Using Topsoil Thickness to Improve Site‐Specific Phosphorus and Potassium Management on Claypan Soil

Abstract

Core Ideas Topsoil depth influences P and K dynamics on claypan soils. Most P and K dynamics were inversely affected by depth to claypan. Accounting for depth to claypan could improve P and K fertilizer management. Precise P and K fertilizer management on claypan soils can be difficult due to variable topsoil thickness, or depth to claypan (DTC), across landscapes, nutrient supply from subsoils, and crop removal. Therefore, a study was performed to determine if DTC could be used to improve P and K management for corn (Zea mays L.), soybean [Glycine max (L.)], and switchgrass (Panicum virgatum L.). Research was conducted on a claypan soil at the University of Missouri's (MU) South Farm Research Center in Columbia, MO, from 2009 to 2016. Corn, soybean, and switchgrass were grown each year on 16 plots with constructed DTC ranging from 0 to 94 cm. Surface soil samples for soil test phosphorus (STP) and potassium (STK) were collected in the early spring of 2009, 2015, and 2016. Fertilizer was applied shortly after soil sampling in 2009 and 2015 based on the MU buildup recommendation. Results in the spring of 2015 showed that STP increased 0.53, STK decreased 4.4, and the phosphorus buffering index (BIP) decreased 0.94 kg ha−1 with each 1 cm increase in DTC. Most importantly, across the 2015 growing season the amount of fertilizer K needed to raise soil test potassium by 1 kg ha−1 (REQK) was four times greater at DTC of 44 than 0 cm. These relationships show that accounting for DTC could improve current fertility guidelines by applying more, or more frequent P, and less K on shallow soils.