ABSTRACT SUBSURFACE drainage systems affect spring and fall trafficability, the timeliness of planting and subse-quent crop harvest and yield on a silty clay loam marine soil. Drainage effectiveness and trafficability response were evaluated by mercury tensiometers for soil moisture, a cone penetrometer for soil strength and by a subjective walk-on trafficability index (TI). Soil moisture and penetrometer resistance measurements were taken over the shallow drains spaced at 3, 6 and 12 m, at the mid-point between drains and at two comparable random locations in undrained plots. Trafficability was found to develop more rapidly in the spring on a slowly permeable representative silty clay loam soil with drainage than on undrained plots. However, we did not observe any clear differences be-tween any of the three drain spacing treatments. A good correlation existed between soil moisture tension and soil strength in the 0-15 cm soil depth which could be match-ed to soil readiness and thus to workday or trafficable conditions. Corn yields vs. drainage for 1980, a year characterized by a dry spring, showed no significant difference between drainage and no drainage. Alfalfa in 1981, when drained plots were ready for cultivation 3 wk earlier than un-drained plots, gave yields significantly greater for the drained over no drainage from two cuttings. There was no difference between drained treatments.