Soil maps and soil survey reports provide the basic material for the land capability classification system.
The system consists of the following three categories:
(a) Land Capability Unit:
It is a grouping of one or more individual soil mapping units (a portion of the landscape that has similar characteristics and qualities and whose limits are fixed by precise definitions) having similar potentials and continuing limitations or hazards.
The soils in a capability unit are sufficiently uniform to:
(i) Produce similar kinds of cultivated crops and pasture with similar management practices;
(ii) Require similar conservation treatment and management under the same kind and condition of vegetative cover; and
(iii) Have comparable potential productivity.
The land capability unit is expressed by putting an Arabic number (1, 2, 3, etc.) at the end of land capability class or subclass. For example: IIIe1, IIIe2.
(b) Land Capability Subclass:
Subclasses are the groups of capability units which have the same major conservation problem or same kind of limitation, such as erosion and runoff (e), excess water or wetness (w), root zone or soil limitations (s), and climatic limitations (c). The suffixes e, w, s, or c are added to the capability class symbol to indicate capability subclass. For example: IIIe, IIw, IIs, etc. No subclasses are made in class I land.
(c) Land Capability Class:
Capability classes are the groups of capability subclasses or capability units that have the same relative degree of hazards or limitations.
Land capability classes are usually divided into two major groups:
(i) Land suitable for cultivation; and
(ii) Land unfit for cultivation but suitable for permanent vegetation like pastures, orchards and forests.
A brief description of the characteristics and safe use of land and soils in each class is presented in the following paragraphs:
Soils in this class have few limitations that restrict their use. They can be cropped very intensively, used for pastures, grazing lands, woodlands, on even for wildlife preserves. The soils are deep, having good water holding capacity, well drained and the land is nearly level.
These soils are either naturally fertile or possess such characteristics which encourage good response of crops to fertilizer application. The soils in class I need only ordinary crop management practices to maintain their productivity such as fertilization, liming, etc. Crop rotations may also be followed.
Soils in this class have some limitations that reduce the choice of plants or require moderate conservation practices. These soils may be used for the same crops as class I. However, they are capable of intensive cropping systems, or with the same cropping systems they require some conservation practices.
The use of soils in class II may be limited by one-or more factors such as:
(1) Gentle slope;
(2) Moderate erosion hazards;
(3) Adequate soil depth;
(4) Less than ideal soil structure and workability;
(5) Slight to moderate alkali or saline conditions; and
(6) Somewhat restricted drainage.
The management practices required for class II land include terracing, strip cropping, contour tillage, rotations involving legumes and grasses. The management practices employed in class I land are also generally required for soils in class II.
Soils in class III have severe limitations that restrict the choice of plants or require special conservation practices or both. The same crop can be grown in class III land as on classes I and II. The area and intensity of clean cultivated land is restricted, however, to the choice of the particular crop to be used. Crops which provide soil cover, such as grasses and legumes, must find place in the rotations.
Limitations in the use of soils in class III may be due to factors such as:
(1) Moderate steep slopes;
(2) High erosion hazards;
(3) Very slow water permeability;
(4) Moderate depth and restricted root zone;
(5) Low water holding capacity;
(6) Low fertility;
(7) Moderate alkali or salinity, and
(8) Unstable structure.
Soils in class III often require special conservation practice. Those mentioned for class II land must be employed frequently in combination with restrictions in kinds of crops. Improvement in drainage may also be needed.
Soils in this class can be used for cultivation but there are very severe limitations on the choice of crops. Also, very careful management may be required. The alternative uses of these soils are more limited than for class III. Close growing crops must be used extensively and row crops cannot be grown safely in most cases. The choice of crops may be limited by excess moisture as well as by erosion hazards.
The most limiting factors on these soils may be one or more of the following:
(1) Steep slopes;
(2) Severe erosion susceptibility;
(3) Severe past erosion;
(4) Shallow soils;
(5) Low water holding capacity;
(6) Poor drainage; and
(7) Severe alkali or salinity.
Soil conservation practices must be applied more frequently than on soils in class III.
Soils in class V are limited in their safe use by factors other than erosion hazards.
These limitations are:
(1) Subject to frequent overflow;
(2) Growing season too short for crop plants;
(3) Stony or rocky soils; and
(4) Ponded areas where drainage is not feasible.
Class VI lands have severe limitations that restrict their use largely to pasture or range, woodland or wildlife. The physical limitations are the same as those for class IV land but severity of limitation is more.
Soils in class VII land have very severe limitations which restrict their use to grazing, woodland or wildlife. The physical limitations are the same as in class VI except that they are to severe to pasture improvement.
The soils in this class of land are such that they should not be used for any kind of commercial plant production being severely eroded. The lands included in this class are tidal lands, swamps, river wash, sand dunes and barren mountain tops. These lands should be used only for recreation, wildlife, water supply or aesthetic purposes. A land capability class can be upgraded if the limitations are overcome.
On a map, the land capability classes are usually shown by standard colours as follows:
In deciding about the land capability class of a given soil and landscape unit the weightage to various properties/features are given as per the following guidelines:
Similarly, other soil, landscape, environmental or land use factors that are of importance be given due weightage in arriving at land capability classification. On the basis of consideration for each individual property of a soil mapping unit, several capability classes of the same unit will be arrived at. But the actual capability class for the unit will be decided on the basis of the class of the inferior most property.
For example, a moderately deep, well drained, loam soil occurs in an arid climate, then its capability class will be decided as follows:
depth of soil â€” class III, drainage â€” class I, texture â€” class I, climate â€” class IV; so capability class of this soil will be class IV.
Soil conservation measures in relation of land capability classification:
The system of land classification based upon the soil and site characteristics enables one to plan and match soil conservation measures according to the intensity and nature of the problem. As one moves from land class I to land class IV, the intensity of the problem increases and greater intensive land treatment would be required to keep it under permanently productive condition.
This would require greater investment, whatever may be the nature of the problem. In fact the various soil and land characteristics which determine the classification of land into various land classes should have an economic bias also, instead of purely physical factors. This will enable the land planners to assess the quantum of investment needed for land management.
The land classes V to VIII have severe hazards in respect of the problems given above and are, therefore, unsuitable for agriculture. As one moves from land classes V to VIII increasing hazards are noticed in respect of use for pasture and afforestation (Fig. 5.2).
The permissible intensity of grazing will also have to be cut down for land classes VII and VIII. Classification of land into units enables one to divide the land into specific land management groups and spell out the quantum of treatment needed to obtain maximum productivity from each parcel of land (Table 5.1).