A. Evolution, Extent and Economic Land Classification of Salt Affected Soils

Paper 1 - 1. Evolution of salinity, alkalinity and waterlogging
Paper 2 - 2. Present and potential salt affected soils - an introduction
Paper 3 - 3. Economic land classification for the prevention and reclamation of salt affected lands

Paper 1 - 1. Evolution of salinity, alkalinity and waterlogging^1/

^1/ Notes taken during Prof. Kovda's speech.

by

V. A. Kovda

Director, Institute of Agrochemistry & Soil Science of the Academy of Sciences of the USSR

Why are the basis principles of land reclamation and development, which have been known for a long time, not put into practice in the same way as the new knowledge we have gained? Some factors effecting this lack of application of the principles are:

i. the need for simplified presentation of the scientific material to farmers, surveyors, engineers and planners;

ii. dissemination of what is already known is lagging far behind;

iii. there is a “domination” of ignorance - almost a determination to resist change in some sectors;

iv. the lack of training and extension services;

v. the lack of infrastructures and organizational set-up;

vi. social problems.

It is true that there are natural difficulties beyond the influence of man to delay and retard the reclamation of saline, alkali and saline-alkali soils. Examples are found in USSR as well as in Texas and other places. In the past ages great differences have been created by nature in the humid, sub-humid, arid and extra-arid areas and all these can be affected by salinization. Today we have marshalled much information on factors affecting the soils: on their fertility, their physical destruction (some of the best chernozoms in USSR have suffered), salinity, secondary salinization, micro-structure, pH, soluble salt content, crusting - particularly after watering, and what could be called “vertisolization” below the arable horizon which is not due to heavy machinery.

During this work of desalinization and reclamation, the great benefit of leaching, what we call in some oases “capital leaching”, was recognized. Leaching under good management conditions not only decreases salinity, but stabilizes it for future control, reduces the groundwater level and also the salinity of the groundwater.

Even when salinity has been stabilized, extensive drainage is necessary to maintain the condition of the soil. In many cases vertical drainage alone is not sufficient but must be accompanied by deep horizontal drainage (e.g. the central lowlands of Armenia). Tubewell drainage can also be of use.

Care must always be taken to desalinize groundwater and not only in the soil root zone. It has taken too long to realize the important role played by groundwater in salinization and it is about time to agree on the importance of desalinization of the groundwater. Nowadays, we know the critical concentration in groundwater and that the salt content must be kept below certain levels. It depends on the chemistry of the soil, the climate, capillary action, etc. And we also know that in reclamation work and to prevent secondary salinization the water table level must be reduced.

For many years work has been done in Tajikistan and in the heavy soils near the Caspian Sea to reclaim soils and to prevent salinization and alkalization. In these areas the reduction of the groundwater level, even to a depth of 4 m played a major part in reclaiming them. In USSR a minimum depth of 5 m to groundwater is favoured.

We recently discovered the importance of artesian effect and as a result, in some extreme cases, it may be necessary to investigate the possibilities of reducing groundwater depths to 100 m, but this of course will be a very expensive procedure.

In the discussion it became clear that working on dissemination of available data does not mean that continuing collection of basic information for technology should stop. Rather that one must follow the other; there is much information already available, now organizations like FAO and Unesco can help to ensure that it is passed through the national infrastructures right down to the farmer and peasant who must apply it.

In this regard a suggestion was made to recommend the establishment of regional centres for land reclamation with sub-regional or national applied research institutes. Field demonstrations and experimental farms are essential in speeding up the delivery of results to farmers, planners and other users. The aim is to have a network according to the economic possibilities of the country and the variability of local conditions.

Paper 2 - 2. Present and potential salt affected soils - an introduction

by

I. Szabolcs

Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences Budapest

The global demand for food and raw materials produced by agriculture makes the further study and optimal utilization of soil resources of the earth imperative and urgent. In science and politics the opinion prevails that the soils of different continents can supply not only the recent demands of mankind, but may fulfil all future food and agricultural product requirements of the ever-growing population. In order to meet the requirements, the further study of soil resources must be given paramount importance, with particular regard to soils and soil forming processes that are associated with unfavourable fertility.

Salt affected soils belong to those types of soils that have low fertility. They occur to such an extent in many countries that they hinder or prevent agricultural production.

The distribution of soils affected by salt at present is closely related to environmental factors such as arid or semi-arid climate, accumulation of products of weathering in groundwater near the surface, etc. On a world-wide scale there is a considerable amount of data, maps and other materials, showing the extent of salt affected soils. Among these, the FAO/Unesco Soil Map of the World (scale 1:5 000 000) should be mentioned particularly, because it is the first to give a world-wide inventory of these soils and their distribution. Salt affected soils cover such an area in many countries in arid and semi-arid regions of Asia, Africa and South America that they cause considerable problems regarding not only the natural environment of these areas, but also the national economy.

However, we do not have yet a World Soil Charter of these soils, although the approximate size of the earth's crust covered by salt affected soils is well known. Obviously, the follow-up of the resolutions adopted during the World Food Conference held in Rome in November 1974, to prepare an assessment of the land capability will help a lot in obtaining exact data on the distribution of presently salt affected soils.

