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Assessing the Australian soil classification using cladistic analysis.

Introduction

Before the introduction of the Australian Soil Classification (ASC; Isbell 1992, 2002), two schemes were in operation in Australia: the Handbook of Australian Soils (Stace 1968) and The Factual key (Northcote 1971). In the former, Stace (1968) described 43 Great Soil Groups differentiated by central concepts of morphology, distribution and land utility, which, although easy to understand, were not explicitly defined (Mazaheri et al. 1995). The classification therefore had numerous misidentifications (McKenzie and Austin 1989). To overcome these, Northcote (1971) developed a key, marking a shift from classifying external factors towards intrinsic soil morphological properties (Northcote 1984). The bifurcating hierarchical design, which used mutually exclusive soil morphological properties, most notably the Principle Profile Form (PPF), removed uncertainty (Fitzpatrick et al. 2010).

The ASC is a general-purpose soil classification that uses clearly definable soil morphological and non-morphological properties. It is similar to Northcote (1971) in that it functions as a sequential key, with mutually exclusive criteria. Its usefulness lies in its ability to identify soils, rather than in capturing shared relationships between them. However, the scheme's lack of mutually exclusive characteristics has led to groups, such as the Hydrosols, having the same characters used that define other groups. Situations such as this arise from artificial classifications and highlight the need for the application of alternative methods to create a natural classification. The benefit of natural classifications is that they are independent of the interpretation of the classifier and represent only what occurs in nature (Wilkins and Ebach 2014). Moreover, they are predictive in that they can accommodate soils from other classifications, or the discovery of new soils.

The importance of natural classifications lies in what can be inferred from them. If taxa (units of classification) are more closely related to each other than they are to any other taxon, they form a natural group or homology (Table 1). For example, a natural soil group (homology) is evidence of a common history, process or mechanism, namely soil genesis. There is a common consensus that soils undergo traceable and explainable development or evolution (i.e. soil genesis), making them amenable to a natural classification assessment (Wilkins and Ebach 2014). The problem, however, has always been the lack of a reliable methodology to discover natural classifications.

Biological systematics offers an avenue in this endeavour, namely, cladistics (from the Greek klados, i.e. 'branch'). The concept began with Phylogenetic Systematics (Hennig 1966), a method that uncovers relationships between taxa based on shared derived character-states. The simple premise that two taxa are more closely related to each other than they are to a third taxon underpins cladistics (Schuh and Brower 2010). The power of cladistics lies in amalgamation of parts of taxa in the form of characters and their states, to create a branching diagram (cladogram) supported by character-states at nodes. Cladistics has proved successful in the natural grouping of birds and dinosaurs (Clark et al. 2002) in geology (Echeverry et al. 2012) and in astronomy (Fraix-Bumet et al. 2006).

Here, we use cladistics to examine whether existing Soil Orders, Suborders and Great Groups in the ASC form a natural classification, by conducting a cladistic analysis using TNT version 1.0 (Tree analysis using New Technology; Goloboff et al 2000). A natural classification occurs when clades (relationship between Soil Groups, Suborders and Orders) are formed in the analysis. A secondary aim is to identify the informative shared characters in the Suborders and Great Soil Groups, based on their distinguishing morphological and non-morphological character-states, and the impact of nonmorphological, continuous and colour characters on the ASC.

Material and methods

Australian Soil Classification

The ASC (Isbell 2002) is a hierarchical key and consists of five tiers: Orders, Suborders, Great Groups, Subgroups and Families. Table 2 shows the 14 Soil Orders. The Suborders are based on subsoil colour (i.e. Red, Brown, Yellow, Grey, Black), with Great Groups differentiated on a range of characters (Table 3). The Subgroups are a continuation of Great Groups. The Family criteria are common to all Soil Orders, including A-horizon thickness, A1 gravel and texture as well as B horizon texture and depth.

To assign an unknown soil to a taxa, the user employs a series of morphological (e.g. soil colour) and non-morphological characters. By using this approach, and considering all tiers, an unknown profile can fit in a maximum of 53 845 soil taxa. This reduces to a maximum 11076 at the fourth tier (i.c. Subgroups), 565 at the third tier, 78 at the second tier (i.e. Suborders), and 14 for the Soil Orders. Typically, soil is classified into Soil Orders.

One way in which these Soil Orders are increasingly being related to each other is shown in Fig. 1a, which is a plot of climate (i.e. rainfall) and parent material, and in lower parts of a landscape (Gray and Murphy 2002). This is because climate is seen as one of the most powerful soil-forming factors through its influence on the rate of chemical weathering of minerals from parent material. Figure 1 a can be represented as a cladogram to recognise relationships, based on shared homologies of climate and parent material, by the branching of taxa (Fig. 16).

Although it is inappropriate to classify soil solely on the basis of parent material, climate and topography (Basinski 1959; Marbut 1951), it does illustrate the potential of cladistics in identifying homologies (i.e. natural groups) based on a common ancestry (i.e. parent material) and relationship with climate as a function of morphological and non-morphological characters. For example, one homologous character of Vertosols and Ferrosols is their mafic parent material. However, Ferrosols form in higher rainfall and in situ and elevated areas associated with the south-east Australian Cenozoic Volcanic Sequence, whereas the Vertosols form as weathered and eroded products in semi-arid areas to the west and on the lower lying and expansive clay alluvial plains of the Murray-Darling Basin. A similar explanation can be provided for the siliceous Soil Orders, whereby, Podosols form from aeolian sands and along high-rainfall coastal areas of south-eastern Australia, whereas Tenesols form in the arid areas of central Australia.

Characters and coding

Forcladistic analysis, the morphological and non-morphological characters used to identify the soil taxa need to be coded into binary or multiple character-states. For most characters, coding is 'absent' (0) or 'present' (1). For continuous characters, it involves discrete classes. For example, exchangeable sodium percentage (ESP) or Character 77 (C.77), indicating non-sodic (<6), sodic (6-15%), moderately sodic (15-25%) and extremely sodic (>25%), are designated character-states of C.77:0 (absence) and C.77: 1, C.77:2, and C.77:3 (presence), respectively. The characters, character-states and coding are listed in Table 3.

The exponential growth in potential soil taxa with each additional tier makes it impractical to include all tiers. This exclusion is supported by the reasoning that important characters are located in the upper tiers (Isbell 2002). For these reasons, the cladistic analysis is limited to the Order, Suborder and Great Group tiers. The analysis was also undertaken with the removal of the Anthroposols, because their characters are unnaturally formed. The base cladogram (BC or cladogram 1) includes all taxa from 13 Soil Orders, with the 113 characters described in Table 3.

