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The expected result of land evaluation is the generation of classes indicating land potential. Land use planners and land managers use these outputs to develop reasonable strategies for the improvement, use, and management of the land. In a worldwide context, there have been a range of classification systems to determine land potential, which depend on different specific situations. However, the most commonly used and most standardised are two systems, one generated by the FAO (1976, 1983, 1984, 1985, 1991a, 1993) and one by the USDA (Klingebiel and Montgomery, 1961; USDA, 2001a; 2001b; 2005; 2010a).

3.6.1 The USDA land capability system

In the USDA system (Klingebiel and Montgomery, 1961; USDA, 2010a), the land potential is classified by land capability. Land capability classification is a system that groups soils, primarily based on their capability to produce common cultivated crops and pasture plants without deterioration over a long time. The land capability classification comprises three major categories: capability classes, capability sub- classes, and capability unit (USDA, 2010a).

Capability classes: are groups of land capability into areas having the same relative

degree of hazard or limitation. The risks of soil damage or limitation in use become progressively greater from class I (1) to class VIII (8). The capability classes are presented with more detailed information on a soil map. The classes show the location, area, and general land suitability for agricultural use. Land in classes 1-4 are suited to cultivation and other uses, whereas classes 5-8 are limited in use, generally not suited to cultivation.

Capability subclasses: are groups of capability units which have the same major conservation (such as e: erosion and run-off, w: excess water, s: root-zone limitations, and c: climatic limitations). The capability subclass provides information on the kind of conservation problem or limitations involved. The integration of class with subclass provide information about both the degree of limitation and kind of problem involved

Capability units:are groups of individual soil mapping units having similar potential and continuing limitations or hazards. In a capability unit, soils are sufficiently uniform to (1) produce similar planted crops and pasture plants using similar management practices; (2) require similar conservation treatment and management in the same kind and condition of vegetative cover, (3) have comparable potential productivity.

Below is a brief description of classes and sub-classes in the USDA system (USDA, 2010a, wp):

“Class 1: have few limitations that restrict the use. Soils in this class are suited to a wide range of plants and may be used safely for cultivated crops, pasture, range, woodland, and wildlife.

Class 2: have some limitations that reduce the choice of plants or require moderate conservation practices. Require careful management, including conservation practices, to prevent deterioration or to improve air and water relations when the soils are cultivated. The limitations are few and the practices are easy to apply. The soils may be used for cultivated crops, pasture, range, woodland, or wildlife food and cover.

Class 3: have severe limitations that reduce the choice of plants or require special conservation practices, or both. Soils in class 3 have more restrictions than those in class 2 and, when used for cultivated crops, the conservation practices are usually more difficult to apply and to maintain. They may be used for cultivated crops, pasture, woodland, range, or wildlife food and cover.

Class 4: have very severe limitations that restrict the choice of plants, require very careful management, or both. The restrictions in use for soils in class 4 are greater than those in class 3 and the choice of plants is more limited. When these soils are cultivated, more careful management is required and conservation practices are more difficult to apply and maintain. Soils in class 4 may be used for crops, pasture, woodland, range, or wildlife food and cover.

Class 5: have little or no erosion hazard but have other limitations impractical to remove that limit their use largely to pasture, range, woodland, or wildlife food and cover. Soils in class 5 have limitations that restrict the kind of plants that can be grown

and that prevent normal tillage of cultivated crops. They are nearly level but some are wet, are frequently overflowed by streams, are stony, have climatic limitations, or have some combination of these limitations.

Classes 6: have severe limitations that make them generally unsuited to cultivation and limit their use largely to pasture or range, woodland, or wildlife food and cover. Physical conditions of soils placed in class 6 are such that it is practical to apply range or pasture improvements, if needed, such as seeding, liming, fertilizing, and water control with contour furrows, drainage ditches, diversions, or water spreaders.

Class 7: have very severe limitations that make them unsuited to cultivation and that restrict their use largely to grazing, woodland, or wildlife. Physical conditions of soils in class 7 are such that it is impractical to apply such pasture or range improvements as seeding, liming, fertilizing, and water control with contour furrows, ditches, diversions, or water spreaders.

Class 8: have limitations that preclude their use for commercial plant production and restrict their use to recreation, wildlife, or water supply or to aesthetic purposes. Soils and landforms in class 8 cannot be expected to return significant on-site benefits from management for crops, grasses, or trees, although benefits from wildlife use, watershed protection, or recreation may be possible.

Subclass (e) erosion: is made up of soils where the susceptibility to erosion is the dominant problem or hazard in their use. Erosion susceptibility and past erosion damage are the major soil factors for placing soils in this subclass.

Subclass (w) excess water: is made up of soils where excess water is the dominant hazard or limitation in their use. Poor soil drainage, wetness, high water table, and overflow are the criteria for determining which soils belong in this subclass.

Subclass (s) soil limitations: within the rooting zone includes, as the name implies, soils that have such limitations as shallowness of rooting zones, stones, low moisture- holding capacity, low fertility difficult to correct, and salinity or sodium.

