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2.1 General
2.2 Land
2.3 Land use
2.4 Land characteristics, land qualities and diagnostic criteria
2.5 Requirements and limitations
2.6 Land improvements
2.7 Land suitability and land capability
Certain concepts and definitions are needed as a basis for the subsequent discussion. These concern the land itself, kinds of land use, land characteristics and qualities, and improvements made to land.
For the sake of clarity, some definitions are given in the text in simplified form. Formal definitions of terms used in a specialized sense are given in the Glossary.
Land comprises the physical environment, including climate, relief, soils, hydrology and vegetation, to the extent that these influence potential for land use. It includes the results of past and present human activity, e.g. reclamation from the sea, vegetation clearance, and also adverse results, e.g. soil salinization. Purely economic and social characteristics, however, are not included in the concept of land; these form part of the economic and social context.
A land mapping unit is a mapped area of land with specified characteristics. Land mapping units are defined and mapped by natural resource surveys, e.g. soil survey, forest inventory. Their degree of homogeneity or of internal variation varies with the scale and intensity of the study. In some cases a single land mapping unit may include two or more distinct types of land, with different suitabilities, e.g. a river flood plain, mapped as a single unit but known to contain both well-drained alluvial areas and swampy depressions.
Land is thus a wider concept than soil or terrain. Variation in soils, or soils and landforms, is often the main cause of differences between land mapping units within a local area: it is for this reason that soil surveys are sometimes the main basis for definition of land mapping units. However, the fitness of soils for land use cannot be assessed in isolation from other aspects of the environment, and hence it is land which is employed as the basis for suitability evaluation.
Suitability evaluation involves relating land mapping units to specified types of land use. The types of use considered are limited to those which appear to be relevant under general physical, economic and social conditions prevailing in an area. These kinds of land use serve as the subject of land evaluation. They may consist of major kinds of land use or land utilization types.
2.3.1 Major Kinds of Land Use and Land Utilization Types
A major kind of land use is a major subdivision of rural land use, such as rainfed agriculture, irrigated agriculture, grassland, forestry, or recreation. Major kinds of land use are usually considered in land evaluation studies of a qualitative or reconnaissance nature.
A land utilization type is a kind of land use described or defined in a degree of detail greater than that of A major kind of land use. In detailed or quantitative land evaluation studies, the kinds of land use considered will usually consist of land utilization types. They are described with as much detail and precision as the purpose requires. Thus land utilization typos are not a categorical level in a classification of land use, but refer to any defined use below the level of the major kind of land use.
A land utilization typo consists of a set of technical specifications in a given physical, economic and social setting. This may be the current environment or a future Betting modified by major land improvement e, e.g. an irrigation and drainage scheme. Attributes of land utilization types include data or assumptions on:
- Produce, including goods (e.g. crops, livestock timber), cervices (e.g. recreational facilities) or other benefits (e.g. wildlife conservation)
- Market orientation, including whether towards subsistence or commercial production
- Capital intensity
- Labour intensity
- Power sources (e.g. man's labour, draught animals machinery using fuels)
- Technical knowledge and attitudes of land users
- Technology employed (e.g. implements and machinery, fertilizers, livestock breeds, farm transport, methods of timber felling)
- Infrastructure requirements (e.g. sawmills, tat factories, agricultural advisory services)
- Size and configuration of land holdings, including whether consolidated or fragmented
- Land tenure, the legal or customary manner in which rights to land are held, by individuals or groups
- Income levels, expressed per capita, per unit of production (e.g. farm) or per unit area.
Management practices on different areas within one land utilization typo are not necessarily the same. For example, the land utilization type may consist of mixed farming, with part of the land under arable use and part allocated to grazing. Such differences may arise from variation in the land, from the requirements of the management system, or both.