Measures used within cladistic analysis: consistency index, retention index and confidence levels

When interpreting cladistic analysis, all cladograms are evaluated in terms of a consistency index (CI), retention index (RI) and confidence levels. CI and RI are measures aimed at determining how well data fit a cladogram (Rexova et al. 2003). CI identifies the amount of character-state conflict, demonstrating how well a character fits onto a cladogram. CI is calculated by the number of steps on the cladogram (M) divided by the actual number of steps that could be on the cladogram if perfectly resolved (S), multiplied by 100: CI = M/S x 100 (Kitching et al. 1998). A non-conflicting cladogram of three Soils Orders (Orders A, B and C) and one soil character (0 = absence, 1 = presence; Orders A = 0, B and C = 1) will recover the soil cladogram (A,(B,C)), resulting in one step, supported by character-state 1. The cladogram can be calculated as M/S x 100 = 1, meaning data fit the tree perfectly, without conflict. If, for instance, another soil order D and another soil character (0 = absence, 1 = presence; Orders A = 0, B, C and D= 1) are added, we would have enough data to form two steps; however, only character 1 forms a step, namely B, C and D form the soil cladogram (A,(B,C,D)). Therefore, M = 1, S = 2, CI = 0.5, Soil Orders A and D = 0, B and C = 1, would form two steps (CI = 1).

Retention index measures how many shared characterstates in a given cladogram are retained on the cladogram; that is, the RI measures how well the character-states explain the cladogram. It is defined as (G - S)/(G - M), where G represents the greatest number of times a character-state can occur (Kitching et al. 1998). For example, if the maximum number of times a character-state can occur is three (G = 3), and the minimum number of occurrences is one (M= 1) and it occurs twice (S = 2), this results in (3 - 2)/(3 - 1) = 0.5. As the example illustrates, RI reflects the characters occurrence as a shared character-state between soil taxa, with less regard for character conflict. The closer RI is to 1, the more occurrences of shared character-states, which represents a better fitting cladogram.

All cladograms are subjected to bootstrapping, which enables the determination of confidence levels of the Soil Orders. The confidence levels of Soil Orders is determined by randomly removing characters and re-running the analysis to see if the Soil Orders still group together. These characters are returned to the analysis and new characters are removed and the analysis re-run (Siebert 1992). This removal of characters occurs 100 times, meaning the confidence level is on a scale of 0-100. A high confidence level indicates inclusive groups (clades) that are supported by multiple characters. The higher the confidence levels, the more confident one can be that the Soil Order identified is natural.

Coding of matrices: base and ASC tiers

The list of characters shown in Table 3 represents those used in the BC or cladogram 1. Table 4 summarises the cladogram names, number of possible taxa and characters of the other matrices. To determine the effect of removing morphological and nonmorphological characters to distinguish the Soil Order tier, cladogram 2 includes characters from Suborder and Great Group tiers only. Cladogram 3 is aimed at assessing the influence of characters in the Suborder tier, whereas cladogram 4 considers the effect of removing all characters from the Great Group tier.

Coding of matrices: continuous, non-morphological and soil colour characters

Cladistics works most effectively when either absence-presence and/or other qualitative characters are used (e.g. C.52, clear and abrupt textual B horizon). Continuous characters such ESP are problematic because the scale values for various states are arbitrary (Pimentel and Riggins 1987) and because they result in polymorphisms, which are poorly resolved nodes at ends of trees causing conflict in the analysis (Nixon and Davis 1991). To assess the effect of polymorphism, all continuous characters are removed in cladogram 5.

Cladistic analysis relies upon the use of characters, that is to say, intrinsic features that are observable in a taxon (Kitching et al. 1998), for example, C.52 (clear and abrupt textual B horizon). Applying this cladistic requirement of applying only morphological characters to the ASC means that characters such as C.39 (profile saturated for at least 2-3 months) are not used. Non-morphological characters describe the environmental conditions under which taxa are found, rather than using morphological properties. To identify the effect of nonmorphological characters, we removed them in cladogram 6.

The use of soil colour to identify relationships between taxa is problematic for two reasons. The first is the arbitrary nature of how colour is determined. The colour chart used is determined by hue, so if it is 5 YR or redder there is a choice of three colours, whereas if the hue is yellower there is a choice of four colours (Isbell 2002). Second, the same colours appear multiple times in Soil Orders (i.e. Vertosols, Kurosols, Sodosols, Chromosols, Ferrosols, Dcrmosols, Kandosols), namely red, brown yellow, grey and black. The frequent use of colour makes it a conflicting character. To establish its impact, it is removed in cladogram 7. Cladogram 8 is aimed at determining the combined effect of removing continuous, non-morphological and colour characters.

Coding of matrices: Soil Orders

To determine the naturalness of each Soil Order, and the impact each has on the others, we excluded each in turn. Table 4 summarises the 13 matrices developed, which include cladograms 9-21. After the initial BC analysis, a final cladogram was created; cladogram 22 saw the removal of the seven Soil Orders that registered no confidence level on the BC analysis.

Cladistic analysis of matrices

In all 22 matrices, an out-group is created, rooting the cladogram. To achieve this, an extra soil taxon (OG) is added, which comprises all Os. The 22 matrices are run through a parsimony algorithm using the cladistics program TNT version 1.0 (Goloboff et al. 2000). The analysis is run using the ratchet setting within the New Technology Search. For all analyses, the ratchet is set at 1000. For each analysis, a combined consensus cladogram (including cladogram length), CI, RI and confidence levels (bootstrapping) are generated.

Results

Description of base cladogram

A well-resolved, natural classification would be discernible in a cladogram if clades of each taxon exist on a radiating node away from a common node (see Fig. 1 b). This is not the case in Fig. 2, which shows the BC (i.e. cladogram 1) that considers all three tiers and morphological and non-morphological characters. This is because most Soil Orders emanate from one mother node. This means that the BC has poor structure and few relationships can be inferred between the different Soil Orders. This lack of structure indicates that the ASC is not a natural classification.

The result does show that the Organosols, Podosols, Vertosols, Calcarosols, Kandosols, Rudosols and Ferrosols are identified as separate clades, and indicates that the tiers and morphological and non-morphological characters used to identify these Soil Orders are equally effective in a cladistic analysis. Although not completely identified as a separate cladc, some Tenosols are resolved, with the majority grouping into a clade based on C.58 :1, C.58 :2, C.58 :3 (i.e. A1 horizon overlies calcrete pan, or partially weathered rock, or unconsolidated materials, respectively).

The remaining Soil Orders, Hydrosols, Dermosols, Sodosols, Chromosols and Kurosols, do not form separate clades. The reason is that their defining characters occur multiple times. The lack of grouping means that the characters used to identify these Soil Orders are not as exclusive as those used by Organosols, Podosols and Vertosols.

One of the most problematic is the Hydrosols, because it groups out into five different clades (e.g. Hydrosols 1, 2, etc.) with Kurosols, Sodosols (2), Chromosols (2), Kandosols, Rudosols and Tenosols (2, 3) (Fig. 2). These groupings result from the use of several morphological and non-morphological characters in different tiers. For example, the morphological characters C.52 (clear textural B horizons) and C.53 (acidity of the major part of upper 0.2 m of B2) used to identify Kurosols, Sodosols and Chromosols are also used within the Great Group tier to identify Hydrosol taxa. A thorough revision of each of the Soil Orders to accommodate the Hydrosols will result in a more natural classification.

