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3. Classification scheme for the 1:1,000,000 map of China's land resources
Commission for Integrated Survey of Natural Resources, Chinese Academy of Sciences, Beijing
China's land resources are being mapped at the scale of 1: 1, 000, 000 in order to compile and present land-survey data and the results of research undertaken since 1949, to estimate the quantity and quality of the nation's land resources, and to indicate their current uses and future potential. On the basis of their suitability and limitations and potential for development for primary use, the land resources of China are classified into a number of orders (regions), classes, sub classes, groups and types.
One of the important projects outlined in the national "Survey of Agricultural Natural Resources and Agricultural Regionalization Programme, " specified by the Scientific and Technological Development Plan of the People's Republic of China (1978-1985), was the mapping of China's land resources at the scale of 1 : 1,000,000. The aim of the mapping project is systematically to compile and present data showing the achievements of land surveys and research undertaken since the establishment, in 1949, of the People's Republic of China, to map and briefly estimate the quantity and quality of all types of land resources in China, and to show the conditions under which they are used at present, as well as their potential for farming, forestry and animal husbandry. Such a project is necessary to provide basic data for the effective development and use of land resources, for agricultural regionalization, which can lead to better farming performance, and for the long-range planning and development of agriculture.
Experimental mapping at a scale of 1:1,000,000 has been
conducted on the land resources of the Harbin, Qiqihar and
Shenyang areas in China's North-east Region. The classification
scheme discussed in this chapter was designed while the maps of
those three areas were being compiled. This scheme has been used
in mapping parts of South China. Owing to China's size and given
that the project is still in its initial stage of development,
other complications, the classification scheme described here is preliminary and future revisions are anticipated.
The classification of land resources is an essential
prerequisite to their mapping and to land evaluation. Land
resources were classified on the basis of current land-use
patterns together with the productivity of the land. Since the
chief purpose of compiling 1:1,000,000 maps is to facilitate the
development of primary resources, the main considerations used in
devising the classification scheme were:
1. The natural productivity of the land and whether this corresponds to actual production performance.
2. The degree of land suitability for particular primary uses and whether present land-use patterns best fit the quality of the land.
3. The limitations of the land on the development of primary resources and whether the measures taken, or that should be taken, to transform, conserve and improve productivity are justified.
4. Whether the productivity of the land is consonant with production costs.
Based on Chinese conditions and foreign experiences, a multiple grade scheme of orders (regions), classes, subclasses, groups and types was adopted for the classification of China's land resources. The sub-classes supplement classes, so four grades are actually used in the scheme. Each is discussed below.
Land Orders (Regions)
This is the highest unit in the classification. Orders are differentiated on the basis of moisture and thermal conditions, and reflect inter-regional differences in productivity. The orders may be used to assess interregional productive potentials. Within an order, land use is relatively homogeneous. Based on agroclimatic characteristics, 14 regions can be distinguished in China (see the Preface to this volume) (table 3. 1), which correspond with the orders of land resources.
TABLE 3.1. Criteria Used to Distinguish Land Orders (Regions) in China
|Land order (region)||Accumulated
|1. Humid cool temperate||<1,700||400-500||<1.0|
|2. Humid moderate temperate||1,700-3,200||400-800||<1.0|
|3. Subhumid moderate temperate||1,700-3,200||400-600||1.0-1.5|
|4. Semi-arid moderate temperate||1,700-3,200||200-500||1.5 2.0|
|5. Arid moderate temperate||1,700-3,500 (or 4,000)||<200 (300)||>2.0|
|6. Humid warm temperate||3,200 4,500||600-900||<1.0|
|7. Subhumid warm temperate||3,500-4,500||400-700||1.0 1.5|
|8. Semi-arid warm temperate||3,500-4,500||300-500||1.5-2.0|
|9. Arid warm temperate||3,500-4,500 (or 5.000)||<200||>2.0|
|10. Humid cool sub-tropical||4,000(4,500)-5.000 (5,500)||700-1,200||<1.0|
|11. Humid moderate subtropical||5,000 (5,500)-6,000 (6,500)||1,000-1,800||<1.0|
|12. Humid warm subtropical||6,000(6,500)-7,500(8,000)||1,600-2,000||<1.0|
|13. Humid and subhumid tropical||>7,500 (8,000)||1,400-2,000||<1.0|
|14. Alpine - Arctic||<1,000||-||-|
a. Aridity index
E = average annual potential evapotranspiration
r = average annual precipitation
Land Classes and Sub-classes
The second category is land classes, which reflect the suitability and potential productivity of the land. Land classes form the core of the assessment of land resources. Within a class the suitability and potential productivity of the land should be virtually homogeneous. Suitability of the land is divided into five categories: (1) farming, forestry and animal husbandry; (2) forestry and animal husbandry; (3) animal husbandry; (4) forestry; and (5) temporarily unsuitable for any primary resource use. Land suitability may be further divided into three subclasses; very suitable, generally suitable and marginally suitable. To reduce the number of possible combinations and also to emphasize land for cultivation, land suitable for primary-resource use has been divided into three subcategories in terms of degree of suitability for farming: good-quality land with no limitations for farming; mediumquality land generally suitable for farming, but with some limitations; and poorquality land with many or severe limitations and only marginally suitable for farming. Thus at the land-class level, seven degrees of land suitability and potential productivity may be distinguished (fig.3.1).
