Biomass and Macronutrient Accumulation and Losses in Switchgrass During and After the Growing Season

Current switchgrass fertilizer recommendations in Arkansas are based on native warm-season grasses used as forages, normally harvested as hay in early to mid-summer when NPK removal rates are typically much greater than when harvested in the fall after the crop senesces and dries down. Fall harvest of a grass crop for bioenergy returns much of the macronutrients to the soil or to the roots and crowns for recycling back into subsequent year regrowth. This phenomenon probably would result in P and K recommendations that are lower than when the same grasses are utilized and harvested as forages.

2014 was the third year of switchgrass N, P and K fertility studies conducted in eastern Arkansas. In 2014, mean moisture content of biomass at harvest was 52.5% across all trials. For the fertilizer trials at Pine Tree, Arkansas, there were no significant differences in biomass yield due to P and K rates. The P trial treatment yields ranged from 12 to 15 t/ha, but yield differences were not significantly different among the P fertilizer rates (0 to 120 kg/ha). The K trial treatment yields ranged from 13 to 14 t/ha among the K fertilizer rates (0 to 240 kg/ha), but yield differences were not significantly different. The N trial treatment yields ranged from 6.7 to 19 t/ha, which was a statistically significant treatment effect. Nitrogen application rates ranged from 0 to 150 kg/ha. All N application rates resulted in significantly greater yield than the control. Furthermore, the 100 and 150 kg N/ha application rates resulted in significantly greater yield than the 50 kg/ha rate. Switchgrass responded to application of N in 2014 as it did in 2013, whereas in 2012 it did not. In none of the study years did P or K rate have any impact on yield.