The grouping of the Sodosols (1), Chromosols (1) and Dermosols (2) is due to the shared Great Group characters C.71 : 1 (upper 0.2 m of the B2 horizon has a strong blocky or polyhedral structure), C.72 : 1 (very weak adhesion between peds), C.73 : 1 (usually high salt contents resulting in weak dry strength), and C.74: 1 (some B2 horizons may be weakly subplastic). The fact that four characters group them together adds considerable strength to their relationship, allowing cladistics to isolate them from other soil taxa and group them into a single clade. This indicates the need to use multiple characters when defining taxa, and not to rely on one.

Base cladogram: confidence levels, consistency index and retention index

We undertook a bootstrapping of the BC in order to give a level of confidence for the 13 Soil Orders. Table 5 shows Organosols (64), Podosols (58), Vertosols (61), Calcarosols (29), Kandosols (2) and Rudosols (33) all registered confidence levels. The remaining seven Soil Orders had far lower values. Given the significantly higher confidence levels (>50) of the Organosols, Podosols and Vertosols, it can be concluded these Soil Orders are the only natural groups.

The greater confidence levels are based on their numerous unique characters. Specifically, Organosols only contain three characters that are found in other Soil Orders (i.e. characters 4, sulfuric materials occur in upper 1.5 m; 5, sulfidic materials in upper 1.5 m; 6, at least some part of the B horizon is calcareous). The Podosols only have one shared character C.17 (long-term saturation in B horizon), whereas the Vertosols share character 17 with Podosols, as well as sharing the five colour characters with seven other Soil Orders (e.g. Kurosols and Sodosols). The remaining characters in the Organosols, Podosols and Vertosols are unique to their respective Soil Order.

Values of CI and RI are calculated for the BC, to determine the occurrence of character-state conflict and shared character-state forms, respectively. Table 6 shows that the BC generated a low CI (0.196) and relatively high RI (0.753). The high RI indicates characters used to identify relationships between different soil taxa. However, the reoccurrence of the same character-states (i.e. low CI) is indicative of the artificial nature of the ASC.

The Soil Orders that registered low or no confidence, such as the Chromosols, Ferrosols and Dermosols, have highly conflicting characters in the Suborder and Great Group tiers. These Soil Orders rely exclusively on one or two characters located in the Order tier to group them into a single clade. For example, the Ferrosols rely exclusively on the sole characterstate C.94 : 1 (major part of B2 horizon has a free iron content >5% in fine earth). The removal of this character means that Ferrosols no longer group together with confidence. This is due to the remaining characters in the Suborders and Great Groups being found in multiple Soil Orders. The example of the Ferrosols illustrates the need for Soil Orders to be defined by multiple unique character-states in all tiers of the classification, not just the upper most tier.

Removal of ASC tier characters

The removal of the 26 characters that define the Soil Order tier and from the BC (cladogram 2) resulted in an increase in both the CI (0.282) and R1 (0.763). The increase indicates less characterstate conflict and more shared character-states, resulting in a better fitting classification. However, the removal of the Soil Order tier characters means none of the 13 Soil Orders form clades with any confidence, with all Soil Orders registering 0 (Table 5), revealing that the 13 Soil Orders rely exclusively on the characters in the Order tier in order to group.

Without Order-tier characters, the taxa grouped out into smaller clades in cladogram 2 (not shown) and based on their Suborder or Great Group characters. Of the multiple clades, only five scored confidence levels >50, including the grouping of all 13 Podosol Suborder and Great Group taxa into three groups based on their Suborder characters: Aerie Podosols (91), Semiaquic Podosols (79) and Aquic Podosols (54).

There was also a grouping of five Sodosols, five Chromosols and five Dermosols with a confidence level of 67 and based on the Great Group C.71 : 1 (upper 0.2 m of the B2 horizon has a strong blocky structure), C.72 : 1 (very weak adhesion), C.73 : 1 (usually high salt contents) and 74:1 (B2 horizons may be weakly subplastic). It is also worth noting two taxa from the Calcarosols and Rudosols grouped together with a confidence level of 63 based on C.85: 1 (dominantly consist of gypsum crystals). Although these results are unsurprising, the groupings described show the strength of various Suborder and Great Group characters and identify them as useful characters that should be retained.

Removal of the Suborder characters (i.e. cladogram 3) leads to a higher CI (0.373) than for the BC. This reduction in character-state conflict is to be expected because seven of the Orders (i.e. Vertosols, Kurosols, Sodosols, Chromosols, Ferrosols, Dermosols, Kandosols) use the same five colour characters, with their removal reducing their reoccurrence, and improving the fit. The RI (0.680) is also lower than for the BC. This indicates that these Suborder characters are identifying relationships between soil taxa. An example of a valuable character that was removed is multi-state character 3 (75% volume of organic material is peat; Fibric (1), Hemic (2), Saparic (3)), because its removal resulted in a decrease in confidence level of the Organosols (53) compared with the BC.

The removal of the Suborder characters saw significant increases in the confidence levels of the Podosols (89), Vertosols (93) and Kandosols (20), however. The removal of the Subgroup characters also resulted in the Ferrosols (19) registering a confidence level. The increase in the confidence levels for the Vertosols and Kandosols and the emergence of the Ferrosols is due to the use of colour, and because this is removed, there is reduced character-state conflict. This allows Soil Orders to group out with greater confidence.

Table 6 shows that the removal of the Great Group characters had the greatest impact on the CI relative to BC. This is because cladogram 4 (Fig. 3) had a CI of 0.602, which is more than triple that of the BC (0.196). This large increase in CI highlights the high level of character-state conflict that the Great Groups characters contribute when all tiers and all morphological and non-morphological characters are considered (i.e. cladogram 1). This is because ~50% of the characters found within the Great Group tier occur within 2-8 different Soil Orders. For example, C.69: 1 (red-brown hardpan either in or directly underlying B horizon) and 70:1 (B horizon that is not calcareous and overlies a calcrete pan) occur multiple times in the Sodosols, Chromosols, Calcarosols, Dermosols, Kandosols, Rudosols and Tenosols. This indicates that characters located in the Great Group tier require the greatest amount of revision, with the aim to increase significantly the number of unique characters used in this tier.

The removal of the highly conflicting Great Group characters also results in significantly higher confidence levels. The Organosols (89), Calcarosols (72), Kandosols (37) and Rudosols (56) all increase by at least 22 relative to the BC. There is also the emergence of the Hydrosols (57), Ferrosols (5) and Dermosols (9). The increase in confidence levels for the majority of the Soil Orders demonstrates the benefits and need to revise the characters used in the Great Groups.

The only Soil Orders to register lower confidence levels from the removal of the Great Group characters are Podosols (42) and Vertosols (39). This reduction may be due to the high percentage of unique characters in the Great Group tier. For the Podosols, this includes characters 20 (with only a Bs horizon) and 21 (with only a Bhs horizon), and for the Vertosols, characters 33 (surface that is moderately self-mulching) and 37 (massive or weakly structured surface).