In many countries the frequency of salt affected soils makes their utilization necessary, mainly by irrigation and application of chemical amendments. In other places though, where salt affected soils are not so extensive, or the country has large areas of non-salt affected, lands, agriculture could be developed, apart from the salt affected areas; but not much practical attention has been paid to the reclamation of these soils. The utilization of present salt affected soils is hindered, mainly by practical economic problems.

Besides soils affected by salt at present, we have to distinguish potential salt affected soils. Soils considered potentially salt affected are those which are not, or only to a very low degree, saline and/or alkaline at present but human intervention, especially irrigation, could cause their considerable salinization and/or alkalization.

Theoretically, there are many factors effecting salt affected soils, however, practically it is irrigation which leads to the formation of many millions of hectares of saline and/or alkali soils in different parts of the world. Paradoxical situations can often happen in irrigation systems established even after thorough work and expensive planning and construction, the soils, instead of increasing in fertility, transform into poor saline land. This process is known as secondary salinization and/or alkalization, which is as old as irrigated agriculture.

Many thousands of km² of fertile irrigated lands were transformed into saline and alkali deserts during the history of mankind by the influence of improper irrigation. Unfortunately, this has happened not only in the past, but secondary salinization and/or alkalization is showing a disastrous increase, parallel with the construction of new irrigation systems in many countries all over the world, particularly in arid and semi-arid, regions, or in regions with mineralized ground-water near the soil surface.

The extent of irrigation - influenced by the demand for an increase in food production - makes it imperative to pay attention to potential salinization and/or alkalization in order to study and characterize this process, as well as to predict and prevent it everywhere if possible.

It is well known that the majority of irrigated territories in the world are exposed to the hazard of secondary salinization, alkalization and waterlogging. According to estimates by the UK and affiliated agencies (FAO, Unesco, etc.) more than 50% of all irrigated lands of the world have been damaged by secondary salinization, alkalization and waterlogging. In the same estimation, many millions of productive hectares in irrigation systems have to be abandoned yearly owing to these causes.

While existing salt affected soils can be recognized on the basis of a few morphological, chemical and physicochemical observations and determinations, the recognition of potentially salt affected soils as well as determination of the long-term hazard of salinity or alkalinity on any given territory necessitates special survey and methods.

Due to the paramount importance of irrigation and the close relationship existing between irrigation, drainage and the salinity and/or alkalinity of soils, it was deemed necessary to delineate on the maps of salt affected soils, whenever possible on the basis of available data, those areas which are exposed to the hazard of salinity or alkalinity owing to the introduction, the present practice, or the further extension of irrigation. The Map of Salt Affected Soils in Europe (sponsored by Unesco, FAO and ISSS) shows the areas where potential salt affected soils are developing. This map demonstrates that even in Europe, where the extent of salt affected soils is smaller than in some other continents, the surface covered by potentially salt affected soils is equal or more than the territories where soils at present affected by salt occur. Evidently, in continents with more arid conditions this rate is much higher.

Secondary salinization and alkalization take place mainly in one or more of the following situations:

i. accumulation of salts from poor quality irrigation water;

ii. increase in the level of groundwater:

a. the salt content of the groundwater accumulates in the deeper soil layers;

b. the rising groundwater transports the salts from the deeper soil layers to the surface or surface layers, or

c. the rising water table limits natural drainage and hinders the leaching of salts.

iii. lack or low effectiveness of drainage systems in irrigated soils.

a. climatic, such as; temperature, rainfall, humidity, vapour pressure and evaporation and their fluctuations and dynamics;

b. geological, geomorphological, geochemical, hydrological, hydrogeological and hydrochemical, such as: natural drainage, depth and fluctuation of water table, direction and velocity of horizontal groundwater flow, salt content and composition of the groundwater, etc.;

c. soil, such as: soil profile, texture, structure, saturated and unsaturated water conductivity, soluble salt content, salt composition and salt profiles, exchangeable cations, pH, etc.;

d. agrotechnics, such as: land use, crops, cultivation methods, etc.

e. irrigation practices, such as: the amount of irrigation waters method, frequency and intensity of irrigation, salt content and composition of irrigation water, natural and artificial drainage, etc.

The basic aims of the survey and study of potentially saline or alkaline soils are to predict the harmful processes and to elaborate, whenever possible, methods suitable to prevent the occurrence of secondary salinization and alkalization. In order to develop a reliable method of predicting salinization and alkalization the following problems have to be solved:

1) the main sources of water soluble salts (irrigation water, groundwater, surface waters, salty deep soil layers, etc.) must be identified;

2) the main features of the salt regime must be characterized (salt balances); and the whole range of natural factors influencing the salt regime must be analysed;

3) the effect of irrigation and drainage on the water and salt regimes of the soil must be predicted and determined.

