Evaluating Topsoil Depth Effects on Phosphorus and Potassium Nutrient Dynamics of Grain and Switchgrass Cropping Systems.

Understanding the effects of fertilizer addition and crop removal on long-term change in soil test phosphorus (STP) and soil test potassium (STK) is crucial for maximizing the use of grower inputs on claypan soils. Due to variable nutrient supply from subsoils and variable crop removal across fields, accurate P and K fertilizer management can be challenging.  Current Missouri fertilizer recommendations rely on a single buffering capacity for all soils throughout the state for P, and modified by only cation exchange capacity for K.  The objective of this research was to evaluate the effect of depth to claypan (DTC) and cropping system (annual grain v. perennial grass) on soil P and K buffering capacity.  Research was conducted at the University of Missouri’s South Farm Research Center in Columbia, MO from 2009 to 2015.  Data were compared using a nutrient buffering index that accounted for soil test change, removal, and fertilizer additions.  Each input into the buffering index calculation was analyzed independently.  Results showed that cropping system significantly affected nutrient removal, but not soil test change.  Depth-to-claypan affected P and K removal and the change in STP.  Phosphorus and K removal increased 0.25 and 1.12 kg ha-1, respectively, and change in STP decreased 2% with each 1 cm increase in DTC.  Therefore, adjusting Missouri’s current P and K fertilizer rate calculations to account for DTC should result in improved fertilizer recommendations.