Removal of characters types: continuous, non-morphological and soil colour

The removal of the four continuous characters (i.e. cladogram 5), including C.77 (ESP in upper 0.2 m of B2), C.53 (acidity in upper 0.2 m of B2), C.68 (base status of upper B2 horizon) and C.81 (soil with hard calcrete fragment), resulted in a slightly higher CI (0.288) than for the BC. This result is expected and consistent with the removal of polymorphism, which decreases character-state conflict. The removal of the four continuous characters also results in a higher RI (0.780). This means that the characters remaining identify more relationships between soil taxa. However, whereas both CI and RI are higher, the confidence levels remain the same (i.e. Organosols 64) or decrease. The only increase in confidence is for the Kandosols (12) with the Ferrosols (2) just emerging.

The removal of the 12 non-morphological characters (i.e. cladogram 6) produces a lower CI (0.189). By comparison, the RI (0.755) remains unchanged. The removal of the nonmorphological characters has a mixed effect on confidence levels of Soil Orders. Specifically, the Podosols (67) and Kandosols (19) increased, and the Organosols (53), Vertosols (51), Calcarosols (15) and Rudosols (21) decreased. There is no discernible reason because the majority of these Soil Orders have limited reliance on non-morphological characters.

Although not optimal or desirable in cladistics, it appears that the 12 non-morphological characters are beneficial to the ASC (Pimentel and Riggins 1987). In a revision, one would need to examine whether these non-morphological characters manifest themselves in some observable morphological properties. This could mean changing the non-morphological characters to morphological characters, marking a move away from characters based on environmental conditions towards characters using observable morphological properties.

The removal of the five colour characters (i.e. cladogram 7) produces a higher CI (0.313). This is to be expected because the five colours used occur multiple times and their removal gives rise to lower character conflict (i.e. RI = 0.724). These results indicate that retaining colour is beneficial in grouping soil; however, they need not be used so frequently. This conclusion is reinforced by the confidence levels of the Soil Orders, which changed little, except for the Vertosols (79) and Ferrosols (31), which saw the greatest increase in confidence levels. This increase mirrors their respective increases in confidence levels when the Suborder characters (including colour) are removed. The other Soil Order that was expected to increase significantly with removal of colour, but did not, was the Kandosols (8). The reason is unknown; removal of colour in cladograms 3 and 8 resulted in increases in Kandosol confidence levels.

The removal of continuous, non-morphological and colour characters (i.e. cladogram 8), unsurprisingly, leads to a higher CI (0.358) than removal of these characters independently; however, the RI (0.754) remained relatively unchanged. The removal saw a large increase in confidence levels of Vertosols (94) and Kandosols (25) and strong emergence of Ferrosols (29); however, the remaining Soil Orders showed no change. These results indicate that colour is more important than continuous and non-morphological characters and mean that revision of colour is crucial.

Removal of Soil Orders

The effects of the removal of entire Soil Orders on the CI are shown in Tabic 6. The removal of the Organosols (0.190), Podosols (0.175), Vertosols (0.192) and Rudosols (0.188) results in a slight decrease in the CI relative to the BC (0.196). This is because these four Soil Orders have a high level of unique characters and their removal increases characterstate conflict. This is consistent with the fact that these Soil Orders have high confidence in the BC.

The converse is true when the Kurosols (0.205), Sodosols (0.205), Ferrosols (0.202), Dermosols (0.222) and Tenosols (0.199) are removed, because the removal of their conflicting character-states results in an increase in CI. This is consistent with the lack of confidence levels that the five Soil Orders register in the BC. Removing them improves the classification. This is not the case with regard to removing Chromosols (0.173) and Hydrosols (0.168), which surprisingly leads to a poorer classification in spite of character-state conflict and their lack of confidence in the BC.

With respect to the RI, there was no clear observable pattern with the removal of individual Soil Orders relative to the BC. In this regard and with regard to low variability in the CIs and RIs generated from cladograms 9-21, these results indicate the limited effect that the removal of any single Soil Order had on the BC.

In order to determine what the effect might be of the removal of the seven Soil Orders that registered no confidence levels in the BC (i.e. Hydrosols, Kurosols, Sodosols, Chromosols, Ferrosols, Dermosols and Tenosols), cladogram 22 shows that CI (0.360) and RI (0.809) are substantially higher. This indicates a better fitting classification. Their removal also results in greater confidence levels for the remainder: Organosols (86), Podosols (61), Vertosols (70), Calcarosols (63), Kandosols (50) and Rudosols (85). The removal of the seven Soil Orders created a cladogram with less character conflict, more shared characterstates between soil taxa, and more confident grouping of the remaining six Soil Orders.

Discussion

Potential revision of types of characters

The removal of continuous (cladogram 5, CI 0.288), nonmorphological (cladogram 6, 0.189), and colour (cladogram 7, 0.313) characters indicates that one option to improve the fit of the ASC could be achieved in the revision of the continuous and colour characters. This is because their removal not only increased the fit of the classification (higher CI and RI) but also saw a greater number of Soil Orders generate higher confidence levels. However, the usefulness of colour in determining chemical and physical properties such as element content (i.e. iron), water saturation and content of organic material is well established (Davey et al. 1975).

The usefulness of colour has resulted in its employment in most soil-classification systems, including those of the USA (Soil Survey Staff 2010), New Zealand (Hewitt 1992) and Great Britain (Avery 1973). Therefore, complete removal of colour characters appears problematic given its diagnostic value. Nevertheless, the appropriateness of using all five colours in all seven Soil Orders could be reassessed. For example, the Ferrosols use all five of the soil colours in Suborders, although most Ferrosols are either red or brown (Isbell 2002). Removing the other colours will reduce the number of taxa and reduce the amount of character-state conflict.

Potential revision of ASC tiers

The results obtained from the cladograms where the characters Orders (cladogram 2, CI 0.282), Suborders (cladogram 3, CI 0.373) and Great Groups (cladogram 4, CI 0.602) are removed indicated higher levels of character-state conflict in Suborders and Great Groups, and this represents an opportunity to revise the characters in these tiers. In this regard, greater use of characters based on properties arising from parent material, as is the case in the US Soil Taxonomy (Bockheim et al. 2014), might be appropriate. This is because the use of characters (i.e. homology) creates an initial starting point for divergence of soil taxa (Arnold and Eswaran 2010), and in the case of the ASC, different Soil Orders. This approach was also suggested by Coffey (1912) and Brevik (1999), who argued that characters based on parent material increase the chance of identifying traceable linages.

Potential revision of Soil Orders

The poor resolution in the BC (Fig. 2) highlights the inability of the characters and structure used in the ASC to identify relationships between Soil Orders. The cladistic analysis did identify some natural Soil Orders. Organosols, which key out first, are rare in Australia because the continent is dominated by semi-arid and arid climatic regimes. As such, this Soil Order is confined to very cold and wet areas near Mt Kosciuszko and in north-western Tasmania.