In the books on salinization, alkalization and waterlogging widely used in soil science, some contain valuable data and information on the methods of studying the former two processes. For instance: Diagnosis and improvement of saline and alkaline soils. Handbook No. 60. US Department of Agriculture, Washington, 1954. The most recent and up-to-date approach and methods concerning the problem are condensed in Irrigation, Drainage and Salinity. An International Source Book. FAO/Unesco, Hutchinson/FAO/Unesco. 1973. (Ed. V.A. Kovda, C. van den Berg and R.M. Hagan).

In technical literature numerous publications describe general and local methods for the prediction and prevention of the above processes. For instance, based on these principles (see details in: Symposium on the Reclamation of Sodic and Soda-Saline Soils. Agrokémia és Talajtan. 18. Suppl. pp. 351-376., 1969.), a special survey was made in the eastern part of the Hungarian Lowland in order to predict the influence of existing and projected irrigation schemes on soil salinity and alkalinity. In spite of the many achievements in this field, we still lack a coordinated international guideline and practical handbook describing the necessary methods of assessment in order to fulfil the growing demand by soil science for preliminary survey of irrigated territories.

During the preliminary study of irrigated areas, or areas to be irrigated, in order to elaborate the proper prognosis and possible prevention methods, besides the natural factors, the projected pattern of farming-systems should be taken into consideration mainly in respect to such factors as methods of irrigation, possibility of drainage, economical aspects, etc.

Following the preliminary survey of irrigated areas and their environment, a special survey is necessary parallel with the construction of irrigation systems. In this survey the thorough study and determination of all essential soil and water properties should be included, checking the data and conclusions of the preliminary survey. During the exploitation of irrigation systems, the regular monitoring of factors influencing salinization and alkalization processes is indispensible. All these are pre-conditions for effective prognosis and for the possibility of preventing secondary formation of salt affected soils in irrigated areas.

Paper 3 - 3. Economic land classification for the prevention and reclamation of salt affected lands

William B. Peters

Head, Land Utilization Section, Resource Analysis Branch
Division of Planning Coordination, Engineering and Research Center
U.S. Department of the Interior, Bureau of Reclamation, Denver, Colorado

1. INTRODUCTION

The prevention and reclamation of Bait affected lands are of paramount importance in meeting the food crisis and helping man. The opportunities and ways and means for accomplishment are great despite the multitude of diverse and complex factors and interactions. Further, responsibilities in providing for social well-being and obligations to assure favourable environmental interactions can be accomplished. The investigations for exploration and analyses integrate the activities of the several disciplines including water quality, plant science, drainage, environmentalism, engineering, sociology, geology, soil science, economics, and land and water use and management. Important to these studies is coordination of their independent activities into a meaningful framework of analysis. From the evaluations, alternative plans are developed to indicate required programming, operation and reclamation procedures conforming to area needs and policies. Analysis is made of land use problems and opportunities associated with alternative plans, recognizing the natural and modified resource base; existing and potential land use patterns; zoning regulations; and general relationship to environmental, social and economic aspects and benefits. In this regard, economic land classification is a tool for identifying needs, establishing opportunities and selecting lands for water and salinity control.

The process is somewhat analogous to eating at a large cafeteria or shopping at a large supermarket. The choice of items and combinations are numerous but somewhere along the line, someone has to pick up the “tab”. Choices are influenced by the funds available and financing capability. The cost of investigations for planning is also very important.

Most of the developing countries are not in a position to go “carte blanche”. The exception would be some of the oil producing countries where, in addition to having ample and ready finances, the governments, in order to meet goals to become self-sufficient in food production, are more likely to subsidize on-farm development and operations in addition to project features.

The classification system described in this paper can be adapted to serve needs for various goals, land characteristics and conditions, farm enterprises and financing arrangements irrespective of water supply or control methods; i.e., it is applicable to both rainfed and irrigated lowland or upland agriculture under either private or government ownership and management.

2. LAND SELECTION

The prevention and reclamation of salt affected lands largely involve providing for control of water movement including soil-water and groundwater. This is accomplished through installation and operation of facilities and implementation of measures; i.e., water supply and distribution, water and land use, management and drainage. Formulation of plans can be efficiently guided by an effective system of economic land classification which avoids a rigid or fixed procedure. The general principles followed in the Bureau of Reclamation are applied to fit land classification to the specific environmental situation including economic, social physical and legal patterns existing in the area.

The physical, soil, topographic, drainage, climatic and water quality factors and their interrelationships influence the needed control facilities and measures related to crop production inputs and yield outputs. These are, in turn, controlled by technological levels, economic conditions, social organization, resourcefulness and motivation of people, the goals of development, and means and availability of financing. Planning is accomplished by using the land classification survey as a systematic, integrating process for the determining elements in the plan (Maletic, 1967; and Maletic and Hutchins, 1967).

2.1 Principles

The classification should conform to modern classificatory principles and practice (Sokal, 1974; Tversky and Kahneman, 1974). The system should be based on a single factor or set of factors, and the factors to be classified should be selected and adhered to for the entire classification. This principle is basic and extremely important. It is preferable that the chosen factor or factors be unifying. This principle and the unifying aspect are logically and conveniently met by selecting economic factors to reflect goals. For this purpose, unifying factors such as benefits, net income, or payment capacity to be generated by means in accomplishing goals are generally used.

