observed, but typically were associated with the very high soil test range where lesser total amounts of fertilizer N are recommended. Variations in P fertilizer recommendations among states within a region generally were less than 10%. More significant variations in fertilizer K recommendations suggested that K is less aggressively managed than N or P, likely due to historical economics and limited environmental concern. Management practices such as method of application (band vs. broadcast) can significantly affect recommendations and apparent consistency between/among states. Results indicated that support for enhanced collaboration among LGU nutrient management programs could lead to significant and meaningful improvement in nutrient management recommendations available to agricultural producers, land managers, and state and federal water resource management agencies.