Native Warm-Season Grasses Show Limited Response to Phosphorus and Potassium
2025
Eric Bisangwa | Jonathan D. Richwine | Patrick D. Keyser | Amanda J. Ashworth | David M. Butler | Utsala Shrestha | Forbes R. Walker
Data are needed to identify optimum response to potassium (K) and phosphorus (P) amendment and associated mycorrhizal colonization for native warm-season grasses (NWSGs; big bluestem [BB; <i>Andropogon gerardii</i> Vitman] and switchgrass [SG; <i>Panicum virgatum</i> L.]). To evaluate these responses, experiments were conducted in Knoxville and Springfield, Tennessee, from 2013 to 2019. In twice-annual harvests, we assessed BB and SG dry matter (DM) yield, crude protein (CP), total digestible nutrients (TDNs), P and K removed by grasses (removal), and soil test P and K in response to P (29 to 88 kg ha<sup>−1</sup>) and K (70 to 257 kg ha<sup>−1</sup>) elemental rates, and rates of root colonization by mycorrhizal fungi in response to P. Amendments had no effect (<i>p</i> > 0.05) on DM yield, CP, or TDN for either species. Yield, CP, and TDN fluctuated among years (<i>p <</i> 0.001) for both species, but no consistent temporal trends were observed. Although removal exceeded inputs at the control (no input) for P and K, and at 70 kg K ha<sup>−1</sup>, there was not an associated reduction in soil test K and P values. Phosphorus rate affected (<i>p</i> = 0.02) total mycorrhizal colonization, with an average of 62% colonization across both species and 70% at the highest P rates. Given the lack of response for yield, CP, TDN, or associated soil nutrient test levels, NWSGs appear to offer a low-input option for forage production.
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