Although the Podosols are found in similarly high-rainfall areas, owing to the hotter coastal climates and highly siliceous parent material, the organic matter is broken down quickly, creating chelates with iron, which leads to concentration of organic matter (i.e. 9:1 Bs horizon) and iron oxides (i.e. 10:1 Bh horizon) into B horizons. Conversely, Vertosols are very clayey (>35%) and found in semi-arid areas where 2:1 clay minerals (e.g. smectite) form, enabling shrinkage and swelling and distinct morphological characters (i.e. C.26:1, slickensides of lenticular peds).

This identification of natural Soil Orders in the ASC marks the first step to understanding the relationships between the Soil Orders. Interestingly, Isbell (2002) accurately captured soil at either end of the spectrum of parent material and rainfall (Fig. la), with the Podosols representing extremely siliceous soil with high average rainfall, and the Vertosols representing mafic soils with low average rainfall (Gray and Murphy 2002).

The remaining Soil Orders, with the exception of Organosols, Hydrosols and Rudosols, fall between these. The issue here is in the differentiation of these remaining Soil Orders. The problem is that the Chromosols, Ferrosols and Dermosols have no unique characters defining the soil taxa in their Suborders and Great Group tier. This lack of differentiating characters makes it impossible to discern the different Soil Orders and in turn the relationships between them. In any review, unique characters need to be used to distinguish these and the other Soil Orders in terms of their genesis.

Conclusions

The ASC (Isbell 2002) is an artificial key, the preferred model for classifications of soil around the world. This is because they can be used by a wide range of people. The issue considered here is not whether the ASC is a bad key, rather, whether it is a natural classification that can reflect soils genesis and thereby provide a set of unique character-states for each soil relationship. In natural classifications, the emphasis is on shared derived characteristics (soil-genesis relationships), not unique describers for each taxon (identification). The cladistic method reported here is a first attempt at testing the naturalness of the ASC.

From the cladistic analysis of the ASC, the following conclusions can be made. The ASC is not a natural classification; however, the Organosols, Podosols and Vertosols are natural Soil Orders. It could also be determined that because of the highly conflicting character-states in the Suborder and Great Group tiers, there is low or no confidence in soil Orders such as the Chromosols, Ferrosols and Dermosols because they rely exclusively on one to two characters located in the Order tier to group them into a single clade. A revised ASC based on relationships derived from soil genesis will provide pedology with a natural classification, one that would require little revision over time. Moreover, the revision of the ASC is also the first step in testing other soil classification systems around the world.

In terms of seeking areas where improvements in the ASC can be obtained, the revision of the Great Group tier characters is ideal. In identifying poor characters, we found that removal of continuous characters and soil colour is an area of promise in terms of revising the ASC. Improvements might also be made if soil colour is used more parsimoniously in seven of the 14 Soil Orders, because this is shown to achieve a reduction in the number of potential taxa and reduced character-state conflict.

http://dx.doi.org/10.1071/SRI4323

Acknowledgements

We thank David Edwards, John S. Wilkins and two anonymous reviewers for their critical and constructive comments, which have materially improved the readability and scientific merit of the manuscript.

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Gregory P. L. Miltenyi (A), Malte C. Ebach (A), and John Triantafilis (A,B)

(A) School of Biological, Earth and Environmental Sciences, UNSW Australia, Kensington, NSW 2052, Australia.

(B) Corresponding author. Email: j.triantafilis@unsw.edu.au

Table 1. Cladistic terms proposed by Wilkins and Ebach (2014)

Term               Cladistic term    Definition

Character-         Synapomorphy      A shared derived character
  state                                between at least two taxa
                                       indicating the presence of a
                                       shared common ancestor
Formism            Monophyly;        The grouping of taxa that are
  (formology)        natural group     more closely related to each
                                       other than to any other taxon
                                       (indicating a relationship
                                       between them)

Aformism           Non-monophyly     The grouping of taxa that are
                                       not more closely related to
                                       each other than to any other
                                       taxon

Radogram           Cladogram;        A visual representation of the
                     branching         relationships between the taxa
                     diagram;          analysis. It is the final
                     Consensus         result from the cladistic
                     tree; Tree        analysis

Rade               Clade             A group of taxa defined by a
                                       shared character

Character-state    Homoplasy         The reoccurrence of the same
  conflict                             character-state in multiple
                                    places throughout the radogram

Table 2. Australian Soil Orders and their defining characters
(Isbell 2002)

Soil Order          Ranking   Defining characters            No. of
                    in Soil                                 soil taxa
                      Key                                    in Soil
                                                            Order to
                                                            3rd tier

Organosols     OR      2      >0.4 m of organic                18
                                material within the upper
                                0.8 m

Podosols       PO      3      Bs horizon (iron                 13
                              compounds); Bhs horizon
                                (organic-aluminium and
                                iron compounds); Bh
                                horizon
                                (organic-aluminium
                                compounds)

Vertosols      VE      4      Clay field texture or            24
                                [greater than or equal
                                to] 35% clay through the
                                solum; when dry, open
                                cracks [greater than or
                                equal to] 5 mm occur;
                                slickenside and/or
                                lenticular peds occur

Hydrosols      HY      5      Saturated for at least           64
                                2-3 months in most years

Kurosols       KU      6      Clear and abrupt textural        35
                                B horizon; B2 horizon
                                strongly acid

Sodosols       SO      7      Clear and abrupt textural        55
                                B horizon; B2 horizon
                                sodic and not strongly
                                acid

Chromosols     CH      8      Clear and abrupt textual         70
                                B horizon; B2 horizon not
                                sodic and not strongly
                                acid

Calcarosols    CA      9      Calcareous through the           43
                                solum; carbonate
                                accumulation pedogenic

Ferrosols      FE     10      B2 horizon has a free            25
                                iron oxide content >5% Fe

Dermosols      DE     11      Major part of B2 horizon         70
                                structure more developed
                                than weak

Kandosols      KA     12      Maximum clay content in          64
                                B2 horizon exceeds 15%;
                                B2 horizon massive or
                                only weak structure or
                                grade

Rudosols       RU     13      Minimal developed A1             21
                                horizon; presence of <10%
                                of B horizon material in
                                fissures in the parent
                                rock; A1 horizon apedal
                                or only weakly
                                structured; little to no
                                textural or colour change
                                in the solum

Tenosols       TE     14      Other soils                      54

                                                            Total = 556

Table 3. List of the soil characters used in the base
cladogram Order characters, bold; Suborder characters,
italic, Great Group characters, plain

Cladogram    Character     Characters used in the         Character-
                           base cladogram                 states

2                0       >0.4 m of organic material     Absent (0),
                           within the upper 0.8 m         present (1)
                           (0.4 can be cumulatively
                           taken)#

2                1       Organic materials              Absent (0),
                           extending from the surface     present (1)
                           to a min. depth of 0.1 m
                           that directly overlies
                           rock or other hard
                           surfaces (e.g. gravel,
                           humus)#

