Topsoil thickness impact on corn, soybean, and switchgrass productivity and soil nutrient management
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Topsoil thickness, or depth to claypan (DTC), is a major contributing factor to variability in corn (Zea mays L.) and soybean (Glycine max [L.]) yield, profit, and nutrient dynamics on claypan soils. Understanding how and when DTC affects these factors across the landscape is crucial for improving management on these challenging soils. Site-specific placement of perennial crops, such as switchgrass (Panicum virgatum L.), and accounting for local variation in nutrient requirements could improve overall productivity, and ultimately profitability in the region. Therefore, studies were conducted on a Missouri claypan soil to: i) compare the productivity of switchgrass grown for bioenergy with corn and soybean grown over varying DTC and to ii) determine if phosphorus (P) and potassium (K) fertility management could be improved by accounting for DTC. Research was conducted in Columbia, MO from 2009 to 2016 on plots constructed with DTC representative of those typically found on Midwest claypan landscapes. Yield and nutrient concentration were measured nearly every year for corn, soybean, and switchgrass grown on 16 plots (5.2 or 6.1 x 10 m) with DTC ranging from 0 to 94 cm. Surface (0-15 cm) soil samples for soil test P (STP) and K (STK) were collected in 2009, 2015, and 2016. Fertilizer was applied shortly after the sampling using the University of Missouri's buildup recommendation. Across all years, relative yield increased 27 and 15 percentage points as DTC increased from 0 to 15 cm for corn and soybean, respectively, while switchgrass relative yield was unaffected by DTC. Coefficient of variation in yield was less for switchgrass than corn or soybean across 2009 to 2015. The greatest net return was experienced by corn and soybean at deeper ix DTC. Switchgrass was able to compete only with the net return of corn on very shallow DTC (<5 cm). Soil test P and STK were inversely correlated to one another, where STP increased and STK decreased with DTC. The P buffering index decreased 0.94 kg ha-1 with each 1 cm increase in DTC, and the amount of fertilizer K required to raise STK 1 unit was four times greater for areas of deeper DTC than shallow DTC. These relationships show that switchgrass has the capacity to reduce yield variability caused by DTC but was unable to compete with corn and soybean at current market values. Furthermore, accounting for DTC could help guide variable-rate P and K applications to raise and maintain STP and STK.
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