Phosphorus and Potassium Fertilizer Rate Verification Under Corn-Wheat-Soybean System for the State of Tennessee.

Fertilizer recommendations based on soil testing and targeted yields can be crucial in minimizing fertilizer use without compromising grower’s benefits. In Tennessee, crop yield potential largely depend on soil phosphorus (P) and potassium (K) levels. University of Tennessee (UT) stopped recommending P and K on high test soils in 2008 because of no profitable returns and high soil buildup. Additionally, commercial laboratories fertilizer recommendations are considerably higher than that of UT despite almost equivalent soil testing results. In order to address the discrepancy between UT and commercial labs, field trials were initiated in 2009 to verify P and K fertilizer recommendations for corn-soybean-wheat rotation system. Two field trials were established at the Research and Education Center in Millan, TN and Highland-Rim Research and Education Center in Springfield, TN. Study was conducted using five fertilizer rates ranging from 0-240 pounds per acre (lbs/acre) P₂O₅ and 0-200 lbs/acre K₂O with five replications in Latin Square Design. Specific objectives were (i) to verify P and K fertilizer requirements of Tennessee soils for maximum row crop productivity and, (ii) to determine whether the current P and K fertilizer recommendation for low testing soils is sufficient to build soil test levels out of the low range. No yield responses were obtained from high P and K testing sites but responses were more frequent since 2012 at low testing sites. Data suggests that over-fertilization on even low P and K testing sites are not profitable. Initial increase in yields observed with increase in amount of fertilizers but slight decrease or no increase with higher treatments. Application of 120 lbs/acre P₂O₅ was brought low soil P into high P level whereas 150 lbs/acre K₂O brought low soil K levels to medium K ranges. Optimization needed to maximize economic yield and maintain soil P and K levels.