Soil Survey: Objectives, Use and Techniques

After reading this article you will learn about:- 1. Definition of Soil Survey 2. Objectives of Soil Survey 3. Use 4. Types 5. Techniques 6. Interpretation 7. Importance.

Definition of Soil Survey:

Soil survey is a definite study of soil morphology in the field, corroboration of diagnostic soil properties in the laboratory, classification of soils of the area in well-defined units, plotting their extent and boundaries on a map, and prediction of the adaptability of these soils to various uses.

Objectives of Soil Survey:

Since the initiation of surveys in 1899 the principal objectives were to predict whether new crops could be grown on soils where they were never grown before and to learn enough about certain soils to predict how they would respond when irrigated with a known quality and quantity of irrigation water.

These objectives have been refined and expanded to include a rational means of transferring technology from one soil to another, interpretations for predicting land use for every soil mapped and to serve as a scientific base for taxation and zoning laws. In a broad sense the objective of soil survey and both fundamental and applied.

Fundamental soil survey includes in expanding knowledge and understanding of different types of soils in relation to their genesis, development, classification and nomenclature. Applied soil survey includes soil maps in addition to fundamental objective of soil survey.

Soil survey and mapping help in:

(i)The transfer of technology under different soil conditions and management practices.

(ii) Providing information needed for developing optimum land use plans and for bringing new areas under agricultural uses.

(iii) Delineating the problematic soils, such as saline-sodic, waterlogged, eroded and waste lands and in suggesting soil and water conservation measures.

(iv) Land settlement, rehabilitation, tax appraisal, locating air-ports and other engineering structures etc. and in public sanitation works.

(v) Demarcating disease infested and nutrient deficient areas which may indirectly help in controlling diseases and correction of particular nutrient deficiency.

Use of Soil Survey:

Soil survey bulletins and maps are useful as a basis for other scientific work. Land evaluation and appraisal, statistical studies and sociological investigations are other interests served. Nutrient index coupled with soil survey may be used for assessing fertilizers requirement and potential productivity of an area.

The extension specialists and agricultural agents find the survey maps and bulletin a guide in making suggestions and recommendations. Predictions can be made of hydrologic changes in relation to modification in land-use patterns. Prospective roadbeds can be selected from soil survey maps. Estimates of water runoff and infiltration can be made on the basis of soil characteristics enumerated in soil survey bulletins.

Types of Soil Survey:

Depending upon the objective, method, type of base map available and the intensity of observations, four types of soil survey are recognised which are given below:

(i) Detailed soil survey

(ii) Reconnaissance soils survey

(iii) Detailed-reconnaissance soil survey

(iv) Semi-detailed soil survey.

(i) Detailed Soil Survey:

In detailed soil survey boundaries of soil units are delineated from observations by actual traverses throughout the course of the boundary. Soils are examined in detail and the close intervals in an area to detect differences that can be significant in their use and management.

Detailed soil surveys are conducted to furnish information required for a proper assessment of soil properties, terrain features, erosional aspects and other related factors that can help in working out the use capability and the management practices for soil conservation and better production of crops and maintenance of soil fertility.

Cadastral maps (1: 8,000 or 1: 4,000 scale) or aerial photographs (1: 15,000 scale) are generally used as base material for preparing soil maps for detailed soil surveys. The mapping units on a detailed soil map show soil series, types and phases. Detailed soil surveys are laborious, time consuming and much expensive. Detailed soil survey is of two types, i.e. low and high intensity survey.

(ii) Reconnaissance Soil Survey:

This type of soil survey is undertaken to prepare resource inventory of large areas. It identifies broadly the kinds of soils and their extent of distribution. It enables to assess broad potentialities of soils and recognition of areas of promise that are suitable for intensive and modern agriculture and those requiring priority for amelioration.

In these surveys the soil boundaries are not totally traversed, but drawn partly by extrapolation. The scale of mapping is 1: 50,000, using topographical maps of the survey of India as base material or aerial photographs of similar scale wherever available. Reconnaissance soil surveys give information for detailed soil surveys and broad land use planning and agricultural development.

(iii) Detailed-Reconnaissance Soil Survey:

It is a combination of reconnaissance and detailed soil surveys and is undertaken for understanding distribution of basic soil classes of series and their phases.

(iv) Semi-Detailed Soil Survey:

This kind of soil survey comprises very detailed study of some selected strips cutting across many aerial-photo-interpretation (API) units for developing correlation between API units and soils. This type of soil survey provides adequate information about various kinds of soils, including problematic soils.