In addition, four other basic principles are followed in structuring the classification to needs and goals for specific areas (Maletic, 1962). These are the principles of prediction, economic correlation, permanent-changeable factors, and arable area - service area analysis.

2.1.1 Prediction principle

Under the prediction principle, the classes in the system express the land-water-crop and economic interactions appraised to prevail after resource and management modification. This involves identifying and evaluating the changes anticipated to result from development or reclamation and management.

Examples of changes that can be brought about by modifying water control measures and management are variation in depths to water tables and associated soil moisture, salinity and aeration conditions affecting tillage and crop growth; modification of slope and microrelief by landforming; and alteration of soil profile characteristics by deep ploughing, chiselling or addition of amendments. Soil texture may be modified by sediment in water entering the soil.

The manner and magnitude of water control can effectively serve to regulate salt effect on lands, crops, social and economic conditions, and the environment. The concentration and composition of salts in the soil solution and associated exchangeable ion status on soils can be influenced by numerous factors, including the composition of water applied, the rate of water application and leaching, dissolution and precipitation of soil solution constituents, and the rate and amount of drainage.

Flooding of soil, as practised under rice cultivation, sets in motion a series of physical, microbiological and chemical processes which influence crop growth (Ponnamperuma, 1965). These include retardation of gaseous exchange between soil and air, reduction of the soil, and the electrochemical and chemical changes accompanying the reduction. There is a decrease in redox potential, increase in pH under acid conditions and decrease under alkaline condition, and an increase in specific conductance. Also, the flooding causes dentrification, accumulation of ammonia, reduction of manganese, iron, and sulfates, accumulation of the products of anaerobic organisms and other secondary effects of reduction. Cate and Sukhai (1964) have attributed the decrease in soil acidity upon flooding to the precipitation of aluminium hydroxide, the reduction of ferric iron and the absorption of ferrous iron by the clay.

Water supply and control and related salinity control are determinants of successful agriculture with respect to either diversified upland cropping or wetland rice production. Regulation of water inflow and outflow largely controls salinity, sodicity, acidity, reduction products and aeration. Thus, the prediction principle should be concerned with the quality of water, the soil, subsoil and substrata characteristics and conditions, drainage, and land use and management - ail under specific plans. In prediction, the classification also deals with water requirements, soil productivity following landforming and expected soil profile modification practices, flood hazard, soil erosion, quality or return flow and crop production inputs and outputs.

2.1.2 Economic correlation principle

The economic correlation principle involves relating, within a given setting, the physical factors of soil, topography and drainage with an associated economic value. The economic basis for the land classification is usually chosen to contribute toward determining the feasibility of water control planning for increasing net farm income, to achieve benefits and to evaluate the interrelationships of investment feasibility and the water use.

More specifically, the economic values chosen to define land class depend upon the purposes to be served by the land classification. As applied to the United States, the economic value is defined in terms of net farm income and payment capacity. Net farm income measures the benefits directly accruing to the farmer, while payment capacity after making allowances for farm returns represents the residual available to defray the cost of water (USDI Bureau of Reclamation Manual, 1953). For initial planning studies in the developing countries, it has generally been the policy to use direct benefits (net farm income), as the economic parameter in land classification rather than payment capacity (USDI Bureau of Reclamation, 1967). This is done to eliminate the need for immediate resolution of the repayment policies and the extent of irrigation subsidies that might be applicable. With newly developing countries, it can be expected that in the early period of project investigation, the development of a repayment capacity is not sufficiently firm to base confidently the land classification on repayment criteria.

With land classes defined as economic entities, a set of relevant and mappable land characteristics is chosen for the time and place to provide a physical definition of the land specifications. The land class determining range of these characteristics varies with the economic, ecological, technological and institutional factors expected to prevail in the area. As a consequence, land classes express, in terms of economics, the local ranking of land for modified use; e.g., best suited, moderately suited, poorly suited and unsuited for irrigation development.

2.1.3 Permanent-changeable factors principle

The permanent-changeable factors principle recognizes that changes in land arising from water and land development impose a need to identify characteristics that will remain without major change and also to identify those which will be significantly altered. This identification permits construction of a consistent set of land class determining factors assuring uniform appraisal of land conditions by the various disciplines engaged in making the land classification surveyed. Most land factors, including soil depth, are changeable at a cost. Typical changeable factors include salinity, sodicity, titratable acidity and exchangeable aluminium, depths to water table, relief, brush and tree cover, rock cover, drainage and flood hazard. Particle-size distribution of subsoils and substrata occuring at depths not disturbed by tillage and landforming is about the only factor that may not be altered.

Whether given characteristics will be changed usually depends upon economic considerations. The land classification survey thus deals with two aspects of this principle. Can the change be accomplished, and what degree of change is economically feasible? This is largely dependent on the climatic and economic setting of the project. For example, a large investment may be made to reclaim a saline, sodic or acid soil which after improvement will yield a net farm income of US$500 per hectare. In another setting, where net income after improvement would only be US$75 per hectare, the soil having similar conditions would be regarded as non-reclaimable. In the latter case, it may be infeasible to make the change.

