Among the countries using a capability classi
fication system the United States is unique in the extent
to which the soils of the country have been surveyed.
This allows a further modification, which is not possible
in countries with less detailed soil survey information.
When the soils of a state or country are well known it
may be found that the land falling within a particular
capability class usually consists of one of a group of soil
types, which is sufficiently different to be recognized
and identified. These would then be classified land
capability units, and land would be classified into these
smaller more precise categories as well as into the broad
capability classes. The land capability unit is often the
same as a soil series in the pedological sense, but need
not necessarily be so. Even when soils have not been
broken down into series, the principle of smaller
divisions within 'capability classes can be applied
whenever distinctive combination of physical
characteristics can be identified.
Two case studies from Chandigarh area and
Chamba Block, Tehri district are presented here. In our
country class I land is not very common because of
climatic factors.
Land irrigability classification
The aim of land irrigability classification is to
categorise the different land units to bring the land under
sustained irrigation which is one of the most important
activity to increase productivity. Considerable
importance has been given to the development and
extension of irrigation since the time of green revolution.
This is accomplished by the interpretation of soil
survey data for grouping the soil and land units for
sustained use under . irrigation. The interpretative
groupings of soil and land characteristics for irrigation
suitability is primarily concerned with predicting
behaviour of soils under the greatly altered water regime
brought about by irrigation practices. The requisite
criteria are soil depth, soil texture, soil permeability/
infiltration, coarse fragments, chance of water logging
etc. This is accomplished in two steps: (1) soil
irrigability classification which is based on the
permeability; (2) by integrating soil irrigability
properties, topography and irrigation water quality, land
irrigability classes following the contextual approaches
in GIS environments. The detailed methodology
followed here is as suggested by AIS&LUS (1970).
Soil Irrigability Classes
According to the suitability of soils for sustained
use under irrigation, they are grouped into different soil
irrigability classes. This grouping is carried out without
taking into consideration the available quantity or
quality of water or even non-soil related factors like land
development costs, drainage facilities etc.
Land Irrigability Classes
The suitability of land for irrigation depends on
physical factors like quality and quantity of irrigation
water and socio-economic factors like land development
costs, provision of drainage facilities and production
costs of individual crops. The definition for classifying
soils into different irrigability classes and land
irrigability classes are shown in Table 5 and 6 (AIS and
LUS, 1970).
The criteria adopted for irrigability classification of
soils are given in Table 7, and for land irrigability in
Table 8. Overlaying soil, drainage and topography using
GIS one can derive these classes through querry shell.
Table 6. Soil irrigability classes and their definitions.
Class
Definition
A
None to slight limitations for sustained use under
irrigation
B
Moderate soil limitations for sustained use under
irrigation
C
Severe soil limitations for sustained use under
irrigation
D
Very severe soil limitations for sustained use
under irrigation
E
Not suitable for irrigation
Table 7. Land irrigability classes and their definition.
Class
Definition
Class 1
Lands that have few limitations of soils, topo
graphy or drainage for sustained use under irrigation.
Class 2
Lands that have moderate limitations of soil, topo
graphy or drainage for sustained use under irrigation.
Class 3
Lands that have severe limitations of soil, topo
graphy or drainage for sustained use under irrigation.
Class 4
Lands that are marginal for sustained use under
irrigation because of very severe limitations of either
soil topography or drainage.
Class 5
Lands that are temporarily classed as not suitable for
sustained use under irrigation.
Class 6
Lands not suitable for sustained use under irrigation.