OLD-WORLD BLUESTEM RESPONSE TO FIRE AND NITROGEN FERTILIZERS

Old World bluestem (Bothriochloa ischaemum L.) is being extensively established on marginal farmland in the Southern Plains. This 4-year field study in western Oklahoma developed guidelines for burning and N fertilization of Old World bluestem on calcareous and noncalcareous soils. Plots were on 'Iron Master' Old World bluestem on a calcareous soil (Quinlan loam, shallow Typic Ustochrepts) and a noncalcareous soil (Carey loam, Typic Argiustolls). On each soil 4 blocks were split (spring burned, unburned) and N treatments (none, urea, ammonium nitrate) and time of N application (early, late) were randomly assigned within each burn treatment. Burning decreased (P<0.01) herbage yields by 6 to 30% per year. Nitrogen fertilizer broadcast at the rate of 50 kg N ha-1 increased herbage production about threefold. Ammonium nitrate fertilization resulted in 20% more herbage production than urea fertilization 1 year, and in equal production 2 years. The 4th year, application of ammonium nitrate in early April increased production by 20% compared to early April application of urea, urea was as effective as ammonium nitrate when either was applied in late April. Burning or calcareous soil had no adverse influence on the effectiveness of urea as compared to ammonium nitrate. Management implications for western Oklahoma and adjacent areas include: burn Old World bluestem only when necessary to remove substantial amounts of standing dead herbage, and broadcast urea 3 to 4 weeks after grass initiates growth when seasonal rains are more likely to move the urea into the soil, thereby decreasing potential for N loss by volatilization.