2.1.4 Arable area - service area analysis principle

The arable area - service area analysis principle relates to the selection of lands to be served and involves a two-step process. In the initial step, land areas of sufficient productivity to warrant consideration for service are identified. Upon this determination, there is superimposed the selection of the lands to be specifically included in the plan of development. The former may be termed arable lands and the latter service area or lands selected for service. The selection of arable lands is guided by farm production economics; i.e., the economic value such as benefits, net income or payment capacity, as chosen to define the land classes. The service area or land selected for service is guided by the economic goals selected to guide plan formulation. The scope of plans may be influenced by relationships to purposes served other than water supply to lands such as in the case of multipurpose projects.

The application of plan formulation criteria to the classification generally leads to successive elimination of identifiable increments of arable lands from the plan of development. Typical adjustments include (i) elimination of non-economic increments such as those that are too costly to serve, drain or manage; (ii) conformance of land area to utilizability, serviceability and manageability; (iii) exclusion of isolated segments, odd-shaped tracts and severed areas that cannot be efficiently fitted into the farm unit pattern; (iv) deletion of proposed public rights-of-way; and (v) elimination of areas unable to meet minimal criteria for economic returns under the plan. Of these factors, items (i) and (v) are goal-dependent.

2.2 Water Suitability

Water quality evaluations are approached by analysis of the environmental setting in the context of predicted water use (USDI Bureau of Reclamation, updated 1975). The determination of the suitability of water involves integrating land and water factors. In this process, land classification surveys are utilized to delineate land classes that would favourably respond to a water supply of a given quality. This selection of land as a potential part of a water development is then tested as to feasibility by application of plan formulation criteria.

Water quality standards per se are not applied in appraising the usability of water for irrigation. As has been stated by Fireman (1960): “Its usability depends on what can be done with the water if applied to a given soil under a particular set of circumstances. The successful long-term use of any irrigation water depends more on rainfall, leaching, irrigation water management, salt tolerance of crops and soil management practices than upon water quality itself.”

2.3 Application of Methodology

The application of procedures for planning requires thorough study to assure fitting resource developments to the goals and social, economic and physical settings. Methodology should be developed for application to local needs using the principles previously discussed. In several countries, there has been a tendency to adopt rather than adapt, i.e. to attempt transfer of procedures rather than develop systems based on the principles. Usually, the transfer approach will not work satisfactorily, thus it is essential to go through the rigours embodied in applying the principles.

The system is also applicable to either state or privately owned or operated farm enterprises irrespective of subsidies involved. This is accomplished by defining the land classes in terms of relevant economic parameters. Therefore, it is necessary to explore fully and consider the controlling policies in structuring and implementing the land classification for local application.

The system is applicable to either diversified cropping and wetland rice production for all situations and ranges in water supply and control including rainfed agriculture, irrigated agriculture, water regulations in flood plains and reclamation of marshlands and tidelands. The principal differences in requirements are the source, quality and control of water. All the principles and components, particularly economics with respect to productivity, land development, flooding and drainage are highly relevant.

The major basis for physically, chemically and economically differentiating diversified croplands from wetland ricelands is the ability of the soil to attain optimum soil submergence, susceptibility to soil puddling and control of water table (Grant, 1964). Therefore, the differentiating characteristics of wetland rice classes are primarily predicated on water control as related to soil characteristics and conditions of adequate drainage and differentiating soil characteristics that have a strong influence on the yield and cost of producing diversified crops.

Many of the soil parameters, especially with respect to prediction and productivity, are different in classifying lands for the two types of irrigated cropping. In contrast to upland agriculture, the classification of soils for paddy rice production involves the prediction of ultimate soil conditions that will occur from flooding. Some of the conventional soil tests used in surveying soils for diversified crop production are not applicable to the characterization of soils for paddy rice production because of the drastic changes in soil properties, their dynamic state and the wide differences among soils produced by flooding (Ponnamperuma, 1965).

In regard to productivity, the ability to attain optimum soil submergence or saturation and control of water tables would be requisite for rainfed wetland rice production on soils high in neutral salt exchange acidity (in cases where the exchangeable aluminium and other acidity can be sufficiently neutralized by processes associated with reduction). Income from rice grown under upland conditions on such soils could be affected by reduction in yields of variations sensitive to the prevailing acidity or the cost of neutralizing the acidity with amendments. Acid sulphate soils could require amendments for flooded rice culture. The predictive and permanent-changeable factors, along with land development costs, are very important in land levelling and terracing operations exposing subsoils. Flooding and inadequate drainage are major factors affecting cultural practices and production of paddy rice in vast areas of Monsoon Asia. These can be highly significant economically from the standpoint of either living with the situation or providing measures for water control measures. Numerous other facets could be cited.

