Department of Plant Sciences

Switchgrass (Panicum virgatum L.) has been targeted for cellulosic ethanol production. Our objective was to evaluate effects of P and K fertilization on switchgrass biomass yields. Experiments were established in switchgrass (cv. Alamo) fields planted at three Oklahoma locations in 2007. Interactions of N and P fertilizer rates were evaluated on two sites that had low soil test P (6 to 9 kg/ha). Interactions of N and K fertilizer rates and harvest system were evaluated on a site that had low soil test K (134 kg/ha). In the N and P rate experiments, biomass yields during establishment were unaffected by treatments, averaging 2145 kg/ha in 2007. During 2008, main effects of location, N, and P rate were significant. Application of 134 kg N/ha increased yields from 7945 to 9967 kg/ha. Applying 101 kg P2O5/ha increased yields from 8528 to 10242 kg/ha. In the harvest system, N, and K rate experiment, biomass yields were unaffected by treatments in 2007 and 2008, averaging 4603 and 16145 kg/ha, respectively. Cutting biomass twice per year as opposed to once a year, however, removed more N,P, and K. Uptake of N, P, and K averaged 216, 24, 209 kg/ha within the two-cut system and 133, 13, and 36 kg/ha, within the one-cut system, respectively. Although low soil test P and K did not strongly limit biomass yields of these young stands, continued biomass harvesting and removal without nutrient replacement has the potential to mine soil of nutrients and constrain biomass yields as stands age.