3                2       75% of the volume of           Absent (0),
                           organic material is peat@      Fibric (1),
                                                          Hemic (2),
                                                          Saparic (3)

4/6/8            3       More or less freely            Absent (0),
                           drained and never              present (1)
                           saturated for >7 days, and
                           contain organic materials
                           that occur as in
                           definition (ii) for
                           Organosols (extending from
                           the surface to min. depth
                           of 0.1 m)

4                4       Sulfuric materials within      Absent (0),
                           the upper 1.5 m of the         present (1)
                           profile

4                5       Sulfidic materials within      Absent (0),
                           the upper 1.5 m of the         present (1)
                           profile

4                6       At least some part of the      Absent (0),
                           B or the BC horizon            present (1)
                           calcareous

4                7       Major part of the organic      Absent (0),
                           material not calcareous        present (1)
                           but not strongly acid

4                8       Major part of the organic      Absent (0),
                           material strongly acid         present (1)

2                9       Bs horizon: visible            Absent (0),
                           dominance of iron              present (1)
                           compounds#

2               10       Bh horizon: organic-           Absent (0),
                           aluminium compounds#           present (1)

3/6/8           11       Free drainage, no              Absent (0),
                           restriction to through-        present (1)
                           drainage in the B horizon
                           or whole substrate@

3               12       B horizon weakly coherent      Absent (0),
                           and porous@                    present (1)

3               13       Often boringly coloured        Absent (0),
                           and lack evidence of           present (1)
                           seasonal reduction@

3/6/8           14       Short-term saturation in       Absent (0),
                           the B horizon@                 present (1)

3/6/8           15       Saturation not sufficient      Absent (0),
                           to reduce or remove            present (1)
                           significant amounts of the
                           accumulated iron@

3               16       May be great accumulation      Absent (0),
                           of organic compounds and       present (1)
                           less iron in the zone of
                           maximum saturation@

3/6/8           17       Long-term saturation in        Absent (0),
                           the B horizon@                 present (1)

3/6/8           18       Saturation sufficient to       Absent (0),
                           reduce most iron compounds     present (1)
                           and move them out of the B
                           horizon; hence, Bh
                           horizons are usually
                           prominent@

4               19        With pipey B Horizon          Absent (0),
                                                         present (1)

4               20       With only a Bs Horizon         Absent (0),
                                                          present (1)

4               21       With only a Bhs Horizon        Absent (0),
                                                          present (1)

4               22       With only a Bh horizon         Absent (0),
                                                          present (1)

4               23       With a Bh and Basi horizon     Absent (0),
                                                          present (1)

2               24       Clay field texture or          Absent (0),
                           [greater than or equal to]     present (1)
                           35% clay throughout the
                           solum except for thin,
                           surface crusty horizons
                           [less than or equal to]
                           0.03 m thick#

2               25       When dry, open cracks          Absent (0),
                           occur at some time in most     present (1)
                           years, these are >5 mm
                           wide and extend upward to
                           the surface or to the base
                           of any plough layer, self-
                           mulching horizon or thin,
                           surface crusty horizon#

2               26       Slickensidc and or             Absent (0),
                           lenticular peds occur at       present (1)
                           some depth in the solum#

3               27       May be indicated by the        Absent (0),
                           presence of mottling and       present (1)
                           grey colours@

3/7/8           28       Dominant colour class in       Absent (0),
                           the major part of the          present (1)
                           upper 0.5 m of the B2
                           Horizon is Red@

3/7/8           29       Dominant colour class in       Absent (0),
                           the major part of the          present (1)
                           upper 0.5 m of the B2
                           Horizon is Brown@

3/7/8           30       Dominant colour class in       Absent (0),
                           the major part of the          present (1)
                           upper 0.5 m of the B2
                           Horizon is Yellow@

3/7/8           31       Dominant colour class in       Absent (0),
                           the major part of the          present (1)
                           upper 0.5 m of the B2
                           Horizon is Grey@

3/7/8           32       Dominant colour class in       Absent (0),
                           the major part of the          present (1)
                           upper 0.5 m of the B2
                           Horizon is Black@

4               33       Soils with a surface that      Absent (0),
                           is moderately to strongly      present (1)
                           self-mulching; when the
                           soil is dry the
                           self-mulching layer should
                           be [greater than or equal
                           to] 10 mm think

4               34       Initial drying may form a      Absent (0),
                           thin surface flake which       present (1)
                           readily disintegrates to a
                           mulch on further drying

4               35       A pedal (stronger than         Absent (0),
                           weak grade, commonly           present (1)
                           blocky or polyhedral) A
                           horizon that is either not
                           or only weakly
                           self-mulching, and there
                           is no surface crusty
                           horizon

4               36       After wetting and drying,      Absent (0),
                           may form a thin, 5-10-mm       present (1)
                           surface flake, which
                           cracks into irregular
                           polygons 0.03-0.1 m
                           diameter

4               37       Massive or weakly              Absent (0),
                           structured surface crusty      present (1)
                           horizon [less than or
                           equal to] 0.03 m thick,
                           often of lighter texture
                           (lower clay content) than
                           underlying pedal clay
                           (blocky or polyhedral),
                           which is not self-mulching

4               38       Massive or weak blocky         Absent (0),
                           (usually >0.05 m peds) A       present (1)
                           horizon and no surface
                           crusty horizon

2/6/8           39       Profile saturated for at       Absent (0),
                           least 2-3 months in most       present (1)
                           years#

3/6/8           40       Inundated by saline tidal      Absent (0),
                           water@                         infrequent
                                                          water table
                                                          (1),
                                                          infrequent
                                                          storms (2),
                                                          frequent (3)

3/6/8           41       Fresh water inundation@        Absent (0),
                                                          uncommon
                                                          (1), common
                                                          (2)

3/6/8           42       Soils of the saline playa      Absent (0),
                           lakes@                         present (1)

3/6/8           43       Saline groundwater table       Absent (0),
                           (salinised soils)@             present (1)

3/6/8           44       Other soils with a             Absent (0),
                           seasonal or permanent          mottled (1),
                           water table@                   whole
                                                          coloured (2)

4               45       Dominated by organic           Absent (0),
                           material to a depth of 0.5     present (1)
                           m

2               46       Calcareous through the         Absent (0),
                           solum#                         present (1)

4               47       Dominant sediment size to      Not
                           depth of 0.5 m                 described
                                                          (0), clay
                                                          (1), sand
                                                          (2), silt
                                                          (3)

4               48       A gypsic horizon occurs        Absent (0),
                           within the upper 0.5 m of      present (1)
                           the profile

4               49       Colour of major part of        Absent (0),
                           the upper 0.5 m of the         mottled (1),
                           profile                        whole colour
                                                          (2)

4               50       Major part of the upper        Absent (0),
                           0.5 m of the profile           present (1)
                           consists of materials
                           dominated (>50%) by halite
                           crystals