In the case of non-irrigated diversified or upland crop production in arid or humid areas, the land class determining features would include soil, topography, drainage and associated management factors influencing ranges in productivity with available moisture plus any land development measure to be taken to modify productivity or ease production. In appraising land potential of swamps, land development costs for drainage and tree removal would enter into the classification as well as productivity of the lands after reclamation. The structuring of the land classification depends on the purpose of the survey, constraints and conditions.

Economic studies and consultations with international agencies, governments involved and other authorities assist in determining economic and financial criteria to be used in project planning and developing land classification specifications. These include period of analysis, interest and discount rates, repayment capacity and means of financing. It is also necessary to estimate the approximate project coat, operation and maintenance cost, and farm costs and returns. These elements provide the information needed to establish the minimum level of land quality which should provide benefits sufficient to meet project costs. As a product of this determination, also provided are the maximum permissible land development costs. After this cutoff point is established, it is possible to finalize the land classification into ranges of net income.

Before the land classification is started, the matter of handling land development costs is determined. Methodology between countries may vary according to whether the government expects the farmer or landowner to pay for all development costs or the government does all of the on-farm development with no direct cost to the farmer. The land classification is varied to show a reduced payment capacity in net farm income and lower land class where land development costs are borne by the farmer. When development costs are handled as a government expense, they do not influence the land class except when the maximum permissible expenditure would be exceeded.

After identifying with the policies to prevail, the classification is guided by a series of somewhat interrelated stages. These may be identified as the pre-survey, survey and post-survey stages.

2.3.1 Pre-survey stage

The pre-survey stage involves study of the land resources, associated productivity and drainage capability experiences in a fully developed area having physical and climatic conditions similar to the area of investigation. In developing, fitting and testing the land classification specifications to project conditions, farm enterprise studies are made to determine the net farm income for the various classes (Seldon and Walker, 1968). Establishing these standards for classifying land for net farm income, involves the projection of representative farm enterprise and representative levels of farm management. These, of course, must stem from information of the present day situation and trends in the development and application of technology in agriculture. In the latter regard, it is recommended that adaptive research programmes be undertaken to help guide the land classification work by providing answers to questions pertaining to management systems, fertility, amendments, water management practices and other factors.

The characteristics and qualities of lands which determine suitability for warranting modification in water supply, control, management and use vary with each project. The land class determining factors represent selected and correlated ranges for such characteristics as texture, depth to bedrock, hardpan, sand, gravel, caliche or other root-limiting influences, structure, consistence, colour, and mottling, kinds and amounts of coarse fragments, and kind, thickness and sequence of horizon. In addition, the prediction aspect of selecting arable lands requires many laboratory measurements. Performance qualities are also either measured or inferred. These would include factors such as fertility, productivity, erodibility and drainability, as well as such measurable factors as infiltration rate, hydraulic conductivity, moisture characteristics and moisture-holding capacity.

In investigating lands consisting of highly leached and weathered soils, a strong soil characterization programme should be conducted. The chemical status of such soils needs to be carefully evaluated along with observable characteristics in making sound selections of irrigable land. The problems met with these soils are usually fertility related chemical characteristics requiring special appraisal. They include status of weathering of the clay minerals, soil acidity, charge status, soluble and exchangeable iron, aluminium and manganese, base saturation, and nutrient status of the soils. Such characterizations identify infertile soils having limited suitability for continuous crop production because of high inputs of both money and management. On other soils they indicate the type and level of production inputs required to attain specified yield levels of particular crops. Of course, other soil characteristics such as texture, structure, depth, water-holding capacity, infiltration rate, permeability and claypans are evaluated as are water quality, climate, topography and drainage conditions. Salinity reclamation and control can be a factor in high rainfall areas including the tropics.

In considering low base status soils, Sanchez and Buol (1975) recommend that: “related inputs should be optimized by (i) selecting crop varieties and species more tolerant to nutritional deficiencies or toxicities, (ii) applying fertilizers at lower rates than those recommended by classic marginal analysis, and (iii) increasing the efficiency of applied fertilizers in such soils.” Should such criteria be adopted, the analyses need to be made in relation to the economics of water control for specific settings. The ranges in soil properties comprising land classes can vary among and within countries.

Topographic characteristics considered consist of the degree of slope, relief and position. These factors are evaluated as they influence land development needs and costs, method of water distribution, design of on-farm conveyance systems, erosion hazards, crop adaptability, drainage requirements, water use practices and selection of management systems. It is necessary to make decisions regarding the extent to which slope and relief will be modified by landforming, and to make estimates regarding the amount, type and cost of land development.

2.3.2 Survey stage

In the survey stage, appropriate land classification specifications are applied in the performance of the arable classification. This involves field traverse, soil and substrata observation and sampling, laboratory analysis of soil samples, delineation of the land classes, subclasses, informative appraisals and the related procedures necessary to accomplish the field survey work. Performance of the fieldwork is guided by the type of investigation being performed. These may be of appraisal, feasibility, pre-construction, post-construction or post-development grade. If the appraisal studies show promise of achieving the development goals, then more detailed studies are subsequently performed.

