Native warm-season grasses show limited response to phosphorus and potassium

Authors: BISANGWA, ERIC - University Of Tennessee; RICHWINE, JONATHAN - Arkansas State University; KEYSER, PATRICK - University Of Tennessee; Ashworth, Amanda; BUTLER, DAVID - University Of Tennessee; SHRESTHA, UTSALA - University Of Tennessee; WALKER, FORBES - University Of Tennessee

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2025
Publication Date: 3/7/2025
Citation: Bisangwa, E., Richwine, J.D., Keyser, P.D., Ashworth, A.J., Butler, D., Shrestha, U., Walker, F. 2025. Native warm-season grasses show limited response to phosphorus and potassium. Agronomy. 15(3), 669. Article 15030669. https://doi.org/10.3390/agronomy15030669.
DOI: https://doi.org/10.3390/agronomy15030669

Interpretive Summary: Understanding the role of finite fertilizers such as phosphorus and potassium in native warm season grass production as part of an overall soil fertility program is critical for production, cost, and environmental sustainability. To address this data gap, researchers conducted an experiment over a seven-year period (2013 – 2019) at two locations in Tennessee to evaluate the yield response of big bluestem and switchgrass across a range of P and K amendment levels. Our objectives were to evaluate response to phosphorus and potassium rates, specifically, dry matter yield, forage nutritive values (i.e., crude protein and, total digestible nutrients), nutrient removal and remaining soil nutrient levels, and mycorrhizal colonization rates, as they are known to aid in the retrieval of soil phosphorus and potassium. Researchers found that neither nutrient affected yield for these two grasses, a finding in agreement with previous studies. With respect to both yield and forage nutritive value, there was no consistent trend indicating any cumulative effects from long-term application of these nutrients, even at the lowest and highest rates. Soil test N and P remained stable, indicating that nutrient uptake from deeper profile depths or that other biological or chemical processes were offsetting the removals. Researchers concluded that switchgrass and big bluestem require limited to no P or K fertilization to maintain adequate yields even on low fertility sites like those in our study, although nutrient removal rates may become unsustainable in the long term with no nutrient additions through fertilization or recycling of removed nutrients. Therefore native grasses are sustainable, low input summer forage options in highly weathered soils of the Southeast.

Technical Abstract: Little work has been done to identify optimum fertilizer response to finite fertilizers like potassium (K) and phosphorus (P) and identify resulting mycorrhizal colonization on native grasses. To evaluate responses of two native warm-season grasses (NWSGs; big bluestem [BB; Andropogon gerardii Vitman] and switchgrass [SG; Panicum virgatum L.]), experiments were conducted in Knoxville and Springfield, Tennessee, from 2013-2019. We assessed BB and SG dry matter (DM) yield, crude protein (CP), and total digestible nutrients (TDN), nutrients removal, soil test P and K, and root colonization rates by mycorrhizal fungi in response to P and K rates. Fertilization with K and P had no effect (P > 0.05) on yield, CP, or TDN for either species. Annual fluctuations in yield, CP, and TDN (P < 0.001) were observed for both species, but none of these showed consistent temporal trends or interacted with fertilizer rates. Although removal of K exceeded inputs at the two lower rates control (no input) and 67 kg K ha'1 yr-1, (212 and 24 kg K ha'-1, respectively), there was not an associated reduction in soil test K values. Removal of P at those same rates were 231 and 4 kg P ha'1 (SG) and 238 and 66 kg P ha'1 (BB) and similarly were not associated with a decrease in soil test P. Phosphorus rate affected (P = 0.02) total mycorrhizal colonization, with an average 62% colonization across both species and 70% at the highest P rates. Given the lack of response for yield, FNV, or associated soil nutrient levels, NWSG appears to offer a sustainable option for forage production without mining soil of P and K long-term owing to positive mycorrhizal associations with NWSG roots.