SOIL MOISTURE REGIMES (SMR)
CONCEPT– cumulative and consecutive periods of dryness and moistness in the soil moisture control section.
Dry – water potential less than –1500 kPa (- 15 bars).
Moist – water potential greater the –1500 kPa.
SOIL MOISTURE CONTROL SECTION (SMCS)
Upper boundary – depth to which dry (but not air-dry) soil is wet by 2.5 cm of water in 24 h.
Lower boundary – depth to which dry soil is wet by 7.5 cm of water in 48 h.
Rule of thumb estimates of SMCS:
- Sandy textures: 30–90 cm
- Coarse-loamy textures (< 18% clay, but not sandy): 20–60 cm
- Other textures: 10–30 cm
Soil Moisture Regimes
Aridic (or torric) – arid climate, usually dry, irrigation required for crop production. SMCS dry > 1/2 the time that T50 > 5 °C and not moist for 90 consecutive days when T50 > 8° C.
Ustic – semiarid climate, rainfall during a growing season. Not applied to cryic or pergelic STR. SMCS moist 1/2 to 3/4 of time or moist 90 consecutive days. SMCS dry < 45 consecutive days in summer.
Xeric – semiarid climate, Mediterranean climate, cool, moist winters, dry summers, dry-land crop possible from stored soil water. Not applied to hyperthermic or iso- STR. SMCS moist 1/2 to 3/4 of time, moist > 45 consecutive days in winter, and dry > 45 consecutive days in summer.
Udic – humid climate, usually moist, generally irrigation not required for crop production. SMCS dry < 90 cumulative days and < 45 consecutive days in summer.
Perudic – precipitation exceeds evapotranspiration in all months, but soil is not saturated for long periods.
Aquic – soil saturated long enough to cause anaerobic conditions, not used as a criterion for differentiating taxa.
Aquic conditions – requires saturation, reduction of Fe and Mn, and redoximorphic features (including redox depletions, redox concentrations, and reduced matrix). Saturation conditions include episaturation, endosaturation, and anthric saturation.