The requirement for investigative detail is set not only by the type of investigation being formed but also by the complexity of the landscape being investigated. In accomplishing the field survey, the Bureau of Reclamation generally uses not more than five land classes defined on the basis of their range in payment, capacity or net income. In short growing season areas, fewer land classes are sufficient. Class 1 lands have the highest level of suitability. Class 2 lands have intermediate suitability. Class 3 lands have the lowest suitability for general farming. Class 5 is used as a temporary designation for lands requiring special studies before a final land class designation can be made, and class 6 is land not suitable for development.

Subclasses are used to indicate the reasons why land is placed in classes lower than class 1. This is shown by appending the letter s for soil deficiency, t for topographic deficiency and d for drainage deficiency to the land class designation. Subclasses of the land classes 2, 3, 4, and 6 are s, t, d, st, td and std. The mapping unit symbol also provides for showing the present land use productivity level development cost, water requirement, drainage requirement and, as needed, special appraisals to indicate specific deficiencies.

For drainage evaluation purposes, the fieldwork involves numerous observations and measurements of conditions of the substrata as well as the true solum and superficial parent materials. Observations to a depth of 3 metres or less in case of a barrier are used in all investigations for irrigated diversified cropping, and to greater depth as needed depending upon the particular type of landform encountered.

2.3.3 Post-survey stage

In the post-survey stage, the arable land classification may be modified as additional pertinent physical, engineering, hydrologic and economic information is obtained. Arable classification adjustments are needed if the final project plan and costs for water and drainage are significantly different from original estimates. During the post-survey stage, application is made of tests for engineering feasibility and project formulation criteria of benefits and costs, repayment, and the operation, maintenance and replacement costs as needed to select the plan and related service area under the development goals.

Results of the land classification are applied to (i) selection of service area, (ii) determination of water management requirements, (iii) selection of land use and size of farm, (iv) determination of project payment capacity, (v) determination of water control benefits, and (vi) development of layouts for water supply and drainage system.

2.4 Laboratory Support

In addition to field measurements for water movement and retention in soils, a certain amount of characterization by laboratory methods is required to support the land classification. Because laboratory studies should serve to substantiate field appraisals, it is essential that laboratory work be closely coordinated with fieldwork. The number and type of studies are determined by the controlling project or area specifications and needs. There should be a joint plan between field and laboratory investigations before taking samples if maximum use is to be made of data obtained. Problems should be studied rather than standard or routine tests made.

This testing own be very expensive. Unfortunately, the results are frequently misleading. In many areas, there has been a tendency to “overtest,” i.e., perform too many or unnecessary tests on certain soils at the expense of not performing essential or critical testing on particular soils. Also, there have been shortcomings in proper interpretations.

In submitting soil samples for laboratory characterizations, the laboratory should be furnished with pertinent field appraisals including soil textural class along with the tentative land class designation. The soil samples should represent genetic horizons with no more than 60 cm depth per sample.

2.4.1 Approach

The first priority in laboratory characterization should be directed toward direct and indirect measurements that evaluate soil structure and its stability, effective soil cation exchange capacity and soil reaction. After this is accomplished then consideration should be given to testing that confirms, explains the causes of phenomena previously observed or predicted, reveals the presence of toxic elements (salinity level, boron content, sodicity, acidity, reduction products, etc.), and indicates what and how much is required to cope with the soil deficiency under eventual field conditions with water control and management.

The laboratory testing might include determinations for soil structure stability by measurements of floe volume and hydraulic conductivity of fragmented samples; moisture retentivity at 15 bars pressure; soil reaction by measurement of pH in 0.01M calcium chloride (1:2) and pH in water (1:1) or (1:5); soil salinity by measurement of specific electrical conductance of soil-water extracts; soil acidity by measurement of exchange acidity including exchangeable aluminium (that portion of soil acidity that can be replaced with a neutral-unbuffered salt); titratable acidity (amount of acid neutralized at a selected pH); soluble aluminium; soil solution concentrate and composition including sodium and calcium plus magnesium; exchangeable cation status; organic matter; available phosphorus; and others.

Laboratory operations and characterizations for moisture retentivity at pressures lower than 15 bars are not recommended unless suitable correlations with field conditions are developed and then only in relation to diversified cropping. In general, the nonroot zone depth soil samples need not be characterized for acidity hazards. Soils of near neutral or basic reaction need not be characterized for exchange acidity. In the initial screening of samples for screenable characterization, soil-water suspension of 1:1 ratio may be substituted for the time-consuming saturated soil pastes. The blanket laboratory analysis for soil textural class is neither required nor desired.

Particle size analysis should be limited to master size characterization, the occasional confirmation of field textural appraisals and the training of new employees.

2.4.2 Screenable testing

After establishing interrelationships that exist for various properties, it is usually desirable to implement screenable testing. Depending on the relationships and degree of correlation, a procedure for sequence of testing and screening of samples might encompass the following phases. Under phase I of the scheme, all samples would be characterized for soil structure stability through measurement of hydraulic conductivity on a fragmented sample basis during an initial and elapsed time interval and volume of wet settled floccules.

