Time as a Factor
B- Yellow apedal Sandy loam subsoil
2.5 Classifying Soils
Thus far the basic soil forming processes and main physical properties have been presented which will enable the wide range of soils that occur in global sugar industries to be simply classified through a description of color, texture, structure, depth and other features as follows:
Red and grey medium-grained sands
Black, heaving clays which are occasionally saline
Shallow grey sandy loams on steeply sloping land in parts of the coastal hinterland
Dark alluvial soils
Neutral red clays
Wet gley soils, clayey and sandy
Moderate to very acid, porous, red and yellow soils, some aluminous and/or humic.
It has also been shown that differences between these types of soils can be attributed to various soil forming factors such as the parent material, rainfall, temperature, topography, drainage and
biological conditions. The length of time that the parent materials have been exposed to the combined actions of these environmental factors or weathering processes is also important. While using a string of words to describe a soil or groups of soils in terms of color and other properties appears to be a simple and easy way to classify soils, using a description such as ‘a black, blocky structured, heavy clay soil, rather shallow, overlying shale rock’, is far too cumbersome and
subjective to describe the soil to another person. The many permutations that can be made up from the four properties described and varied deductions one can make about a soil from a description is the main reason for wanting to classify soils into easily recognizable groups.
Systems of soil classification
Unlike plants and animals that can be classified as separate entities, the global soil mantel is a continuum. Soil is a three dimensional system with properties that mirror the influence of climate, flora and fauna, and the topography formed from the underlying parent material over a variable period of time. Grouping and naming of soils has developed and changed considerably over the years.
Historically, the Russian school classified soils on the basis of climate as climate is the dominant soil forming factor in that country. However, grouping soils of sugarcane areas located mainly in the tropics and subtropics into podsols (soils of the coniferous forests), pedalfers (soils rich in aluminum and iron), pedocals (soils rich in calcium and magnesium) and desert soils, simply did not work out. A number of further soil classification systems were developed and reviewed by Butler (1980), such as the map of Africa (D’Hoore 1964), for former French colonies Duchaufour 1982), but they all ended up being mainly of academic value.
Only two systems of soil classification enjoy international recognition and are used in soil surveys needed in feasibility studies of new green fields sugarcane related projects. These are Soil Taxonomy, originally developed in the USA in 1938 and now in its 11th edition (USDA Soil Taxonomy) and the World Reference Base (WRB) system of classification, which is a revision of the FAO soil classification system (FAO 1974). A number of countries such as Australia, Brazil, India, Argentina and South Africa have developed their own national systems of classification that are best suited to their environmental conditions. Selected examples covering the principles and relative merits of some of these classification systems for sugarcane production are briefly discussed.
Soil taxonomy
Since 1938, when soil taxonomy was first conceived by soil survey staff of the US Department of Agriculture to group many thousands of soil series into meaningful, exclusive groups, the
classification has evolved though a number of stages or approximations, to the extent that the 11th edition of Soil Taxonomy was published in 2010. The system has changed from being a classification that between 1938 and 1960 was based mainly on soil forming factors as criteria for identification, to becoming almost totally generic, focusing on soil morphology in the form of topsoil and subsoil diagnostic horizons as criteria for classification.
Soil Taxonomy can be considered a hierarchical system which classifies soils into six levels of detail, from the most detailed level, soil series, to the broadest category, soil orders. The 12 Soil Orders at the broadest level are summarized in Box 2.4. Further details may be found in USDA (1975, 1982), and the system is very well documented on the website
http://soils.cals.uidaho.edu/soilorders/orders.htm.
Box 2.4 Soil Taxonomy’s 12 soil orders are listed in the sequence in which they key out Gelisols − soils with permafrost within 2 m of the surface.
Histasols − contain a high organic matter content.
Spodosols − acid forest soils with accumulation of metal-humus complexes in subsoil.
Andisols − formed from volcanic ash.
Oxisols − highly weathered subtropical and tropical soils.
Vertisols − usually black cracking clays with high shrink/swell potential.
