Four categories, namely. Orders, Classes, Subclasses and Units are recognized in the FAO Framework classification:
I. Land Suitability Orders reflect kinds of suitability (i.e. Suitable and Not Suitable);II. Land Suitability Classes reflect degrees of suitability within Orders (i.e. S1, S2, S3, N1 and N2);
III. Land Suitability Subclasses reflect kinds of limitation or kinds of inputs and improvements required within Classes (i.e. S2d, etc.);
IV. Land Suitability Units reflect minor differences in the required management within Subclasses (e.g. S2d-2, etc.).
Each category retains its meaning with respect to any classification, whether of 'present' suitability, 'potential' suitability, the classification of 'provisionally-irrigable' land, or of 'irrigable' land.
I. LAND SUITABILITY ORDERS
There are two Orders: Suitable (S), and Not Suitable (N). Suitable land is land on which sustained use of the kind under consideration is expected to yield benefits which "justify the inputs and development costs, without unacceptable risk of damage to land resources. Not Suitable indicates that the land has qualities that appear to preclude sustained use of the kind under consideration. (Note that 'Suitable' in a 'provisionally-irrigable' classification can be used where information about the water supply and costs are uncertain, but in a classification of 'irrigable' land these uncertainties must be resolved.)
II. LAND SUITABILITY CLASSES
Within Orders, land suitability classes reflect degrees of suitability. The classes are given arabic numbers. The designations are:
Class S1, Highly Suitable; Class S2, Moderately Suitable; Class S3, Marginally Suitable; Class N1, Marginally Not Suitable; and Class N2, Permanently Not Suitable. Fewer or more Classes can be designated as appropriate. Only classes with significant economic differences should be distinguished.
Class S1 land has no significant limitations to sustained application of a given use. It may include minor limitations that will not reduce productivity, benefits or costs below the lower boundary set for the class.
Class S2 land has limitations which in aggregate are moderately severe for sustained application of a given use. The limitations may reduce physical productivity, benefits or costs compared with S1 land to a lower limit set for the class.
Class S3 land has limitations which in aggregate are severe for sustained application of a given use and will so reduce physical productivity, benefits or costs that the expenditure will only be marginally justified.
Note: A Conditionally Suitable (Sc) class may be used in certain instances of uncertainty, or where factors relevant to suitability are not yet fully understood. However, this is discouraged, and in classifying 'provisionally-irrigable' land the uncertainty about water supply and costs are assumed to be unresolved. The use of class Sc may seem convenient to an evaluator, but in excess it would greatly complicate the understanding of the classification by users.
Class N1 indicates that the land is marginally not suitable and has limitations that may be surmountable in time but which cannot be corrected with the existing knowledge or under present social conditions to give acceptable physical productivity. It indicates the economic limit defining the boundary between 'Suitable' and 'Not Suitable'.
Class N2 indicates that the land is permanently not suitable for the given use usually because of physical limitations. N2 land should be delineated early in the study to avoid unnecessary studies on land which will never be developed for the given use. In contrast, the boundary between the two Orders, S and N, may be variable over time due to changes in economic and social conditions.
See Section 2.4 for further discussion on the measures of suitability used to define classes. They are a physical measure of productivity, namely, a land productivity index, and two economic measures, namely, net farm income and net incremental irrigation benefit.
III. LAND SUITABILITY SUBCLASSES
These reflect kinds of requirements or limitations and are indicated by lower case letters with mnemonic significance e.g. S2m, S3xy, etc. There are no subclasses in Class S1. The lower case letters used must be decided before any evaluation; there is no unique list. Table 17, Chapter 6 presents a listing for classifications of 'provisionally-irrigable' and 'irrigable' land. For example, m indicates that the factor 'Water Quantity' is the reason for downgrading from S1 to S2m. The letters x and y indicate limitations due to salinity and sodicity, respectively. The number of Subclasses recognized and the limitations chosen to distinguish them will differ in classifications for different purposes.
IV. LAND SUITABILITY UNITS
These 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 requirements or limitations at the subclass level. The units differ from each other in their production characteristics or in minor aspects of their management requirements (often definable as differences in detail of their limitations or their position in relation to irrigation canals, etc.). Their recognition permits detailed interpretation at the farm planning level. Suitability units are distinguished by arable numbers following a hyphen, e.g. S2e-l, S2e-2, etc. There is no limit to the number of units recognized within a subclass.
Land suitability units should not be confused with land units. The former are a category of the classification structure and the latter represent an area of land.
The FAO Framework classification structure is shown in Table A.1 with the US Bureau of Reclamation irrigation classification system appended for comparison.
Table A.1 FAO FRAMEWORK LAND SUITABILITY CLASSIFICATION COMPARED WITH USBR SYSTEM
|
SUITABILITY CATEGORY |
FAO FRAMEWORK |
USBR CLASSIFICATION |
||
|
ORDERS |
S Suitable |
- |
||
|
N Not Suitable |
- |
|||
|
CLASSES |
S1 Highly Suitable |
Class 1 |
||
|
S2 Moderately Suitable |
Class 2 |
|||
|
S3 Marginally Suitable (Sc Conditionally Suitable) |
Class 3 |
|||
|
|
Class 4 Special Uses |
|||
|
|
Class 5 (requiring further study to determine whether suitable or not) |
|||
|
N1 Marginally Not Suitable |
- |
|||
|
N2 Permanently Not Suitable |
Class 6 |
|||
|
SUBCLASS |
Class |
Subclass |
Class |
Subclass |
|
S2 |
S2m |
Class 2 |
2s |
|
|
S2d |
|
2d |
||
|
S3md |
|
2sd |
||
|
|
etc. |
|
etc. |
|
|
UNITS |
Subclass |
Unit |
|
|
|
S2d |
S2d |
Special study areas (informative appraisal areas) (for which management and development recommendations are given) |
||
|
S2d |
||||
|
S2d |
||||
|
|
etc. |
|||
Notes: (1) Subclasses, reflecting a requirement or limitation are denoted by a letter suffix (see Table 17)- in the USBR system these are s, t or d indicating a soil, topographic or drainage deficiency respectively; (2) See text for use of Sc (Conditionally Suitable) in the FAO system; (3) Special use lands (USBR Class 4) are classified 1, 2 and 3 to reflect relative payment capacity with a letter designating the land use (crop); and (4) Class 5 land (USBR) requires further study to determine whether it is suitable or not. Class N1 (FAO) is marginally not suitable at present.
