Range Fertilization: Plant Response and Water Use

During the 10-year study, herbage production on an unfertilized, mixed prairie range site in eastern Montana averaged 1,047 kg/ha and ranged from 720 to 1,321 kg/ha. Elimination of nitrogen (N) and phosphorus (P) deficiencies by fertilizing increased herbage yields an average of 114% (ranging from a low of 32% in a "dry" year to a high of 218% in a "wet" year). Nitrogen was the major growth-limiting plant nutrient with measurable responses to P occurring only when N was nonlimiting. Single high-rate applications were about equal to annual N applications when compared on an annual rate equivalent basis. Species composition varied as much among years as among fertilizer treatments. At N rates of 336 kg/ha or less, cool-season grasses increased in about the same proportion as did forbs and shrubs, maintaining a relatively constant composition of the major species groups. On unfertilized plots, herbage yields and water use reached maximum values of about 1,250 kg/ha and 265 mm, respectively, regardless of further increases in available water. Unfertilized plots produced an average of 2.60 kg/ha for each 1 mm of precipitation received as compared with 5.81 kg/ha on fertilized plots.