Aridsols − lime containing soils of arid environments.
Ultisols − strongly leached soils with clay accumulation in the subsoil with < 35 % base saturation.
Mollisols − grasslands soils with high base status.
Alfisols − moderately leached soils with a subsurface accumulation of clay with > 35 % base saturation.
Inceptisols − soils with weakly developed subsoils.
Entisols − soils with little or no morphological development.
World Reference Base (WRB) soil classification
This system was developed with international collaboration coordinated by the International Soil Reference and Information Centre (ISRIC) and sponsored by the International Union of Soil Science (IUSS) and the FAO via its Land and Water Development division. It is essentially a follow-up to the FAO Legend for the Soil Map of the World. The classification is based mainly on soil properties defined in terms of diagnostic horizons and characteristics which should be observable and
measurable as best as possible in the field. The WRB is a two-level classification system (FAO 1998):
The first level contains 30 Reference Soil Groups, e.g. Histols, Fluvisols, Luvisols.
The second level is based on the use of one or more qualifiers or adjectives, from a range of 121, that are used to subdivide a Reference Group. The qualifier may relate to color, chemical
conditions or a number of other properties, e.g. Leptic Umbrisols (weakly developed), Chromi- Vertic Luvisols (red colored with vertic properties). The subdivisions do not generally take into account factors such as climate, parent material, vegetation, depth of water table or drainage, and physiographic features such as slope, geomorphology or erosion, with the exception where they have affected soil morphology. These features can be used locally to define mapping phases, but they are not considered soil properties to be classified as such.
Driessen (1991) has gone a step further and simplified the 30 reference groups into nine sets using identifiers that incorporate the soil forming factors. The nine sets are shown Table 2.4 with examples of Reference Soil Groups that are likely to be encountered in sugar industries worldwide.
Table 2.4. Major WRB Reference Soil Groups simplified in nine sets.
Sets Dominant identifiers Reference
soil group Description
1 Organic soils Histosols Soils containing more than 20% soil organic matter (SOM).
2 Mineral formation due to human influence
Anthrosols Soils conditioned by human influence.
3 Mineral soils conditioned by parent material
Andosols Arenosols Vertisols
Young soils from volcanic deposits.
Sandy soils with weak profile development.
Dark cracking and swelling clays.
4 Mineral soils conditioned by topography – lowlands/high elevations
Fluvisols Gleysols Leptosols Regosols
Young alluvial derived soils.
Permanently or temporarily wet soil.
Shallow soil on hard rock or gravel.
Very limited profile development.
5 Mineral soils conditioned by limited age
Cambisols Weak to moderate profile development.
6 Mineral soils conditioned by climate in wet tropical and subtropical regions
Plinthosols Ferralsols Nitosols Acrisols Alisols Lixisols
Wet soils with subsurface plinthite horizon.
Deep highly weathered red to yellow soils.
Deep dark red, brown or yellow acid clays.
Soil with low activity clay and low base satn.
Soil with high subsurface activity clay and Al.
Soil with low activity clay and high base satn.
7 Mineral soils conditioned by climate in arid and semi-arid regions
Solonchaks Solonetz Gypsisols Calcisols
Strongly saline soils.
Soils with high clay subsurface.
Soils with an accumulation of gysum.
Soils with an accumulation of limestone.
8 Mineral soils conditioned by climate in steppes and steppe regions
Kastonozems Chernozems Phaeozems
Soils rich in SOM but with lime/gypsum in subsoil.
Black topsoil rich in SOM over subsoil with lime.
Dark topsoil rich in SOM, and leached subsoil.
9 Mineral soils conditioned by climate in sub-humid forest and grassland regions
Luvisols Planosols Podzols
Soils with subsurface high activity clays.
Topsoil over a bleached layer on impermeable gley.
Acid soil over with a back/brown/red subsoil.