Some examples of land utilization types are:
i. Rainfed annual cropping based on groundnuts with subsistence maize, by smallholders with low capital resources, using cattle drawn farm implements, with high labour intensity, on freehold farms of 5-10 ha.
ii. Farming similar to (i) in respect of production, capital, labour, power and technology, but farms of 200-500 ha operated on a communal basis.
iii. Commercial wheat production on large freehold farms, with high capital and low labour intensity, and a high level of mechanization and inputs.
iv. Extensive cattle ranching, with medium levels of capital and labour intensity, with land held and central services operated by a governmental agency.
v. Softwood plantations operated by a government Department of Forestry, with high capital intensity, low labour intensity, and advanced technology.
vi. A national park for recreation and tourism.
Some descriptions of land utilization types are given in Chapter 5.
Where it is wished to relate agricultural land utilization types to a general classification, the Typology of World Agriculture of the International Geographical Union may be considered (Kostrowicki, 1974). The role of land utilization types in land evaluation is discussed further in Beek (1975).
2.3.2 Multiple and Compound Land Use
Two terms, multiple and compound land utilization types, refer to situations in which more than one kind of land use is practiced within an area.
A multiple land utilization type consists of more than one kind of use simultaneously undertaken on the same area of land, each use having its own inputs, requirements and produce. in example is a timber plantation used simultaneously as a recreational area.
A compound land utilization type consists of more than one kind of use undertaken on areas of. land which for purposes of evaluation are treated as a single unit. The different kinds of use may occur in time sequence (e.g. as in crop rotation) or simultaneously on different areas of land within the same organizational unit. Mixed farming involving both arable use and grazing is an example.
Sometimes an appropriate land utilization type can be found by making several land mapping units part of the same management unit, e.g. livestock management which combines grazing on uplands in the rainy season and on seasonally flooded lowlands in the dry season.
Land utilization types are defined for the purpose of land evaluation. Their description need not comprise the full range of farm management practices, but only those related to land management and improvement. At detailed levels of evaluation, closely-defined land utilization types can be extended into farming systems by adding other aspects of farm management. Conversely, farming systems that have already bean studied and described can be adopted as the basis for land utilization types.
2.4 Land characteristics, land qualities and diagnostic criteria
A land characteristic is an attribute of land that can be measured or estimated. Examples are slope angle, rainfall, soil texture, available water capacity, biomass of the vegetation, etc. Land mapping units, as determined by resource surveys, are normally described in terms of land characteristics.
If land characteristics are employed directly in evaluation, problems arise from the interaction between characteristics. For example, the hazard of soil erosion is determined not by slope angle alone but by the interaction between elope angle, slope length, permeability, soil structure, rainfall intensity and other characteristics. Because of this problem of interaction, it is recommended that the comparison of land with land use should be carried out in terms of land qualities.
A land quality is a complex attribute of land which acts in a distinct manner in its influence on the suitability of land for a specific kind of use. Land qualities may be expressed in a positive or negative way. Examples are moisture availability, erosion resistance, flooding hazard, nutritive value of pastures, accessibility. Where data are available, aggregate land qualities may also be employed, e.g. crop yields, mean annual increments of timber species.
Table 1 gives an illustrative list of land qualities related to productivity from three kinds of use and to management and inputs. It is not exhaustive, nor is each land quality necessarily relevant for a particular area and type of land use. The qualities listed in B and C are in addition to those of A, which may be relevant to all three kinds of use (based in part on Beek and Bennema, 1972). There may also be land qualities related to major land improvements. These vary widely with the types of improvement under consideration. An example is land evaluation in relation to available supplies of water where irrigation is being considered.
A land quality is not necessarily restricted in its influence to one kind of use. The same quality may affect, for example, both arable use and animal product
There are a very large number of land qualities, but only those relevant to land use alternatives under consideration need be determined. A land quality is relevant to a given type of land use if it influences either the level of inputs required, or the magnitude of benefits obtained, or both. For example, capacity to retain fertilizers is a land quality relevant to most forms of agriculture, and one which influences both fertilizer inputs and crop yield. Erosion resistance affects the costs of soil conservation works required for arable use, whilst the nutritive value of pastures affects the productivity of land under ranching.