Subclass (c) climatic limitation: is made up of soils where the climate (temperature or lack of moisture) is the only major hazard or limitation in their use”.

Concisely, the USDA land potential classification is presented in Figure 3.1

Category

Order Class Sub-class Unit

Classes 1 Unit 1

Arable land  Classes 2 Unit 2

Classes 3 Sub-class (e) Unit 3

Classes 4  Sub-class (w)  Unit 4

Classes 5 Sub-class (s) Unit 5

Generally not suited

to cultivation  Classes 6Classes 7 Sub-class (c) Unit 6Unit 7 Classes 8

Figure 3.1: The USDA land potential classification systems

Figure 3.1 shows that capability classification in the USDA system includes two major groups. Class 1 to class 4 are suited to agricultural production, and class 5 to class 8 are not suited to cultivation with respect to the decline in the capability from class 1 to class 8. This description reveals that the land capability in the USDA system is based on the purpose of the agricultural use.

In summary, the USDA land potential classification system includes three main levels: classes, sub-classes, and units. The level of classes has 8 ratings, ranging from few limitations (class 1) to very many limitations (class 8) that restrict the use for arable purposes. The level of sub-classes has 4 key limitations being erosion, excess water, soil limitations, and climate limitations. The capability of each class is impacted by subclasses, and as a hierarchy, the capability of sub-classes is influenced by units.

3.6.2 The FAO land suitability classification system

In the FAO (1976, p. 22; 1983; 1984; 1985; 1991a; and 1993) system, the land potential is classified by land suitability. Land suitability classification includes four categories of decreasing generalization: land suitability orders (kinds of suitability), land suitability classes (degrees of suitability within orders), land suitability sub-classes (kinds of

limitation or main kinds of improvement measures required, within classes), land suitability unit (minor differences in required management within subclasses).

Land suitability orders indicate whether land is assessed as suitable or not suitable for the use under consideration. There are two orders represented: S and N.

S (suitable): Land on which sustained use of the kind under consideration is expected to yield benefits which justify the inputs, without unacceptable risk of damage to land resources.

S1 (highly suitable): Land having no significant limitations to sustained application of a given use, or only minor limitations that will not significantly reduce productivity or benefits and will not raise inputs above an acceptable level.

S2 (moderately suitable): Land having limitations which in aggregate are moderately severe for sustained application of a given use; the limitations will reduce productivity or benefits and increase required inputs to the extent that the overall advantage to be gained from the use, although still attractive, will be appreciably inferior to that expected on Class S1 land.

S3 (marginally suitable): Land having limitations which in aggregate are severe for sustained application of a given use and will so reduce productivity or benefits, or increase required inputs, that this expenditure will be only marginally justified.

N (not suitable): Land which has qualities that appear to preclude sustained use of the kind under consideration.

N1 (currently not suitable): Land having limitations which may be surmountable in time but which cannot be corrected with existing knowledge at currently acceptable cost; the limitations are so severe as to preclude successful sustained use of the land in the given manner.

N2 (permanently not suitable):Land having limitations which appear as severe as to preclude any possibilities of successful sustained use of the land in the given manner.

Land suitability sub-classesreflect kinds of limitations, e.g. moisture deficiency (m), erosion hazard (e)

Land suitability units are subdivisions of a subclass. All the units within a subclass have the same degree of suitability at the class level and similar kinds of limitations at the subclass level. Suitability units are distinguished by Arabic numbers following a hyphen, e.g. S2e-1, S2e-2”.

Examples of land suitability sub-classes:

S2e: land assessed as S2 on account of limitation of erosion hazard S2w: land assessed as S2 on account of inadequate availability of water N2e: land assessed as N2 on account of limitation of erosion hazard Shortly, the FAO land potential classification is shown as Figure 3.2

Category

Order Class Sub-class Unit

S1 S2w S2e-1

Suitable (S)  S2  S2e  S2e-2

S3 S2ew S2e-3

etc etc etc

N1w

Not suitable (N)  _N1  N1e

N2 etc

Figure 3.2: The FAO land potential classification system

Associated with the two main FAO and USDA land potential classification systems, many researchers also modify or develop new ones, to be suitable to particular

circumstances, such as those by Rowe et al. (1981), The State Planning Commission

(1989), Grose (1999), The ACT Parliamentary Counsel (1999), CEMA (2006). In general, most are based on these key principles:

(1) Land potential is indicated by the three major categories, including units, sub- classes, and potential classes to arrive at a higher level of order (arable land or not suitable to cultivation)

(2) Land potential classes decrease gradually corresponding to a gradual increase in physical, socio-economic, and management limitations, these constitute constraints for the use of the land resources

(3) The number of land potential classes is flexibly modified to suit different situations.

To summarize, the FAO land potential classification system also includes three key categories (class, sub-class, and unit) likes the USDA system. However, at the class level the FAO system distinguishes separate suitable (S) and non-suitable classes (N). Particularly, the non-suitable class is divided into two more sub-classes, currently and permanently not suitable.