Land Class I: This is high-quality land with no or little limitations for farming. It is also suitable for forestry and animal husbandry. Class l land is characterized by low relief, high fertility, and good conditions for mechanization. It is the main farm land with high and stable yields, or land that can easily become basic farmland. The uncultivated land of this class needs little or no improvement and after cultivation it easily becomes basic farmland. Land in this class does not deteriorate with use.
Land Class II: Land in this category has certain limitations for farming and its quality is moderate. Generally, class Il land is suitable for primary resource exploitation. Among its limitations for farming are thermal and/or moisture deficiencies, poor relief, alkaline and/or saline soils, swampiness and soil erosion. Certain improvement measures must be taken before the land can be cultivated or changed into basic farmland. Conservation measures are needed to prevent deterioration.
Land Class III: This class has limitations for farming. It is poor quality land, only marginally suitable for farming. Among its major limitations are the uncertainty of obtaining even one crop a year owing to moisture and water deficiencies, unstable yield of dry farming, steep slopes, thin soils, serious soil alkalinity or salinity, swampiness and severe soil erosion. Class Ill land requires major rehabilitation before it can be cultivated and converted to basic farmland, but it can be used for forestry and animal husbandry. Stringent conservation measures must be enforced to avoid further deterioration.
Land Class IV: Such land has very severe limitations for farming but has few if any for forestry or animal husbandry. Under exceptional conditions and with vigorous rehabilitation measures, some land in this category could be used for farming, for improved pasture, or for specialized agricultural uses.
Fig. 3.1. Relationship between Land Suitability and Land Potential
Fig. 3 2. Land Resources of the Anda Area, Heilongjiang Province
TABLE 3.2. Criteria for Rating Factors that Limit Land Use
|Limiting factor||Rating||Land use|
|Erosion (percentage of area occupied by gullies)||None (e0)||I||A||a|
|Soil texture||Homogeneous loam (m0)||I||A||a|
|Homogeneous clay (m1)||II||A||a|
|Soil depth (cm) (¡ = developed on rock or consolidated bedrock; s = developed on soft bedrock)||Very deep (l0) - ¡ 100; s 50||I||A||-|
|Deep (I1) -¡ 100 50; s 50-30||II||A||-|
|Shallow (13) - ¡ 30-10; s < 10||None||B|
|Very shallow (14) - Y <10||None||C||-|
|Hydrological and drainage conditions||No-occasional flooding. Well-drained (w1)||I||A||a|
|Seasonal flooding. Poorly drained (w2)||II||B||b|
|Extended flooding. Poor prospect||III||C||None|
|for improved drainage (W3)|
|Severely extended flooding||None||None||None|
|Improvement very difficult (W4)|
|Soil salinization||None/slight (s1)||I||A||a|
|Lime hardpan (depth from surface in cm thickness >20 cm)||<40 (h1)||I||B||-|
|6-4 5 (a2)||II||B||-|
|4 5 (a3)||III||B/C||-|
Land Class V: Land in this class suffers severe limitations for farming and forestry, but few or none for animal husbandry. Under exceptional conditions and with major rehabilitation measures some areas could be made suitable for farming, forestry, or improved pasture.
Land Class Vl: Such land has very severe limitations for farming and animal husbandry, but is suitable for forestry. In exceptional cases small areas could be used for animal husbandry, and major improvements are required before any could be used for special agricultural activities.
Land Class Vll: This refers to land unsuitable for farming, forestry and animal husbandry owing to its present dedicated use or particular natural characteristics. Among the different types of land in this class are shifting sand dunes, gobi, land with rare and endangered biota, glaciers, protected sources of water, tourist areas, areas producing industrial raw materials, mines, and urban land.