Land qualities can sometimes be estimated or measured directly, but are frequently described by means of land characteristics. Qualities or characteristics employed to determine limits of land suitability classes or subclasses are known as diagnostic criteria.
A diagnostic criterion is a variable which has an understood influence upon the output from, or the required inputs to, a specified use, and which serves as a basis for assessing the suitability of a given area of land for that use. This variable may be a land quality, a land characteristic, or a function of several land characteristics. For every diagnostic criterion there will be a critical value or set of critical values which are used to define suitability class limits.
Table 1 EXAMPLES OF LAND QUALITIES
A. LAND QUALITIES RELATED TO PRODUCTIVITY FROM CROPS OR OTHER PLANT GROWTH
- Crop yields (a resultant of many qualities listed below)
- Moisture availability
- Nutrient availability
- Oxygen availability in the root zone
- Adequacy of foothold for roots
- Conditions for germination
- Workability of the land (ease of cultivation)
- Salinity or alkalinity
- Soil toxicity
- Resistance to soil erosion
- Pests and diseases related to the land
- Flooding hazard (including frequency, periods of inundation)
- Temperature regime
- Radiation energy and photoperiod
- Climatic hazards affecting plant growth (including wind, hail, frost)
- Air humidity as affecting plant growth
- Drying periods for ripening of crops.
B. LAND QUALITIES RELATED TO DOMESTIC ANIMAL PRODUCTIVITY
- Productivity of grazing land (a resultant of many qualities listed under A.)
- Climatic hardships affecting animals
- Endemic pests and diseases
- Nutritive value of grazing land
- Toxicity of grazing land
- Resistance to degradation of vegetation
- Resistance to soil erosion under grazing conditions
- Availability of drinking water.
C. LAND QUALITIES RELATED TO FOREST PRODUCTIVITY
The qualities listed may refer to natural forests, forestry plantations, or both.
- Mean annual increments of timber species (a resultant of many qualities listed under A.)
- Types and quantities of indigenous timber species
- Site factors affecting establishment of young trees
- Pests and diseases
- Fire hazard.
D. LAND QUALITIES RELATED TO MANAGEMENT AND INPUTS
The qualities listed may refer to arable use, animal production or forestry.
- Terrain factors affecting mechanization (trafficability)
- Terrain factors affecting construction and maintenance of access roads (accessibility
- Size of potential management units (e.g. forest blocks, farms, fields
- Location in relation to markets and to supplies of inputs.
2.4.1 Examples
These terms may be illustrated with reference to the land quality "oxygen availability in the root zone". This quality can be most closely estimated by the diagnostic criterion of the period when the redox potential (Eh) in the root zone is less than +200 millivolts. Such information would frequently not be available, in which case the next most direct criterion would be periods when the root zone lay below the water table. For example, oxygen availability might be classed as "moderate" with 3-6 months below the water table, and "low" with over 6 months. Failing information on periods with a high water table, then soil mottling, soil drainage class or natural vegetation could be used as diagnostic criteria for assessing oxygen availability.
Land qualities can sometimes be described by means of a single land characteristic, as in the preceding example. In many cases, however, their rating involves combinations of several characteristics, as in the case of moisture availability illustrated by the following example.
Moisture availability to plants is a land quality that is relevant in a wide variety of circumstances. It can apply to arable cropping, animal productivity (via its influence on growth of pastures) and forest production. It can affect both productivity, e.g. crop yields, and inputs, e.g. mulching measures necessary, or amounts of irrigation water required. Among the land characteristics which affect the quality moisture availability are: amount of rainfall, its seasonal distribution and variability; potential evapotranspiration, and hence the characteristics which themselves affect it (temperature, humidity, wind speed, etc.); and available water capacity of the soil, and the characteristics which affect it - effective soil depth (depth to which roots penetrate) and the field capacity and wilting point of each soil horizon, the latter being in turn influenced by texture, organic matter content, etc. The probable recurrence interval at which the soil moisture level falls to wilting point within the entire rooting zone is a further land characteristic of importance (which can be estimated but not measured within a short period). By no means all these land characteristics would be employed as diagnostic criteria. Supposing, for example, that differences in both rainfall and potential evapotranspiration within the surveyed area were so small as to be of little importance in differentiating types of land, then this characteristic would become part of the physical context of the evaluation and would not be used in defining class limits. The most appropriate diagnostic criterion used to define class limits might be available water capacity of the soil profile. However, where soil data were not available, then some function of effective depth and soil texture, believed to bear a linear relationship with available water capacity, could be used. In the former case, the set of critical values for available water capacity used to define class limits might be such as: over 40 cm, 30 40 cm, 24-30 cm.
