THE USE OF GEOGRAPHIC INFORMATION SYSTEMS
FOR SOIL SURVEY AND LAND EVALUATION

ABSTRACT

Geographic Information Systems (GIS) play an important role in soil survey and land evaluation for land use planning in the Lao PDR nowadays. This paper addresses in particular the use of Geographic Information Systems to help determine which areas in the Central Part of the Lao PDR are most suitable for rainfed lowland rice, cash crops and fruit trees.

The study at hand concentrates on establishing suitability ratings for different types of crops in different parts of the country given different soil, physical and other factors. The methodology proceeds by converting land characteristics into a set of land qualities that are relevant for the land utilization type.

The Geographic Information Systems provides the digital soil map, comprised of digitized polygons of soil units linked to an attribute table of quantitative soil properties. Each land quality is determined by assessing the appropriate land characteristics. The user specifies the criteria for assessing suitability and matches the land quality with the land use requirements for specific crops to be evaluated (based on bio-physical considerations) by using Automation Land Evaluation System (ALES) and map results using GIS.

The results of land suitability assessment from this study area indicate that the most areas evaluated are moderately (S3) to slightly limited (S2) for rainfed lowland rice, cash crops and fruit crops. This is due to the limited available nutrient content and soil reaction. Consistent yield improvements can be expected by a proper choice of crops or alternative land uses and/or by concentrating on an appropriate management on liming and fertilizer application.

INTRODUCTION

The Lao PDR has abundant natural resources, i.e. forest, land and water. In the period 1950-1960, the forest cover of Laos covered about 70% of territory. Since then, however deforestation has increasingly become a problem. Deforestation in Laos is attributable to several factors. In upland areas, the shifting cultivation practices of ethnic groups (253.000 families or 1.5 million) destroys forest at a rate of roughly 250.000-300.000 hectares per annum. In addition, increasing pressure from agricultural expansion, illegal logging as well as traditional land clearing practices where large areas are burning for hunting and /or for other purposes contribute heavily to deforestation.

In order to solve this problem, Party and Lao Government have developed a land use policy, which aims to protect, conserve, and rehabilitate land and forest resources.

Forest and Soil resources inventory and the assessment of land suitability classification is one of the main strategies of land use policy thus, the Soil Survey and Land Classification Center accordingly concentrates it work on soil survey and land evaluation of the physical environment.

At present, the soil survey at a reconnaissance scale has been completed throughout the country. However, most of the survey areas were concentrated on the areas of low-relief, which normally have slopes less than 55 percent. At the same time, a number of semi-detailed and detailed soil surveys were also made to serve the needs of soil interpretation for agriculture planning by various government agencies.

To date, the Soil Survey and Land Classification Center has identified about 16 soil groups and more than 20 soil units in the Lao PDR Unfortunately, these are many soils defined in such a way that users lacking a soil science background find it difficult to understand them. Therefore one of the main priorities of SSLCC has been the establishment of soil interpretation for agriculture use, which can help other agencies in make more meaningful use of soil and land resource management.

OBJECTIVE:

The objective of this study is to present an approach to establishing GIS techniques in conjunction with other computerized land evaluation tools to generate land suitability maps from soil-crop suitability maps and their integration for land resource management.

MATERIALS AND METHODS

Soil study:

The study area is located in Paksan, Bolikhamxay province in the Central part of the Lao PDR and lies on longitudes 103 20’E and latitudes 18 30’N. The topography is flat or almost flat with 2-8% slope to steep land with slope more than 55%. Climatologically the area has a dry period from November to May and wet period from June to October. The average annual rainfall during the growing period is about 1700mm and mean temperature around 28°C.

The system of soil classification used by the Soil Survey and Land Classification Center (SSLCC) is derived from the FAO/UNESCO ‘s legend soil map of the world (1989 revised legend). There are two categories: soil groups and subgroups (units). Classification is based on soil properties (diagnostic horizons and properties) observed in the field or inferred from observation or laboratory measurements. In this context, a soil group consists of soils that are developed on similar materials and under similar environmental conditions (physiography, topography, and slopes and drainage condition). Soil subgroups are differentiated from one to another according to the chemical-physical properties of soils and/or soil diagnostic properties.

The treatment of materials from field survey and from laboratory for map compilation included the following: Soil unit boundaries are delineated (on a topographical map at 1/100.000 scale, produced by Russia with Gauss projection) by transferring information collected from aerial photograph interpretation, field survey results and elements of the interpretative situation from aerial photographs on to the base map; each mapping unit should bear the index of mapping unit featuring soil group, soil subgroups…etc.

The soil pattern of this study area includes six soil groups: Arenosol, Regosols, Alisols, Acrisols, Luvisols and Cambisols with associated soil units: Haplic Arenosols, Dystric Regosols, Gleyic Alisols, Ferric Alisols etc. as show in the soil map legend, below. Land mapping units are presented on the map by soil profile code which refer to the dominant soil group in capital letters, the subgroups (soil units) by lower case letters, and are augmented with information on soil depth class, soil texture, slopes class and soil fertility – as shown below:

P 120: LPd-R-SL-b(M)

Figure 1. Land mapping units identified in the study area.

Application to Land Suitability Evaluation for Cropping System:

This land evaluation methodology proceeds by converting land characteristics or primary land attributes that are recorded by a soil survey, into a set of land qualities that are relevant for the land utilization type. Land qualities are important for determining the physical land suitability for growing crops, i.e., nutrient availability, nutrient retention, erosion hazard, moisture availability, temperature regime and rooting condition. These land qualities can be ranked according to the following classes:

• S1- Lands very well suited for crop production; having no significant limitations that restrict their use for this land use alternative.
• S2- Land well suited for crop production; having slight limitations that restrict their use for this land alternative.
• S3- Land moderately well suited for crop production; having moderate limitations that reduce the choice of crops and/or require special land management for this land use alternative.
• N- Lands not suited for crop production, having very severe limitations that preclude their use for this land use alternative.