Within a class, sub-classes reflecting suitability of forestry and natural pasture may be used as supplementary units. The capital letters "A", "B" and "C" denote suitability for forestry, and minuscules "a", "b" and "c" refer to animal husbandry.
"A" refers to land most suited to forestry. It has a good climate, fertile soils and appropriate slope. The land is suitable for many species, especially those with economic value, forest productivity is high and timber quality good. " B" is land generally suitable for forestry, but has certain limitations for some species. Development of economic forests and valuable species is limited by topography and soils, and forest productivity is not high. " C " refers to land marginally suitable for forestry, but severely limited in the number of species, especially high grade species, that can be grown. Such land may need improvement before forestry use. C-grade land may be suitable for watershed protection forests, windbreak plantations, trees grown to fix drifting sand, and the like.
TABLE 3.3. Micro-relief as a Factor Limiting Land Use
|Moderate hillocks (g2)||Higher
|<1 m <30%||Dense
<1 m >30%
1-2 m <30%
|1-2 m 30-50%||>2 m
TABLE 3.4. Temperature as a Factor Limiting Land Use
crop per year.
TABLE 3.5. Moisture as a Factor Limiting Land Use
on arid and
on arid and
on arid and
Cannot be used
for dry farming
In terms of pasture lands, "a" refers to land of good quality and very suitable for animal husbandry. It is characterized by thick. good quality and highly nutritious grasses palatable to livestock. Such land has high yields and is suitable for many kinds of animals. Pasture land "b" is of moderate quality and generally suitable for animal husbandry. The grasses grow moderately well, but their high quality is off-set by low yield, or vice versa. The nutritional values, especially the N content, are relatively low. The kind of livestock that may be raised on this type of land may be limited. The letter "c" refers to grassland of poor quality, only marginally suitable for animal husbandry. Both the quality and the productivity of the grasses are poor. The grassland is degenerated with a low carrying capacity, and needs improvement. The number and kinds of livestock that can be raised on this category of land is severely limited.
Sub-classes can be combined into as many as are needed. The cultivated fields of classes I and II are not evaluated for animal husbandry and forestry.
Land groups follow land classes and sub-classes and are differentiated on the basis of limiting factors and the corresponding measures needed for their amelioration. There are similar limiting factors within groups, and similar ameliorative measures are needed. In compiling the map of land resources of the Anda area, in Heilongjiang Province (fig. 3.2), at a scale of 1:1,000,000, primary consideration was given to the main limiting factors that fluctuate little and which play different roles in different groups. Eleven limiting factors and one non-limiting factor have been used on the map. These 12 factors are: erosion (e), slope (p), soil texture (m), soil depth (I), hardpan (h), hydrological and drainage conditions (w), soil salinization (s), soil acidity (a), microrelief (9), temperature (t), moisture (r). and no limitations (0).
TABLE 3.6. Criteria for Rating Pasture
|Quality of pasture|
These limiting factors are arranged according to their degree of stability. The factors of erosion and slope are stable whereas acidity and micro-relief factors are relatively more liable to change. Although the temperature and precipitation factors are fairly stable, they do have regional significance, as has been reflected in the classification of orders (regions). At the group level moisture and temperature are significant only in exceptional cases. In addition, each limiting factor is further rated in terms of severity. For example, in evaluating erosion (e), five subgroups (e0, e1, e2, e3, e4, e5) are recognized. Examples of some factor ratings are given in tables 3.2, 3.3, 3.4, 3.5, and 3.6.
Two or more limiting factors often appear simultaneously in dividing the groups. Owing to the constraint of the small map scale, the main limiting factor is clearly indicated under such circumstances. The relative importance of the factors may be indicated by the rating codes. If necessary, two factors can be given at the same time, but the main factor must be identified first.
Types of Land Resources
Below groups, land-resource types are differentiated. They are the smallest unit in the classification and constitute the foundation of land-resource evaluation. The characteristics of the land in a type should be relatively uniform, including landform, soils, vegetation and the present use status. In addition, within a type, the measures needed to up-grade the productivity and the landmanagement methods should also be similar. There is no limit to the number of land-resource types in a given area, the number being flexible to fit local conditions. The names given to landresource types may include compound terms, such as flood plain - meadow swampy soils - Deyeuxia angustifolia-Carex meadow, but local terminology may also be used.
