The udic (L. udus, humid) moisture regime implies that, in 6 or more out of 10 years, the soil moisture control section is not dry in any part for as long as 90 cumulative days per year. If the mean annual soil temperature is lower than 22°C and if the mean winter and mean summer soil temperatures at a depth of 50 cm from the soil surface differ by 5°C or more, the soil moisture control section, in 6 or more out of 10 years, is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice. In addition, the udic moisture regime requires, except for short periods, a three-phase system, solid-liquid-gas, in part or all of the soil moisture control section when the soil temperature is above 5°C.

The udic moisture regime is common to the soils of humid climates which have well-distributed rainfall, or which have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration. Water moves down through the soil at some time in most years.

In climates where precipitation exceeds evapotranspiration in all months of most years, the moisture tension rarely goes up to 100 kPa in the soil moisture control section, although there are occasional brief periods when some stored moisture is used. The water moves through the soil in all months when it is not frozen. Such an extremely wet moisture regime is called perudic (L. per, throughout in time; L. udus, humid). In the names of most taxa, the formative element ud is used to indicate either a udic or a perudic regime; the formative element per is used in selected taxa.

7/15/98