Nitrogen Fertilization Influences Shear Strength and Quality of Kentucky Bluegrass Sod Grown on Clay

Shipment costs and easiness of handling sometimes make it necessary to produce sod on non-ideal soils, such as clay. The objective of this study was to test the effects of N rate and N-application timing, seeding rate, and mowing height on sod shear strength of Kentucky bluegrass (Poa pratensis L.) produced on a clayey soil under the climate conditions of the upper Midwest in the United States. A blend of four Kentucky bluegrass cultivars were seeded at 50 or 100 kg ha(-1) in 2004 and 2005 and maintained at 4.5- or 7.5-cm mowing height. Annual N rates of 120 or 210 kg ha(-1) were applied from April to September in different splits. Sod was harvested at 14 or 23 mo aft er seeding. By 14 mo aft er seeding, all N treatments had turfgrass visual quality above 6.0 except the unfertilized control and the 120 kg ha(-1) yr(-1) rate applied mainly in summer or spring. Sod shear strength reached a minimum acceptable level of 0.9 kg m(-2) x10(4) about 23 mo aft er seeding. Highest sod shear strength (1.18-1.36 kg m(-2) x10(4)) with acceptable visual quality could be achieved through an N program of 8, 16, 24, 32, and 40 kg ha(-1) in May, June, July, August, and September, respectively. Based on a 23-mo production time line, a seeding rate at 50 kg ha(-1) and a 7.5-cm mowing height is recommended for economic reasons. Applying more than 40 kg ha(-1) of N in June and July is not recommended.