In the second phase of testing, all samples from the root zone depth, i.e., those depths that would prevail after land levelling, would be characterized for moisture retentivity at 15 bars pressure; electrical conductivity of 1:1 soil-water ratio extracts; and soil pH in water (1:1) or (1:5), and CaCl₂ (1:2). Samplings of greater depths in the case of barrier situations should be appraised on salinity levels.

In the third phase, selected root zone samples suspected through the testing results of phases I and II to be highly acid, low in base saturation, or low in cation exchange capacity, should be further characterized for neutral salt exchangeable acidity, aluminium, sodium and calcium plus magnesium. Also, in the third phase, selected root zone soil staples suspected through the testing results of phases I and II to be salt affected should be characterized for electrical conductivity of the saturation extract and sodium absorption ratio and residual gypsum.

In phase IV, selected samples having been characterized during phases I, II and III to be saline-acid would be characterized for soluble aluminium. Also selected soil samples coming from potential ricelands and found to be acid in phase III testing would be characterized for active iron and manganese and organic matter content.

Concurrently with the screenable testing, the master site samples should be characterized on a complete analysis basis, i.e., all samples from all depths should be characterized for the items previously mentioned.

The field and laboratory soil scientists should study the results of field and laboratory characterizations for possible correlation, especially with respect to soil genesis, soil morphology and clay mineralogy.

USING NATURAL SYSTEMS

Many workers trained in mapping natural bodies have great difficulty in initial attempts to adopt and adapt to economic land classification. The difficulty seems to be in conceptualizing the landscape under the conditions expected to prevail under the new land use regime through economic reasoning and installation of control structures. Another difficulty concerns notions that boundaries of natural bodies will coincide with class boundaries, ranking land for use suitability. This rarely occurs because kinds of soil having natural boundaries are commonly found in contrasting economic environments or vice versa. The location, size of tract and other economic characteristics of land are highly significant in land classification.

It can be very difficult to rely upon natural body mapping, as commonly made, for classifying a given area particularly on complex and problem lands consisting of soils and substrata requiring extensive and intensive field and laboratory characterization. Although logical procedures can be advanced for accomplishing the required integration, experience has shown that the procedures necessary for a land classifier to establish class boundaries related to natural body mapping units can be nearly as time consuming as the conduct of a basic land classification without benefit of a soil survey. This is not to imply that soil surveys are not useful. Natural soil bodies, because of their information content, can provide much essential information, including bases for deriving predictors.

The Soil Resources Development and Conservation Service of the Food and Agriculture Organization of the United Nations, Rome, Italy, under the leadership of Dr. R. Dudal, is to be commended for recognizing needs and implementing revised soil survey procedures to serve water resource planning better.

REFERENCES

Cate, R.B. and Sukhai, A.P. 1964. A study of aluminium in rice soils. Soil Sci. 98:85-93.

Grant, C.J. 1964. Soil characteristics associated with the net cultivation of rice. Proc. Symp. The Mineral nutrition of the Rice Plant, held at the International Rice Research Institute, Los Banos, Philippines, February 1964. John Hopkins, Baltimore, Maryland.

Fireman, M. 1960. Quality of water for irrigation. University of California Extension Service, Davis, California.

Maletic, J.T. 1962. Principles involved in selecting lands for irrigation. Paper presented at the International Seminar on Water and Soil Utilization, South Dakota State College, July, 1962.

Maletic, J.T. 1967. Irrigation, a selective function - selection of project lands. Paper presented at International Conference on Water for Peace, Washington, D. C., May 1967.

Maletic, J.T. and Hutchins, T.B. 1967. Selection and classification of irrigable lands. In: R.M. Hagan et al (ed) Irrigation of Agricultural Lands. Amer. Soc. Agron.

Ponnamperuma, F.N. 1965. Dynamic aspects of flooded soils and the nutrition of the rice plant. Proc. Symp. The Mineral Nutrition of thy Rice Plant, help at the International Rice Research Institute, Los Banos, Philippines, February 1964. John Hopkins, Baltimore, Maryland.

Sanchez, P.A. and Buol, S.W. 1975. Soils of the tropics and the world food crises. Science, Vol. 188, No. 4187.

Seldon, T.H. and Walker, L.D. 1968. Economic evaluation and selection of lands for irrigation. Paper presented at the Soil Survey Seminar, Bangkhen, Thailand, August 1968.

Sokal, R. 1974. Classification: Purposes, principles, prospects. Science, Vol. 185, No. 4157.

Tversky, A. and Kahneman, D. 1974. Judgement under uncertainty: heuristics and bias. Science, Vol. 185, No. 4157.

U.S. Department of the Interior, Bureau of Reclamation. 1953. Manual, Volume V - Irrigation Land Use; Part 2 - Land Classification.

U.S. Department of the Interior, Bureau of Reclamation. 1967. Instructions for the Conduct of Feasibility Grade Land Classification Surveys of the Lam Nam Con Project - Thailand.

U.S. Department of the Interior, Bureau of Reclamation. 1975 (updated). Brief Statement - Water Quality Evaluations.