International Committee on Soil Moisture and Temperature Regimes (Icommotr): a Review.
Wayne H. Hudnall, Department of Plant and Soil Science, Texas Tech University, MS 42122, Lubbock, TX 79409
Two soil orders are defined based upon their soil moisture (Aridisols) and soil temperature (Gelisols). Gelisols require permafrost within two meters of the soil surface, which implies that water, even though frozen is incorporated into the definition. Of the remaining orders, all suborders are based upon soil moisture and soil moisture is used to define some subgroup intergrades. Soil temperature is used as family criteria. Both soil moisture and temperature are combined as criteria for some taxa. Soil temperature is easier to measure than soil moisture partly because of its intrinsic properties. Soil moisture is difficult to measure because there are two conceptual measurements: 1. Gravimetric and 2. Potential metric. The problem lies in that we use gravimetric terms (saturated, wet, moist and dry), but use potential metric criteria. The problem is further complicated because Soil Taxonomy and NASIS are not consistent in terms used to describe the conditions and the soil depth or volume used to measure the condition. The present definitions use matric potential, not soil water content, as the soil parameter for determining the soil moisture regime. Currently a soil is classified based upon the number of days, cumulative or consecutive, that an ill-defined zone in the soil (called the soil moisture control section or SMCS) is entirely, or partially, wetter or dryer than -1500 kPa matric potential. The chief difference between the suggested method and the current method is that the current method requires that the water state of all or part of the SMCS be wetter or dryer than some arbitrary matric potential, 1500kPa, for an arbitrary number of days when the soil temperature at a different depth (50 cm) is greater than one of two arbitrary values (5 and 8 degrees C), whereas the suggested method would require that the mean or normal annual and seasonal soil matric potentials measured at an arbitrary depth be within some arbitrary range. It is easier to determine mean annual and mean seasonal soil water consecutively, when the soil temperature at 50 cm is greater than 5 or 8 degrees C, that a nebulous zone in the soil is wetter or dryer than 1500 kPa. The proposed definitions are based on practically no hard data. They need to be tested. The concepts seem sound i.e. soil under an aridic regime should have an average annual soil water state drier than soil under an ustic regime. The matric potential at 15 cm may be needed to make separations of the drier regimes if there is not enough difference in the matric potential at 75 cm between the regimes. This should become evident as the proposal is tested. Soil matric potential is determined at depths of 75 and 15 cm from the soil surface (the air-soil interface). The length of record required is 30 years. The value used in the determination of soil moisture regime normal value for the soil matric potential during the time interval of interest; i.e., the 0.5 probability of the statistical cumulative frequency distribution of the measured values (if the soil matric potential follows a normal frequency distribution then the normal value is numerically equal to the arithmetic average). The normal value is the one most likely to occur. The objectives of the committee are: 1. Develop a statement describing why soil climate is an appropriate soil property to be included in Soil Taxonomy. This conceptual statement will serve as the guide to evaluate ICOMMOTR proposals. 2. Propose standard procedures for measuring soil moisture and temperature as well as a standard site condition. Provide guidance on correlation of other conditions to the standard. Also, consider methods for measuring moisture in Vertisols. 3. Test the use of measurements at fixed points at standard depths to replace the concept of the moisture control section. 4. Define moisture and temperature regimes separately from one another, including seasonal concepts (moist/dry and warm/cool seasons). Utilize combinations of the regimes to define appropriate taxa. Explore the use of near-surface measures of moisture and temperature for further defining some taxa, such as the very cold soils and very dry soils and shallow soils.