2.4.2 The Scarcity Value of Land
The value of a particular type of land may be increased by its scarcity or the rarity of certain of its qualities, within a given region or country. This is often the position with nature reserves. In the extreme case, the presence of a plant or animal species unique to one area may make that land virtually irreplaceable, resulting in strict protection even against highly profitable other uses. Situations whore land acquires added suitability for a particular use by virtue of its scarcity can also arise with productive forms of uses for example where dry-season grazing land is in short supply.
2.5 Requirements and limitations
Requirements of the land use refer to the set of land qualities that determine the production and management conditions of a kind of land use.
Limitations are land qualities, or their expression by means of diagnostic criteria, which adversely affect a kind of land use.
For example, the requirements for mechanized cultivation of wheat include high availability of oxygen in the root zone and absence of obstructions (boulders or rock outcrops); waterlogging and the presence of boulders are limitations. Thus limitations may be regarded as land qualities expressed in such a way as to show the extent to which the conditions of the land fall short of the requirements for a given use.
Land improvements are activities which cause beneficial changes in the qualities of the land itself. Land improvements should be distinguished from improvements in land use, i.e. changes in the use to which the land is put or modifications to management practices under a given use.
Land improvements are classed as major or minor. A major land improvement is a substantial and reasonably permanent improvement in the qualities of the land affecting a given use. A large non-recurrent input is required, usually taking the form of capital expenditure on structure and equipment. Once accomplished, maintenance of the improvement remains as a continuing cost, but the land itself is more suitable for the use than formerly. Examples are large irrigation schemes drainage of swamps and reclamation of salinized land.
A minor land improvement is one which either has relatively small effects or is non-permanent or both, or which lies within the capacity of individual farmers or other land users. Stone clearance, eradication of persistent weeds and field drainage by ditches are examples.
The separation of major from minor land improvements is intended only as an aid to making a suitability classification. The distinction is a relative one; it is not clear-cut and is only valid within a local context. In cases of doubt, the main criterion is whether the improvement is within the technical and financial capacity of individual farmers or other landowners (including small communal owners, e.g. village co-operatives). In many areas improvements such as subsoiling, dynamiting or terracing cannot be undertaken by individual farmers, and are therefore regarded as major land improvements; in countries with large farms and high capital resources coupled with good credit facilities, however, these changes may be within reach of individuals and are therefore considered as minor improvements. Field drainage is another improvement that may or may not be regarded as major, depending on farm size, permanency of tenure, capital availability and level of technology.
2.7 Land suitability and land capability
The term "land capability" is used in a number of land classification systems, notably that of the Soil Conservation Service of the U.S. Department of Agriculture (Klingebiel and Montgomery, 1961). In the USDA system, soil mapping units are grouped primarily on the basis of their capability to produce common cultivated crops And pasture plants without deterioration over a long period of time. Capability is viewed by some as the inherent capacity of land to perform at a given level for a general use, and suitability as a statement of the adaptability of a given area for a specific kind of land use; others see capability as a classification of land primarily in relation to degradation hazards, whilst some regard the terms "suitability" and "capability" as interchangeable.
Because of these varying interpretations, coupled with the long-standing association of "capability" with the USDA system, the term land suitability is used in this framework, and no further reference to capability is made.