4. Supplement to the preliminary standard classification system for land evaluation in China
Commission for Integrated Survey of Natural Resources.
Chinese Academy of Sciences. Beijing
An effort was made in 1979 to rank, classify and map China's land resources. For this purpose 11 agroecological zones were delimited nationwide, each of which was classified into eight land-use classes. This preliminary system of land evaluation is deemed too complex and a new approach based on land suitability orders, sub-orders, types and ranks. together with the factors limiting land uses. is suggested in this chapter.
Since 1978, there has been a clear need in China for maps of land evaluation for use in planning rural development and land use. A classification system for land evaluation was also required for use in compiling maps. This chapter describes briefly the preliminary standards used to evaluate China's land resources and presents a system of land evaluation that may be used to supplement the existing one.
Fig. 4.1. Agro-ecological Regions of China
TABLE 4.1. Criteria for Agro-ecological Recionalization of China
|Accumulated annual temperature and number of days of > 10°C||Aridity index||Cropping system||Frost-free days|
|0.5-1.0||Three crops of rice||Entire year|
|II. Southern subtropical||6,500-7,500°C
|0.5-1.0||Two crops of rice or one crop of wheat plus sweet potatoes||About 330|
|0.5-1.0||Same as above||260 330|
|IV. Northern subtropical||4,500
|0.5-1.0||One crop of rice and one crop of wheat||230-260|
|V. Warm temperate||3,200-4,800°C
|0.7-1.5||Two or three crops in two years||150-230|
|VI. Sub-humid warm temperate||3,200-3,600°C
|1.5-2.0||Three cropsin two years||145-195|
|VIII. Arid temperate||3,200-4,500°C
|Over 4||One to three cropsin two years||210|
|IX. Semi-arid temperate||2,000
|X. Sub-humid and semi-arid temperate||2,700-3,500°C
|2 4||One crop||<180|
|0.5 2+||One crop||<120|
TABLE 4.2. Standard Classes of Land Resources Used in Compiling Land Resources Map of China
|Class I||Superior quality land with no limitations and suitable for agriculture, animal husbandry and forestry.|
|Class II||Good quality land with only minor limitations. Crop yield slightly lower than class I, but well-suited to animal husbandry.|
|Class III||Land of medium
quality. Limited by low soil nutrient levels, imperfect
drainage and salinity. Easily improved.
|Class IV||Poor quality
land. Limitations include steep slopes, soil erosion,
thin soils, salinity, or unavailability of water.
Marginally suitable for crops but suitable for animal husbandry and forestry.
|Class V||Land of very
poor quality owing to severe biological and/or physical
limitations. Difficult to improve.
Unsuitable for crops but marginally suitable for animal husbandry and forestry.
|Class VI||Land at high altitudes, with steep slopes and bare rocks. Marginally suitable for animal husbandry.|
|Class VII||Land on rocky mountains or gravel and sandy deserts with thin pasture. Marginally suitable for animal husbandry.|
|Class VIII||Land of sandy and gravel deserts, permanent snowfields or tundra.|
Preliminary Criteria for Compiling Land Evaluation Maps of China
One of the first steps involved in compiling maps of land evaluation is to establish the agro-ecological zones. The zonation used here is based on: (1 ) the accumulated annual temperature of more than or equal to 10°C; 12) the number of days per year with temperatures of more than or equal to 10°C; (3) the aridity index; (4) the cropping system; and (5) the number of frost-free days per year.
Eleven broad agro-ecological zones were delimited (fig. 4.1 and table 4.1). In the tropical region, where the temperature is high throughout the year and the aridity index is 0.5-1.0, triple cropping of rice is possible. Three zones are located in the sub-tropical region, where the annual accumulated temperature of >=1 0°C is 4,5007,500°C, the number of days with temperatures of >=1 0°C is 180-320, and there are 260-330 frost-free, days, and two crops of rice or one crop of rice plus one crop of wheat are grown. The other six zones are distributed in the temperature belt where the values of such diagnostic criteria are below those of the first four zones. The last zone lies in the Qinghai-Xizang (Tibet) Plateau, which is characterized by a low agricultural productivity.
Each agro-ecological zone was sub-divided into eight classes, based on certain diagnostic criteria that were used to evaluate the suitability of the land for different purposes and to rate the factors limiting land use (table 4.2). The classes are numbered consecutively by Roman numerals in descending order of potential and in ascending order of the limiting factors (cf. chapter 3 of this volume).
