Matric suctions to which soils in South Central England drain

In a survey of soils in South Central England matric suctions were measured frequently during 1962 and 1963. Results for the period October to May are given for sites lying above the influence of the regional ground water table and show the matric suctions to which the soils drain. Sandy soils drain within 2 or 3 days to a definite field capacity at about 50 mbar suction. Loams and clay loams, though from profile appearance regarded as freely drained, take up to about 1 week to drain to a fairly constant suction of 40–50 mbar, which can also be regarded as field capacity. However, suctions prevailing during this time are less – about 20 mbar. The prevailing suctions in clay soils are about 10 mbar in the topsoil (13 cm) and – 10 mbar in the subsoil (38 cm). These soils drain slowly but steadily when rain ceases and no definite field capacity is evident. For the sandy soils, loams and clay loams, the matric suctions characterize the upper limit of the available water capacity (AWC) of the soil. The values are less than almost all previous estimates or assumed values. For the clays the upper limit of AWC remains best estimated by field sampling at the time of interest.