3. THE LAND USE CAPABILITY CLASSIFICATION 48
3.3 Description of Land Use Capability Subclasses 76
3.3.5 Discussion on subclasses 84
Only the dominant limitation is recorded for each map unit. All four subclasses are applicable to each LUC class, with the exception of subclass ‘e’, which is not used with LUC Class 1. Note that subclass ‘e’ is now applicable for the redefined LUC Class 5.
Conventions are observed in determining the subclass. The limitations imposed by erosion, excess water, and soil factors listed previously can be modified or partially
overcome by management and are therefore given precedence over climate in the determination of subclasses. The dominant limitation or hazard determines which of the subclasses ‘e’, ‘w’, or ‘s’ is used.
Subclass limitation ‘c’ is used where climate is the only limitation.
In practice several limitations may affect the map unit. It may therefore be difficult to decide which should be the subclass, especially on non-arable land, particularly where soil limitations and erosion risk are closely related. Where two kinds of limitation are essentially equal, the limitations are given the following priority, ‘e’, ‘w’, ‘s’, and ‘c’. That is, ‘e’ is given precedence over ‘w’ and ‘s’, and w is given precedence over ‘s’, and so on (Figure 12).
e
erodibilitys
rooting zone limitationsc
climatew
excessive wetnessFigure 12: Subclass allocation priority when more than one kind of limitation is considered equal.
3.3.5.1 LUC class and subclass assumptions
When assessing and allocating LUC classes and subclasses the following assumptions are made:
x The permanent physical limitations of the land remain. x The rectifiable limitations may be removed.
x An above-average level of land management is practised.
x Appropriate soil conservation measures will be applied and maintained.
Where it is feasible to either remove or significantly reduce the physical limitation (e.g. installing drainage or permanent irrigation, improving soil fertility, removing surface gravel, stones or boulders, or minimising erosion), then the land is assessed as if the limitation has already been removed or managed. For example, stoney Kopua soils (Plate 79) may be classified as LUC 3s even before stone picking has taken place.
3.3.5.2 Physical limitations
‘Physical limitations’ refer to land characteristics which have an adverse effect on the capability of land. These limitations can be permanent, removable, or modifiable.
Permanent limitations cannot be removed. It is neither practical, nor economic,
nor technologically possible to remove the limitation. Examples include:
x Rock type attributes.
x Adverse climate (e.g. frequency of extreme events).
x Excess wetness even after drainage. x Overflow from major river systems
that cannot be controlled.
x Slope angle.
x Soil attributes, such as plant rooting depth (presence of subsoil pans or other rooting impediments), texture, structure, water holding capacity, type of clay minerals.
Removable limitationsare those that can be removed, but removal is not easy and
often involves a sizeable investment in land development. Removal depends on economics and the availability of appropriate technology, relative to the degree of limitation. Examples include:
x Gravel and boulders on the land surface and brought to the surface by cultivation (stone picking or piling).
x Soil wetness (drainage, water diversion, land reclamation). x Flooding risk (through large community flood control schemes). x Soil moisture deficit (through large irrigation schemes).
Modifiable limitations can be removed, but only through ongoing management
and investment. Examples include:
x Erosion.
x Nutrient deficiencies.
x Soil moisture deficit.
The difficulty of removing or modifying limitations depends on their type and severity. The key words ‘reasonable’, ‘feasible’ and ‘economic’ are considered when deciding on the practicability of removing or modifying limitations. Soil conservation measures, irrigation, farm drainage, stone removal, and fertiliser applications are examples where technology can be used to modify or remove existing physical limitations on individual farms. The Land Use Capability assessment assumes that such improvements have been carried out.
LUC assessment can also be adjusted by major schemes that permanently change the degree of the limitation, such as large scale irrigation, drainage or flood control schemes.
For land where permanent irrigation has been installed (e.g. a centre pivot, border dykes) the classification is made on the basis that the soil moisture limitation has been permanently removed.
Where permanent drainage has been installed or is part of an approved scheme, the classification is made on the degree of limitation that will exist after drainage. Where major community-based flood control schemes designed with protection levels to the 50–100 year return event are in place, the classification is made on the basis that the flood risk has been removed.
The type and severity and/or effective removal of the above physical limitations should be documented in the LUC unit descriptions.
Where works are beyond the ability of individual farmers and require a community scheme the land is classified according to the nature of its present limitations. If in time a large scheme such as irrigation becomes operative, the land can be reclassified into a higher LUC class.
The classification of LUC is independent of such factors as location, distance from markets, processing facilities, land ownership, or the skills of individual farmers.