A Supplement to the Preliminary System of Land Evaluation
The land-evaluation procedures mentioned above in the preliminary standard classification system are too complex and the system has recently been revised at the provincial and county levels. In addition, the approach to land evaluation in the comprehensive surveys of land resources has changed. During the last 30 years, land evaluation was restricted to the physical aspects of the land only, but it was realized recently that land evaluation also requires a cost-benefit analysis of different types of land use. It would be useful to develop a dual approach for land evaluation, one being concerned primarily with physical factors and the other with social and economic factors. These two aspects have been studied simultaneously during multidisciplinary integrated surveys of natural resources in China. There follows a description of this approach, based on the concept of land suitability.
Geographers generally use the term "land evaluation" synonymously with "land suitability" and "land capability." Here "land suitability" is considered appropriate, since it refers to land fit for a given type of use. The process of classifying land suitability involves an appraisal of the degree of suitability for particular uses within a defined area. This process can also be used to distinguish a type of land with limitations from one without limiting factors. It also rates the limiting factors.
TABLE 4.3. Land Suitability Classification
|Suitable (S)||Suitable for
|Rainfed farming (SAr)||1 No limitations|
|Irrigated farming (SAi)||Example: SAr1|
|Paddy rice (SAp)||2. Some limitations|
|Sugar-cane and sugar-beet (SAs)||Example: n1d1|
|Fruits (SAf)||3. Major limitations|
|Mulberry and oak trees (SAm)||Example: n2d1|
|Tea (SAt)||4. Severe limitations|
|Suitable for forestry (SF)||Timber (SF:)|
|Water conservation (SFc)|
|Economic forest (SFe)|
|Shelterbelts and soil protection (SFp)|
|Natural pasture (SPp)|
|Improved pasture (SPi)|
Four categories of land suitability are recognized: orders, suborders, types, and ranks (table 4.3).
1. Land Surtability Orders
Three orders, which indicate whether an area is assessed as suitable (S),unsuitable (N), or conditionally suitable (Sc) for productive use, have been established. Order S means that land is suitable for various kinds of sustained use. Depending on both physical and socio-economic factors, the land is expected to yield benefits without unacceptable risks of damage to the land resources, and inputs are easily justified. Land designated as N is unsuitable for sustained use and anticipated benefits would not justify the expected costs of inputs required. Land defined as Sc means that it may be suitable for a given type of use if certain conditions are fulfilled, after which suitability might increase significantly. Usually, a conditionally suitable land area is small and generally results from such localized phenomena as poor drainage, soil salinity, and the like. Poor choice of crops in relation to market values may also make a piece of land conditionally unsuitable. Such a land area requires physical modification and/or improved management.
2. Land Suitability Sub-orders
Three sub-orders of suitability are recognized: agricultural use (SA). pastoral production (SP). and forestry (SF).
Sub-order SA: Depending on both physical and socioeconomic factors, the land is suitable for sustained cultivation, under which it will not deteriorate. This highly productive type of land is characterized by fertile soils and sufficient water. (Additional categories within a sub-order may be created for special cash crops.)
Sub-order SP: Denotes land suitable for pastoral production, on which, with normal management, degradation would not be expected. Suitability depends mainly on physical factors, but sometimes socioeconomic factors must be considered. Land in pasture is usually inferior in quality SA land.
Sub-order SF: SF land is suitable for forestry, where anticipated benefits justify inputs. Suitability depends mainly on physical factors although socio-economic considerations may be important.
3. Land Suitability Types
Different types of suitability are distinguished within the suborders. Seven types are distinguished under SA five under SF, and two types under SP.
SAr: Rain-fed farming is widespread on the flood-plains in north China and in limited roiling or hilly areas in the centre and south of the country. Water supply is generally limited in the north, where occasional droughts may occur during part of the growing season. In some areas occasional flooding may also occur.
SAi: Irrigated farmland occurs nationwide. This type of land is usually level or gently rolling, with a high soil nutrient content and a thick solum. Cereals have long been grown on this type of land. In general, irrigated land is highly productive, but the tracts are small.
SAp: Paddy rice is widely grown in subtropical and tropical China, but only in a small area of the north. Commonly it is grown on flood plains with a high or medium level of soil nutrients. Recently, waterlogging owing to poor drainage has occurred in subtropical China.
SAs: This type of land is suitable for both sugar-cane, in south China, and sugar-beet, in the north. It occurs usually on flood plains with a high soil-nutrient content and where water is plentiful.
