The ustic (L. ustus, burnt, implying dryness) moisture regime is intermediate between the aridic and the udic regime. Its concept is one of moisture that is limited but is present at a time when conditions are suitable for plant growth. The concept of the ustic moisture regime is not applied to soils that have cryic or pergelic soil temperature regimes (defined below).

If the mean annual soil temperature is 22°C or higher or if the mean summer and winter soil temperatures differ by less than 5°C at a depth of 50 cm below the soil surface, the soil moisture control section in the ustic moisture regime, in 6 or more out of 10 years, is dry in some or all parts for 90 or more cumulative days per year. But the moisture control section is moist in some part either for more than 180 cumulative days per year, or for 90 or more consecutive days.

If the mean annual soil temperature is lower than 22°C and if the mean summer and winter soil temperatures differ by 5°C or more at a depth of 50 cm from the soil surface, the soil moisture control section in the ustic regime is dry in some or all parts for 90 or more cumulative days per year in most years. But it is not dry in all parts for more than half the cumulative days when the soil temperature at a depth of 50 cm is higher than 5°C. If the moisture control section, in 6 or more out of 10 years, is moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice, the moisture control section is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice.

In tropical and subtropical regions that have a monsoon climate with either one or two dry seasons, summer and winter seasons have little meaning. In those regions, the moisture regime is ustic if there is at least one rainy season of 3 months or more. In temperate regions of subhumid or semiarid climates, the rainy seasons are usually spring and summer or spring and fall, but never winter. Native plants are mostly annuals, or plants that have a dormant period while the soil is dry.

7/15/98