Crop and Soil Response from Phosphorus and Potassium Fertilizer Rate Verification for the State of Tennessee.

Soil testing laboratories follow several philosophies when making fertilizer recommendations including maintenance, building and sufficiency. Based on maintenance philosophy, fertilizer recommendations depend on the rate of crop removal without considering the soil test value. Heavy rates of fertilizer applications are recommended based on ‘building’ philosophy which result in high or very high soil test values. Sufficiency philosophy is based on ‘feeding the crop’ concept in which the fertilizer recommendations are based on filling the gap between soil test values and nutrient need for maximizing yield. University of Tennessee (UT) follows a combination of the building and sufficiency philosophy which has proven to be the most profitable approach. Accordingly, UT had stopped recommending phosphorus (P) and potassium (K) fertilizer applications in 2008 on soils testing high for P and K. We started a field study to verify the current soil test calibration and resulting fertilizer recommendations on soils testing low and high in P and K. For low testing soils, we expected yield response to the addition of low testing nutrient, and for high testing soils we expected no yield response to the addition of the high testing nutrient. Field studies with different rates of P and K fertilizer applications have been started since 2009 for corn-wheat-soybean rotation cropping system in both middle and west TN on soils tested low and high for P and K. Sites testing high in P and K have shown no yield response to date. In low P and K testing sites, yield responses become more frequent since 2013. On low testing soils, it was also observed that the rates of P and K fertilizers currently recommended are adequate to build out of low to medium range. Plant uptake results since 2009 will also be discussed in the poster.