The three taxa here represent intergrades/extragrades to other soil moisture regime (SMR) concepts. These soils have a soil moisture regime as well as a soil moisture subclass that is not "typic". Some soils have a moisture regime that is transitional to another regime (intergrades) or one that grades away from the regime they are assigned to (extragrades). See Soil Survey Tech. Note 9 Populating Taxonomic Moisture Class and Subclass in NASIS for more info: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053576
The soil moisture regimes, and the criteria used in the subgroup-level keys for determining soil moisture subclasses, are generally based around the water status of the soil moisture control section (SMCS). The SMCS is defined on page 29 (KST 2014). The official definition of the soil moisture control section boundaries involves depths of infiltration of fixed volume of water over fixed time intervals. In practice the SMCS boundaries are rarely measured via depths of infiltration for a variety of reasons. Guideline depth ranges are given on p. 29 that vary with the particle-size family class and many people use these values directly.
There are methods in the literature for deriving the SMCS boundaries from laboratory water retention data e.g. Zobeck and Daugherty (1982): Calculating the Depths of Soil Moisture Control Sections from Laboratory Data https://acsess.onlinelibrary.wiley.com/doi/epdf/10.2136/sssaj1982.03615995004600040026x which have a similar theoretical basis as the guidelines found on p. 29.
One of the more broadly used methods for estimating the taxonomic quantities ("number of days dry" etc.) is the Newhall model. This model is fairly naive about soil properties and layering--instead leaning on climatic data at the monthly timescale.
- Newhall Soil Climate Simulation Model model: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/?cid=nrcs142p2_053559.
Also see relevant discussion here:
- https://soiltxnmyforum.cals.vt.edu/forum/read.php?3,2043,2043#msg-2043
- https://soiltxnmyforum.cals.vt.edu/forum/read.php?3,1927,1927#msg-1927
The Newhall model simplifies the estimation of SMCS in that it essentially treats the whole soil profile as a single "bucket" divided into 8 equal volume zones. In Newhall's method the whole profile has a fixed storage capacity of 200mm, with two of the 8 zones (or about 50mm of water storage capacity, corresponding to the zone between 25 and 75mm water retention difference) considered to be the SMCS. See Calculation of Soil Moisture Regimes from the Climatic Record: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052248.pdf for additional details on the assumptions and limitations of this method.
If you are interested in calculating taxonomic quantities from monthly climatic data there has been recent work to port Newhall model over to R.The rNewhall project (https://github.com/ncss-tech/rNewhall) completely rewrites the model in R, and will eventually be able to use daily data, but is not done yet. I have been working on a R wrapper that calls the Java model (https://github.com/ncss-tech/jNSMR) and is a convenient interface to the widely-used calculations that involve monthly average temperature and precipitation data.
I will now go through each of the taxa you brought up.
First note that the definition of aridic SMR (p. 30) requires:
1. Dry IN ALL PARTS for more than half the year (5/10ths) when 50cm soil temperature >5 degrees C
2. Moist in SOME OR ALL parts LESS than 90 consecutive days when 50cm soil temperature is >8 degrees C
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Aridic Haplustalfs: have an ustic SMR with aridic soil moisture subclass
- Dry IN SOME PART for 6/10ths or more of year when 50cm soil temperature >5 degrees C (p. 71)
The key thing here is that for Aridic Haplustalfs the soil moisture control section is dry only in SOME parts for 6/10ths or more of the year when >5C. If it were dry in ALL parts in that same time you would be correct that that criterion would match aridic... assuming the second (less than 90 days moist when temperature >8 degrees C) criterion for Aridic SMR is also met.
Aridic Haplustalfs are consistent with an ustic SMR because they may be moist in some part of the soil moisture control section for half or more of the year, or they are moist for more than 90 consecutive days when temperatures are over 8 degrees C. These soil moisture control sections are not fully dry enough long enough, or are not dry in the warm time of year, in order to be considered as having an aridic soil moisture regime. Therefore they are assigned an Aridic subgroup in the Haplustalfs with a soil moisture subclass "aridic" rather than "typic". At NRCS we would say these soils have an "aridic ustic" moisture regime.
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Argiduridic Argixerolls: have an aridic SMR with xeric soil moisture subclass
- By definition Xerolls with an Aridic SMR must "be bordering on xeric" (p. 246)
- By definition all Argiduridic Argixerolls have an aridic SMR (p. 248)
Argiduridic Argixerolls are consistent with an aridic soil moisture regime because soils with mollic epipedons are excluded from the Aridisols. If a Mollisol has an aridic moisture regime, then by definition it is bordering on xeric or ustic. Since the Argiduridic (tending towards Argidurids) subgroup of Argixerolls requires the Aridic SMR as a criterion, we know that the soil must be aridic SMR with a xeric subclass (the latter we derive from Xerolls). At NRCS we would say these soils have a "xeric aridic" moisture regime.
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Aridic Calciusterts: have an ustic SMR with an aridic soil moisture subclass
- Moisture regimes in the Vertisols are based on the duration of times/timing of when cracks are open not on when the soil moisture control section is moist or dry. At NRCS we would say these soils have an "aridic ustic" moisture regime, if the criteria for the Aridic subgroup of Calciusterts (involving time when cracks are open) are met.
The timing and duration of Vertisol cracks being open is related to the water status of the soil but there is not a general numerical relationship between "number of days moist/dry" and number of days that cracks are open. The difficulties with the standard SMCS definition are even more difficult in the Vertisols.
In general: individual instances of subgroups have their own criteria for what constitutes an "Aridic" subclass or subgroup; the definition is not guaranteed to be constant across Great Groups/Suborders/Order; so there isn't a general answer to "what time we use aridic in subgroup"
As far as the meaning of 6/10, that is interpreted as six tenths (6 / 10) of the total number of days per year where the soil temperature at 50cm is greater than 5 degrees C. So, say that at your study site you estimate you have 200 days per year that have soil temperature at 50 cm is >5 degrees C. the minimum number of days that SOME part of the soil moisture control section is dry needs to be greater than 0.6*200 = 120 days, or about 4 months of the year
As with what I said above about the boundaries of the SMCS, the values of number of days are not usually explicitly measured and modeled in the practice of soil mapping. As such there can be significant difference between moisture regimes "as mapped" in the US and model generated values for those areas. Major factors being source/scale of climate data and how the soil and its heat/water balance is parameterized. For soil survey, there are usually relationships between major geographic/physiographic transitions, plant species, and climate that are useful surrogates for these concepts and can be generally related to ranges of moisture condition. When a soil survey is being conducted, the number of meaningful strata are considered relative to the survey goals and local conditions. These strata are then assigned a reasonable value for SMR/STR based on the available local climate data and mapping concepts developed during the survey.