4               51       B horizon containing or        Absent (0),
                           directly underlain by          present (1)
                           ferricrete, a petroferric
                           horizon or a
                           petroreticulite horizon

2               52       Clear or abrupt textural B     Absent (0),
                           horizons#                      present (1)

2/5/8           53       Acidity of the major part      Not
                           of the upper 0.2 m of the      described
                           B2 horizon (or the major       (0), strong
                           part of the entire B2          acid (1),
                           horizon if <0.2 m thick)#      not sodic
                                                          and not
                                                          strong acid
                                                          (2), sodic
                                                          and not
                                                          strong acid
                                                          (3)

2               54       B2 horizon with structure      Absent (0),
                           more developed than weak       present (1)
                           throughout the major part
                           of the horizon#

2               55       Major part of the B2           Absent (0),
                           horizon massive or has         present (1)
                           only weak grade of
                           structure#

2               56       Max. clay content in some      Absent (0),
                           parts of the B2 horizon        present (1)
                           >15% (c.g. heavy sand
                           loams)#

2               57       A peaty horizon#               Absent (0),
                                                          present (1)

2               58       At or A2 horizon directly      Absent (0),
                           overlies a calcrete pan,       A1 horizon
                           hard unweathered rock or       (1), A2
                           other hard material; or        horizon (2),
                           partially weathered or         A humose,
                           decomposed rock or             melacic, or
                           saprolite; or                  melanic
                           unconsolidated mineral         horizon, or
                           materials#                     conspicuously
                                                          bleached A2
                                                          horizon (3)

2               59       A tenic B horizon or a B2      Absent (0),
                           horizon with [less than or     present (1)
                           equal to] 15% clay (SL),
                           or a transitional horizon
                           (ClB) occurring in
                           fissures in the parent
                           rock or saprolite, which
                           contains 10-50% of the B
                           horizon material
                           (including pedogenic
                           carbonate)#

2               60       A ferric or bauxitic           Absent (0),
                           horizon >0.2 m thick#          present (1)

2               61       A calcareous horizon >0.2      Absent (0),
                           m thick#                       present (1)

2               62       Minimal development of an      Absent (0),
                           At horizon#                    present (1)

2               63       Presence of [less than or      Absent (0),
                           equal to] 10% of B horizon     present (1)
                           material (including
                           pedogenic carbonate) in
                           fissures in the parent
                           rock or saprolite#

2               64       Soils are apedal or only       Absent (0),
                           weakly structured in the       present (1)
                           At horizon and show no
                           pedological colour changes
                           apart from the darkening
                           of an A1 horizon#

2               65       Texture or colour change       Absent (0),
                           with depth unless              present (1)
                           stratified or buried soils
                           are present#

4               66       Exchangeable Ca/Mg ratio       Absent (0),
                           <0.1 in the major part of      present (1)
                           the B2 horizon

4               67       Major part of the upper        Absent (0),
                           0.2 m of the B2 horizon is     present (1)
                           sodic

4/5/8           68       Base status in the major       Absent (0),
                           part of the B2 horizon         dystrophic
                                                          (1),
                                                          mesotrophic
                                                          (2),
                                                          eutrophic
                                                          (3)

4               69       Red-brown hardpan either       Absent (0),
                           within or directly             present (1)
                           underlying the B horizon

4               70       B horizon that is not          Absent (0),
                           calcareous and which           present (1)
                           directly overlies a
                           calcrete pan

4               71       Upper 0.2 m of the B2          Absent (0),
                           horizon (or the B2 horizon     present (1)
                           if it is <0.2 m thick) has
                           a strong blocky or
                           polyhedral structure in
                           which average ped size is
                           usually in the range 5-20
                           mm

4               72       Very weak adhesion between     Absent (0),
                           peds (when dry it is very      present (1)
                           easy to insert a spade
                           into the upper B2 horizon)

4               73       Usually high salt contents     Absent (0),
                           resulting in weak dry          present (1)
                           strength and a bulk
                           density of1.3 t [m.sup.-3]
                           or less

4               74       In some soils the B2           Absent (0),
                           horizons may be weakly         present (1)
                           subplastic

4               75       Fine earth effervescence       Absent (0),
                           (1 m HCI) throughout the       present (1)
                           solum

4               76       Major part of the upper        Absent (0),
                           0.2 m of the B2 horizon is     present (1)
                           mottled

4/5/8           77       ESP in the major part of       Absent (0),
                           the upper 0.2 m of the B2      6-15 (1),
                           horizon                        15-25 (2),
                                                          >25 (3)

2               78       Soils with strongly            Absent (0),
                           subplastic upper B2            present (1)
                           horizons#

4               79       Major part of the B2           Absent (0),
                           Horizon is strongly            present (1)
                           subplastic

4               80       Soils in which the             Absent (0),
                           carbonate is evident only      present (1)
                           as a slight to moderate
                           effervescence (1 m HCI)
                           and/or contain <2% soft,
                           finely divided carbonate

4/5/8           81       Soils with a calcareous        Absent (0),
                           horizon containing hard        0-20% (1),
                           calcrete fragments and/or      20-50% (2),
                           carbonate nodules or           >50% (3)
                           concretions and/or
                           carbonate coated gravel

4               82       Soils with a calcareous        Absent (0),
                           horizon containing >20% of     present (1)
                           mainly soft, finely
                           divided carbonate

2               83       Carbonate accumulate must      Absent (0),
                           be judged to be pedogenic#     present (1)

3               84       Soils in which the             Absent (0),
                           profile, with the possible     present (1)
                           exception of the A
                           horizon, is calcareous,
                           either loose or only
                           weakly coherent both moist
                           and dry, and consists
                           dominantly of sand-size
                           fragments of shells and
                           other aquatic skeletons@

3               85       Dominantly consist             Absent (0),
                           ofgypsum crystals, which       present (1)
                           are sand-sized or finer@

4               86       Argic horizon within the B     Absent (0),
                           horizon                        present (1)

4               87       Major part of B horizon        Absent (0),
                           has a grade of structure       present (1)
                           that is stronger than weak

4               88       Overlie a hard siliceous       Absent (0),
                           pan                            present (1)

4               89       Directly overlie hard rock     Absent (0),
                                                          present (1)

4               90       Directly overlie partially     Absent (0),
                           weathered or decomposed        present (1)
                           rock or saprolite

4               91       Directly overlie marl          Absent (0),
                                                          present (1)

4               92       Directly overlie               Absent (0),
                           unconsolidated mineral         present (1)
                           material

4               93       Other soils with a             Absent (0),
                           calcareous horizon             present (1)

2               94       Major part of B2 horizon       Absent (0),
                           has a free iron content        present (1)
                           >5% Fe in fine earth
                           fraction#

4               95       Bulk density appears           Absent (0),
                           relativity low                 present (1)

4               96       Soil with a thick iron-        Absent (0),
                           pan occurring within or        present (1)
                           directly underlying the B
                           horizon