(after Driessen 1991)
According to Rossiter (2001), the WRB has borrowed heavily from modern soil classification concepts, including Soil Taxonomy, the legend for the FAO Soil Map of the World 1988, the Référentiel Pédologique, and Russian concepts. He further recommends that the WRB is not intended to be used in detailed mapping, as many detailed soil properties that are important for land use and soil behavior are not specified in sufficient detail in the two levels of the WRB. For detailed mapping and site characterization, locally-defined soil series would need to be integrated into the system.
Australia
According to Bruce (1999), there are at least three common soil classifications used in the
Queensland sugar industry: (i) the official Australian Soil Classification System (Isbell 1996), (ii) the Great Soil Group and (iii) the Factual Key. In addition, there appears to be a fourth classification that is used by CSR in the lower Herbert River, Stone River, Lannercost and Seymour districts of
Queensland that is based mainly on color, texture, structure and depth (Wood et al. 2003). It is a more user friendly system for growers to follow as it is not based on the national hierarchical system consisting of five levels, ranging from the most general to the most specific level as shown by the following sequence: order, suborder, great group, subgroup, family. There are 14 orders at the top level: Anthroposols, Organosols, Podosols, Vertosols, Hydrosols, Kurosols, Sodosols, Chromosols, Calcarosols, Ferrosols, Dermosols, Kandosols, Rudosols and Tenosols. At the next level there are five suborder color categories: Red, Brown, Yellow, Grey and Black. The remaining soil orders have suborder categories that reflect unique characteristics of the given order (Isbell 1996).
While the philosophy that soils should form the basis for making decisions on good management practices is very strongly followed in the Australian sugar industry and implemented through a number of courses to growers through extension services, there appears to be little reference to using the national system of soil classification in pursuing soil-specific management guidelines.
South Africa
In KwaZulu-Natal, early soil scientists working on the soils of the South African sugar belt found a very strong similarity between the underlying rocks and the soils that were weathered from them.
Many soil profiles were examined, mainly in coastal cane land, and the findings were that dolerite always weathered to form heavy, deep red or shallow black clay; shale produced dark colored base rich clays and granite, and Table Mountain Sandstone produced shallow, coarse grained, loamy sands and somewhat deeper grey medium grained sandy soils respectively. Initially, individual soil series were described and named after the farm or locality at which they were first found, but later it was decided that soil associations, each consisting of soils derived from one parent material, would be the best mapping unit (Beater 1962).The soils were therefore classified according to the rocks from which they had weathered and these were all mapped. Initially, eight major soil parent materials were identified, namely: granite, Table Mountain Sandstone, Dwyka tillite, Ecca sediments, basalt, dolerite, Recent Sands and alluvium. Virtually the entire industry covering close to 400 000 ha has been surveyed in this way to a scale of 1:6 000.
In keeping with the global trend, and stimulated mainly by the publication of the USDA’s 7th Approximation in 1960, soil classification in the South African sugar industry also moved away from using a soil forming factor approach to a generic approach based on the properties of diagnostic horizons. The national Binomial System of Soil Classification that was developed during the late 1960s and early 1970s (MacVicar et al. 1977), was subsequently adopted in the local sugar industry for mapping and use in compiling Land Use Plans. It grouped soils into a general category called ‘soil form’ and a specific category referred to as ’soil series’. Initially 41 soil forms, each made up of a vertical sequence of diagnostic top and subsoil horizons, were described, but this was extended to 73 forms in the revised edition, published in 1991 (Soil Classification Working Group 1991). The names of the five topsoil and 20 subsoil horizons that regularly occur in the South African sugar industry and most other industries, and their relative positions in the soil profile, are
diagrammatically illustrated in Fig. 2.8.
Figure 2.8. Arrangement of master and diagnostic horizons according to the Binomial System (after MacVicar et al. 1977).
The main features of selected top and subsoil diagnostic horizons are summarized Table 2.5. More detailed definitions of these diagnostic horizons may be found in the main text of ’Soil Classification:
A Taxonomic System for South Africa’ (Soil Classification Working Group 1991).
Table 2.5 Simplified description of main diagnostic horizons used in the Binomial System.
