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Effects of North- and South-facing Slopes on Yield of Kentucky Bluegrass (Poa pratensis L.) with Variable Rate and Time of Nitrogen Application¹

Plant responses to differences in slope orientation have been observed for many years, but attempts to identify and measure specific responses and microclimatic factors are limited. Field studies were conducted near Morgantown, W.Va, during the 1966 to 1969 growing seasons to determine the effect of slope orientation on the yield of Kentucky bluegrass (Poa pratensis L.) treated with O, 112, 224, 448, and 672 kg/ha rates of N applied singly and in up to four split applications. Slopes were approximately 35%. Soil on both slopes was Gilpin silt loam. The rate of growth, total yield, seasonal distribution of yield, and N recovery in plants were influenced greatly by slope orientation. Production on the north facing slope was more than twice that on the south-facing slope. Splitting the N applications tended to produce a higher yield and more uniform seasonal distribution. More than 11,000 kg/ha of bluegrass forage were produced on the north-facing slope in 1967 as compared to 4,000 kg/ha on the south-facing slope. A combination of high soil temperatures and low soil moisture levels appeared to limit growth on the south-facing slope. Maximum soil temperature on the north-facing slopes were from 8 to 10 C less than on the south-facing slope. Temperatures were measured 2.5 cm (1 in) below the soil surface under the bluegrass sod. Yield differences due to slope orientation completely overshadowed any yield responses from specific fertility treatments.

Key Words: Cotton • Nitrogen utilization

² Research Investigations Leader and Agronomist, SWC, ARS, USDA, Morgantown, W.Va., and Agronomist, Ohio State University, Columbus, Ohio (formerly professor at West Virginia University), respectively.

Received for publication January 10, 1972.