Flowchart of the operations needed to create a map of suitability classes for crops using FAO land evaluation procedures

Each land quality is defined by specific combination of selected land characteristics (flowchart) i.e. nutrient availability can be derived directly from the soil properties (% organic matter, available phosphorus, available potassium and soil reaction pHH20); rooting condition and erosion hazard can be considered from soil information (effective soil depth, slope class, soil texture) and moisture availability refers to the water requirement in growing period can be derived from rainfall and temperature regime can be derived from mean temperature in growing period. The way in which these combinations are performed must be specified by the user and expressed in the form of decision trees.

A decision tree is a basic component of the model. The decision tree can be a severity level or a subclass decision tree. The severity level decision trees allow one to place each land unit into one of the defined suitability classes, based on how good the LURs of each LUT are met by the prevailing land characteristics. The subclass decision tree assigns specific physical suitability subclasses as a final output of the decision procedure, indicating the major limitations.

The tabulated land use requirements for a list of crops (Qualitative Land Evaluation 1995; SSLCC) have been used for specific suitability rate for each crops and figure below shows a simple decision tree for land use type rainfed lowland rice and land use requirement nutrient retention.

Table1: LAND USE REQUIREMENT OF SOME LAND USE TYPES

Nutrient Retention (nr)

Figure 2. Example of a severity level decision tree for land use type rainfed low land rice and land use requirement nutrient retention

The graphs, the land use requirement " Nutrient Retention" is evaluated through an assessment of cation exchange capacity by sum of bases plus extra. Acidity (CECe) and Basaturation (Bases). A first level decision is made on the basis of the cation exchange capacity by sum of basses plus extr. acidity, with additional subdivisions related to the amount of basaturation (Bases).

RESULTS

The Automation Land Evaluation System (ALES) is used to compute and present the result of land suitability classification. The land suitability for each one of the lands mapping units of a survey area can be demonstrated in the table2 below. This table shows mapping code and approximate extent of those mapping units suitable for rainfed lowland rice, cash crops and fruit crops.

Land mapping units are grouped into classes according to degree of limitation in use or risk of damage when used. Thus, the most serious degree of limitation determines the suitability classes and classes are indicated by numeric 1 to 4 increasing order of suitability and each class is subdivided into subclass according to dominant kinds of limitations. Lower case letters following the class number indicates the dominant limitation.

The overall suitability of the study area for the Rainfed lowland rice, Cash crops and Fruit trees is illustrated in fig.2 ,3 and 4.

Figure 2 Suitability map of the study area for rainfed low land rice

Figure 4. Suitability map of the study area for Citrus.

The results of the land suitability classification in this study area indicated that: the most areas evaluated are moderately limitation (S3) to slightly limitation (S2) for rainfed low land rice, cash crops and fruit crops due to most severe limitation factors in production are the limited available nutrient content (na) and soil reaction (pH). Thus, an appropriate management on water supply, drainage arrangement, liming and fertilizer application should take into consideration.

Generally, there are a lots of land use types in the study area in which some land use types answer the goals of development, some land use types not. Thus, it is necessary to select suitable land use types.

The principle of selecting suitable land use types was based primarily on information gathered from the soil and land capability maps i.e. soils are grouped in term of their response to the same productivity level and/or their requirement common and similar management practices for specific use or crops cultivation. Optimum prospective land use of Thabok, Bolokhamxay province was classified in accordance with 9 land utilization types as the table3 below:

Land Utilization Types (LUT)

Rp-Rainfed lowland rice

Cash crops: Co-Corn, Cot-Cotton, Mb-Mung bean, and Sb-Soy bean, Tob-Tobacco, Su-Sugar
cane…

Fruit crops: Ci-Citrus, Man-Mango

Figure 5. Optimum prospective land use map of the study area

CONCLUSION AND DISCUSSION

The use of Geographic Information Systems in conjunction with other computerized land evaluation tools to generate land suitability assessment has both advantages and drawbacks. However, the advantages are comparatively greater.

The main advantages include that: models can be built based on local expert knowledge, available data and specific objectives, enhancing a better use of local experience to solve local problems and output can be integrated with a GIS to produce required maps.

The main drawbacks include that: a strictly user-defined approach is followed to assign severity levels to combinations of land characteristics values, giving way to some subjectivity in the decision.

Although the land suitability classification system in the Lao PDR has been developed recently, there are still some problems that limit its use. Some problems and reactions to the system can be summarized as follows:

• Although the agronomic data and other observations of crop performance are essential for classifying the land in suitability, they have been used to a limit extent for developing the system. The reason is that there are not so many rigorous research investigations on crop response to soil in the Lao PDR
• Some limitations being used for the determination of classes or subclasses are rather difficult to visualize. For instance flooding regime, moisture stress and erodibility are difficult ones.

REFERENCES:

Soil survey and Land Classification Center (SSLCC), 1995 Qualitative Land Evaluations.

Soil survey and Land Classification Center (SSLCC) " Methodology of soil Survey and Land Classification"

Food and Agriculture Organization of the United Nations " Land Evaluation for Development"

Rossiter and Van Wambeke,1989.ALES:Automation Land Evaluation System. ALES User’s manual, version 2.2, Dept. of Agronomy, Cornell University, NY.

Dr. Ty PHOMMASACK, Land Use/Land cover change overall land use policy in the Lao PDR