HORIZON POSITION DOMINANT FEATURE
TOPSOILS (O and A)+
1. ORGANIC 0: rich in organic matter, at least 20 %.
2. HUMIC A: rich in humified organic matter, at least 3.5 %.
3. VERTlC A: high shrinking swelling clay (> 45 %) with slickensides; strongly blocky structure; wide vertical cracks when dry.
4. MELANIC: black non-swelling clay; moderate to strong blocky structure.
5. ORTHIC A: does not qualify as an organic, humic, vertic or melanic topsoil, although it can be darkened by organic matter.
SUBSOILS (B, E, G, R and C) Hard material or weathering rock
1. HARD PLlNTHlC B: ironpan, laterite or ferricrete.
2. HARDROCK B: continuous hard layer of rock that cannot be cut with a spade.
3. LITHOCUTANIC B: tongues or cones of soil penetrate into the weathering rock.
Structured 4. RED STRUCTURED B: uniform red blocky clay; red cutans and color variation due to faunal activity may be present.
5. PEDOCUTANIC B: moderately to strong blocky structure; prominent clayskins on most ped surfaces result in non-uniform or variegated colors.
6. PRISMACUTANIC B: prismatic or columnar structure; peds non-uniform color.
Uniformly colored, freely drained
7. RED APEDAL B: mainly uniform red color, but color variation due to faunal activity allowed.
8. YELLOW-BROWN APEDAL B: mainly uniform yellow and yellow-brown colors.
Non-uniformly colored, young soils
9. NEOCUTANIC B: has weakly developed structure, variegated colors or clayskins.
Restricted drainage 10. E HORIZON: is light grey or bleached when dry but is sometimes yellowish or pinkish when moist; may contain mottles due to periodic waterlogging.
11. G HORIZON: underlies an organic, vertic, melanic, orthic or an E horizon;
saturated with water for long periods of time unless drained.
12. SOFT PLlNTHlC B: reddish-brown, yellowish-brown mottles or iron.
Material recently transported by water, wind, gravity or man
13. STRATIFIED ALLUVIUM: contains stratifications caused by alluvial or colluvial deposition.
14. REGlC SAND: a recent deposit, usually Aeolian; little development other than a darkening of the topsoil by organic matter.
By determining the presence, sequence and depth of the diagnostic horizons the appropriate soil form may be determined by referring to a special soil form key in which form names are arranged in terms of the defined topsoil and subsoil horizons (see Annexure 1). For example, a soil with an orthic A over a red apedal B horizon will be classified as the Hutton form. Another soil profile with the same topsoil but a red structured B subsoil will be classified as the Shortlands form.
Box 2.5 Examples of representative forms in each of the main soil color groups using the South African Binomial System.
1. HUMIC AND ORGANIC SOILS (about 8 % of the cane belt)
Champagne organic / unspecified
Inanda humic A / red apedal B
2. BLACK STRUCTURED SOILS (about 13 % of the cane belt)
Arcadia vertic A / unspecified
Rensburg vertic A / G
Mayo melanic A / lithocutanic B 3. RED SOILS (about 18 % of the cane belt)
Hutton orthic A / red apedal B
Oakleaf (red) orthic A / red neocutanic B
Shortlands orthic A / red structured B 4. GREY SOILS (about 60 % of the cane belt)
Cartref orthic A / E / lithocutanic B
Glenrosa orthic A / lithocutanic B
Longlands orthic A / E / soft plinthic B
Katspruit orthic A / G
Kroonstad orthic A / E / G
In many ways, the Binomial System of classifying soils is analogous to that of classifying plants and animals according to their generic and specific names. Thus soils are classified by being allocated first to the appropriate soil form and then to the series, so a number of soil series belonging to the same soil form is like a family of soils. Full details of this system of classification, which has been adopted by a large part of the cane growing community in Southern Africa, to identify, name and manage soils, is given in the publication ’Identification and Management of Soils of the South African Sugar Industry’ (SASRI 1999). Briefly, the five steps that are recommended in this bulletin for using the Binomial System is as follows:
Step 1: Expose a profile of the soil to be identified by digging a pit, and mark off the master