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Crops and soils can react to the same level of compaction differently, thus there is a need to assess the impact of compaction under particular environments. The effect of soil compaction as a result of tractor traffic on cowpea (Vigna unguiculata L. Walps) yield was investigated in a sandy loam soil (Regosol) in the semi-arid region of northern Nigeria for two growing seasons, 1998 and 1999. A randomized complete block design of field plots with treatments of 0, 5, 10, 15 and 20 passes of a tractor with 31 kPa contact pressure was used. Each treatment was replicated four times. The soil bulk density, penetration resistance and soil moisture content for each applied load were measured and the yield from each treatment was. determined. Agronomic treatments were kept the same for all plots in both 1998 and 1999. Results showed increased soil dry bulk density, penetration resistance and soil moisture content with increased tractor passes. Plant height decreased, plant moisture content and plant stem diameter increased with increased tractor traffic passes. Highest grain yield was obtained at 10 tractor passes with a mean bulk density of 1.58 Mg m(-3) and penetration resistance of 1.37 MPa with a mean grain yield of 276.5 kg ha(-1), while lowest yield was obtained from 20 tractor passes was 555 kg ha(-1). Statistical models were used to predict grain yield as a function of bulk density, penetration resistance, soil moisture content, contact pressure, and number of tractor traffic passes. Grain yield with respect to moisture content gave the best prediction (r(2) = 0.96). (C) 2002 Elsevier Science B.V. All rights reserved.