EFFECT OF SOIL WATER STATUS AND STRENGTH ON TRAFFICABILITY

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Abstract
Trafficability tests with typical farm vehicles were carried out on three lowland fields at various degrees of wetness. Structural damage after the first and third passes was assessed in terms of bulk density, aeration porosity, pore-size distribution and rut depth. These indices could not be used per se as criteria for trafficable conditions because of lack of information concerning their relationship to plant growth. Instead, a trafficability criterion oriented toward traction efficiency was established by determining for each soil the relationship between its strength (assessed with a cone penetrometer) and traction efficiency measured by wheelslip. A critical value of strength for trafficability was inferred from this relationship. This was then used to obtain soil water tension limits for trafficability from known relations between tension and strength. Soil strength was found to be linearly dependent upon water table depth in spring when evapotranspiration was small and when the water table depth was less than 80 cm. Consideration of these relationships led to the establishment of critical water table limits for trafficability. These were 53, 45, and 60 cm for Lumbum muck, Hallart silty clay loam (SiCL) (grassland), and Hallart silty clay loam (cultivated), respectively.

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