SAf: In general, this type of land is devoted to fruit cultivation. It is either level or gently sloping with high soil fertility and good drainage of both water and air. The water supply is good and there is no flood hazard.
SAm: Land of this type is used for mulberry and oak, the leaves of which feed silkworms. It occurs on floodplains or in hilly areas with gentle slopes. The soils are fertile, water is plentiful, there is no flood risk, and occasional, minor limiting factors neither reduce productivity nor require additional inputs. Such land usually occurs in small pieces, scattered among rice paddies or in other level and rolling areas.
SAt: Such land is suitable for tea bushes and is found in subtropical China. It occurs in gently rolling and hilly areas where the solum is thick, acidic soils are fertile, and the drainage good.
SFt: This land is suitable for timber and occurs throughout China, except for arid and permanent snow areas. Most such land is in upland areas with steep or moderately steep slopes, and stony soils.
SFc: Such land is suitable for water conservation forests and occurs in mountainous areas or in the vicinity of newly built reservoirs, where soils are thin and the slopes steep. Soil erosion is problematical.
SFe: This type of land is fit for forests or for, largescale plantation of crops with a high economic value, such as oil palm, camphor tree, Camellia oleosa, roses, rubber, and the like. It occurs in the subtropical and tropical regions, and only a limited part of the temperate zone is suitable for economic forests. This type of land is characterized by high quality soils and abundant water.
SFw: This is hilly land around rural settlements usually suitable for growing fuelwood. Such land has poor quality soils with a thin solum and is low in nutrients. Soils may have been eroded. Since wood accounts for 80 per cent of the fuel used in Chinese villages, land suitable for its production is urgently needed.
SFp: This type is suitable for windbreak forests and those established to protect against soil erosion. It occurs in several parts of China, especially on the periphery of oases in arid regions, in the Loess Plateau area, and in subtropical areas where the soil has been eroded and the bedrock deeply weathered. Such land has a number of limiting factors and is inferior to any other type of land adapted to forests.
SPp: This land is suitable for natural pasture and occurs in steppe, dry steppe, desert steppe, and mountainous steppe areas. Available water is enough for livestock but cannot support agriculture.
SPi: Such land is suitable for improved pasture, and occurs in both north and south China. It requires fertile soil, a thick solum, and enough water.
4. Land Suitability Rank
Ten factors limiting land use are rated by the degrees of limitation (table 4.4). Four classes of limitation are recognized for each of the ten factors, ranging from zero, or no limitation, to 3, meaning severe limitations. Thus a land type with a low level of soil nutrients is coded n2, whereas an area without drought risk is designated simply as i. The total value of the numerical subscripts accompanying the letters determines the rank of a land type. Land is ranked 1 when there are no limiting factors or when the sum of the numerical subscripts is zero or less than, but including, 1, e.g.. S. and do, which together have a combined numerical value of 1 + 0 = 1. In rank 2, there is usually one dominant limiting factor, and rarely two. A land type is ranked 2 when the combined numerical value is between 1 and 2, e.g. n1d1 (medium soil nutrient with a solum of between 60-100 cm). Rank 3 has a numerical value of between 2 and 3 whereas that for rank 4 is greater than 3 (such as n2d3).
TABLE 4.4. Preliminary Ratings of the Intensities of Limiting Factors Used to Rank Land Suitability
|Slope steepness (s)||15||15-25||25-35||35|
|Solum depth (cm) (d)||100||60 100||30-60||30|
|Soil nutrient (n)||High||Medium||Low||Very low|
|Soil erosion (e)||None||Slight(sheet)||Moderate(rill)||Strong (gully)|
stones (%) (r)
|Drought risk (i)||None||Possible drying
horizon for short period
|Flood hazard (f)||None||Occasional flooding||Occasional heavy
|Drainage status (g)||Well-
|Poorly to very
difficult to improve
|Water supply (w)||Plenty||Medium||Low||Very low|
The relationship between ranks and the values of the limiting factors is summarized as follows:
|Rank 4||3 +|
The four ranks represent different qualities of land assessed. The first rank indicates the most productive land in a given land type, and it may differ significantly from other ranks in the same land type in management requirements or in the potential for improvement owing to different limiting factors. In the present system, the number of limiting factors has been kept to a minimum, and usually only one dominant factor is emphasized. However, if two factors are equally important, then both are used. Ranks may be employed as a fundamental tool for mapping land suitability at the county, provincial or national levels, and such rank symbols as SAr1 or SAr2n2 are easily depicted on maps.
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