4               97       Soils with massive to          Absent (0),
                           weakly structured (10 mm       present (1)
                           subangular blocky parting
                           to finer granules) B
                           horizons that are very
                           porous with a weak
                           consistence strength when
                           moist

3               98       Soils that are highly          Absent (0),
                           saline@                        present (1)

3               99       Frequently stratified but      Absent (0),
                           do not have permanent or       present (1)
                           seasonal water table and
                           do not show any evidence,
                           such as mottling, or
                           episodic wetting by
                           groundwater@

3               100      At least the upper 0.5 m       Absent (0),
                           of the profile is not or       present (1)
                           only slightly gravelly
                           (<10% >2 mm) throughout,
                           either loose or only
                           weakly coherent both moist
                           and dry, and the texture
                           is sandy (up to 10%)@

3               101      Aeolian cross-bedding may      Absent (0),
                           be present but there is        present (1)
                           little if any evidence of
                           other stratification or
                           buried soils@

3               102      At least the upper 0.5 m       Absent (0),
                           of the profile consists        not gravel
                           dominantly of                  (1),
                           unconsolidated mineral         distinct not
                           material@                      gravel (2),
                                                          gravel (3)

4               103      Other soils that have          Absent (0),
                           formed in tephric              present (1)
                           materials that may be
                           visibly stratified

4               104      Ferric horizon and which       Absent (0),
                           overlies ferricrete,           present (1)
                           petroreticulite or
                           petroferric horizon

3               105      Soils which have a peaty,      Absent (0),
                           humose, melacic or melanic     present (1)
                           horizon@

3               106      An unbleached A2 horizon       Absent (0),
                           may be present between the     bleached (1)
                           dark surface horizons and
                           the substrate materials@

3               107      Other soils with peaty,        Absent (0),
                           humose, melacic or melanic     present (1)
                           horizons@

3               118      A conspicuously bleached A     Absent (0),
                           2 horizon is present@          present (1)

3               109      Soils with a ferric or         Absent (0),
                           bauxitic horizon (nodules      present (1)
                           or concretions) [greater
                           than or equal to] 0.2 m
                           thick and occupies >50% of
                           the solum depth; the solum
                           depth excludes cemented
                           layers@

3               110      Soils with a calcareous        Absent (0),
                           horizon (consisting of         present (1)
                           >20% pedogenic carbonate)
                           [greater than or equal to]
                           0.2 m thick@

4               111      A tenic B horizon              Absent (0),
                                                          present (1)

4               112      Andie properties and have      Absent (0),
                           formed in basaltic tephric     present (1)
                           materials that may be
                           visibly stratified

Note: bold indicated with#.

Note: italic indicated with#.

Table 4. List of soil matrices used in the study

Cladogram name                             No.    No. of       No. of
                                                 soil taxa   characters

Base cladogram (BC)                         1       556         113
BC without Orders characters                2       556          87
BC without Suborders characters             3       150          57
BC without Great Groups characters          4       72           37
BC without continuous characters            5       556         109
BC without non-morphological characters     6       556         103
BC without colour characters                7       287         108
BC without continuous,                      8       287          94
  non-morphological
  and colour characters
BC without Organosols                       9       538         107
BC without Podosols                        10       543          99
BC without Vertosols                       11       532         103
BC without Hydrosols                       12       492         100
BC without Kurosols                        13       521         112
BC without Sodosols                        14       501         110
BC without Chromosols                      15       486         112
BC without Calcarosols                     16       513         109
BC without Ferrosols                       17       531         112
BC without Dermosols                       18       486         113
BC without Kandosols                       19       492         110
BC without Rudosols                        20       535         105
BC without Tenosols                        21       502         101
BC without Hydrosols, Kurosols,            22       183          72
  Sodosols,  Chromosols, Ferroso],
  Dermosols, Tenosols

Table 5. Confidence levels of the Soil Orders generated by
the bootstrapping of different cladistics analyses with
altered character

BC, base cladogram

Soil order         BC   Orders   Suborders   Great    Continuous
                                             Groups

Organosols         64     --        53         89         64
Podosols           58     --        89         42         57
Vertosols          61     --        93         39         47
Hydrosols (HY)     --     --        --         57         --
Kurosols (KU)      --     --        --         --         --
Sodosols (SO)      --     --        --         --         --
Chromosols (CH)    --     --        --         --         --
Calcarosols        29     --        23         72         26
Ferrosols (FE)     --     --        19         5          2
Dermosols (DE)     --     --        --         9          --
Kandosols          2      --        20         37         12
Rudosols           33     --        27         56         28
Tenosols TE)       --     --        --         --

Soil order          BC without:    Colour    Continuous,     HY, K.U,
                       Non-                     non-         SO, CH,
                   morphological            morphological   FE, DE, TE
                                             and colour       Orders

Organosols              53           67          56             86
Podosols                67           51          65             61
Vertosols               51           79          94             70
Hydrosols (HY)          --           --          --             --
Kurosols (KU)           --           --          --             --
Sodosols (SO)           --           --          --             --
Chromosols (CH)         --           --          --             --
Calcarosols             15           17          23             63
Ferrosols (FE)          12           31          29             --
Dermosols (DE)          --           --           -             --
Kandosols               19           8           25             50
Rudosols                21           27          23             85
Tenosols TE)            --           --          --

Table 6. Cladogram lengths and calculated consistency index
(Cl) and retention index (Rl) of the cladogram, including
the base cladogram, cladogram without Suborder and Great
Groups characters and unsuitable characters

Cladogram                                     Radogram    CI      RI
                                               length

1     Base cladogram (BC)                       680      0.196   0.753
2     BC without Order characters               369      0.282   0.763
3     BC without Suborder characters            185      0.373   0.680
4     BC without Great Group characters          83      0.602   0.781
5     BC without continuous characters          530      0.288   0.780
6     BC without non-morphological              619      0.189   0.755
        characters
7     BC without colour characters              402      0.313   0.724
8     BC without continuous, non-               279      0.358   0.754
        morphological and colour characters
9     BC without Organosols                     657      0.190   0.752
10    BC without Podosols                       679      0.175   0.740
11    BC without Vertosols                      642      0.192   0.751
12    BC without Hydrosols                      666      0.168   0.720
13    BC without Kurosols                       645      0.205   0.752
14    BC without Sodosols                       623      0.205   0.748
15    BC without Chromosols                     765      0.173   0.656
16    BC without Calcarosols                    628      0.205   0.754
17    BC without Ferrosols                      652      0.202   0.757
18    BC without Dermosols                      599      0.222   0.764
19    BC without Kandosols                      599      0.217   0.760
20    BC without Rudosols                       653      0.188   0.750
21    BC without Tenosols                       607      0.199   0.762
22    BC without Hydrosols, Kurosols,           222      0.360   0.809
        Sodosols, Chromosol, Ferrosols,
        Dermosols and Tenosols
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Article Details
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Author:Miltenyi, Gregory P.L.; Ebach, Malte C.; Triantafilis, John
Publication:Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Oct 1, 2015
Words:10100
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