Recently there are two other types of soil survey have been recognised i.e. exploratory and rapid reconnaissance soil survey. These lead to the preparation of small scale soil maps that are needed for macro level planning for diversified agro-based development programmes.

Soil Mapping Unit:

The distribution of different soil series on the landscape is often intricate and necessitates some simplification for mapping. This is achieved by means of a mapping unit, which is the smallest area of a map that can be delineated by a single boundary at the scale used.

In large scale maps, simple mapping units (narrowly defined soil series) are delineated to attain a purity of 80 per cent or more within that unit. But as the map scale decreases (from 1: 25,000 to 1: 50,000. or 1: 1,00,000), complex mapping units are used.

Techniques used in Soil Survey:

There are usually three techniques as follows:

(i) General-Purpose API Survey:

Used for low-intensity surveys in which mapped boundaries (soils or land resources) are largely or entirely inferred from Air Photo Interpretation (API) with a free survey at low intensities of field observations to characterise physiographic units.

(ii) General-purpose free survey:

For medium-intensity surveys using API, but also with a relatively high intensity of field observations.

(iii) Special Purpose Grid Survey:

In this method, individual soil properties are recorded on a grid pattern and may be mapped parametrically. Observation points may commonly be located on an elongated grid sited to cross the ‘grain’ of the land.

Such systems, with observations along each line spaced more closely than the transets themselves is particularly suited to land with readily identifiable soil catenae, or where dense vegetation makes trace cutting difficult.

However, where soil variation is more random as, for example, in recent alluvial sediments, a regular grid becomes more efficient. Grid surveys may also be used where there is too little surface evidence and/or too few landmarks to allow free survey of representative areas.

Sequence of Survey Operations:

i. Initial Reconnaissance:

It includes preliminary identification of major soil-landform- vegetation relationships, and the study of all relevant existing data.

ii. Main Survey:

To establish distribution of soil and land units. Must be flexible to accommodate changes in definitions based on additional information produced during survey; preliminary classification and mapping to proceed concurrently.

iii. Sampling:

To sample soils of major mapped areas for characterisation.

iv. Consolidation:

‘Filling in’ in areas of complex soils or difficult boundaries; testing of mapped areas, revisions of API; incorporation of soil analytical results; final classification.

v. Reporting:

Production of final report and accompanying maps and diagrams.

Data Recording:

Landform characteristics, soil drainage, profile permeability and crop rooting often receive only a cursory appraisal because they are less easy to quantify than soil colour, depth or texture. Nevertheless, the former characteristics are always of equal, and often of greater importance from the point of view of agricultural development, and must therefore by carefully considered and recorded at each inspection site.

Soil Sampling:

With the present accuracy and reproducibility of physical and chemical analysis data, the weakest link in the whole soil analytical chain is the sampling procedure.

Whereas most reputable laboratories would regard duplicate variation of greater than 10 per cent as unacceptable in most of their chemical analysis it would not be unusual to encounter differences in specific chemical characteristics of more than twice this magnitude between horizons taken from two profiles of the same soil type in the same field.

Moreover, this variation need not be restricted to the immediate topsoil; subsoil clay contents from similar horizons in adjacent profiles of, say, 17 and 22%, or exchangeable sodium values of 4 and 6 g kg^-1, would not be uncommon, even though differences of 29 and 50% respectively are involved.

Therefore, when analyses are being performed in order to assesses the feasibility of a project or to assist in the implementation of agriculture development, the surveyor must take great care to ensure that the samples collected are as fully representative as possible of the defined soil types and that any interpretation of the results takes into account the constraints imposed by the sampling procedure.

Judgment and Random Sampling:

Judgment samples are those selected by the surveyor on the basis of field or API studies as being representative of the soils in a particular area, or a particular type. Random samples are collected in a statistically random manner and, in general, the fewer the number of samples actually collected, the greater the possible representational errors.

With judgment sampling, the possible errors also decrease with sample numbers, but they do so at a slower rate with random sampling. At some point, therefore, random samples become more statistically representative than judgment samples from ‘typical’ sites.

Depth and Intensity of Sampling:

Depending on the soils and crops involved; two possible systems might be as follows (depths in cm):

(a) 0-15, 15-30, 30-60, 60-90, 90-120

(b) 0-10, 40-50, 90-100, 140-150

Deep borings (of about 3 to 5 m) are often required to characterise substrata—particularly with reference to permeability and salinity, and for groundwater sampling.

Composite Samples:

The collection of composite samples from all soil profile horizons should be a standard objective where fertility, rather than pedogenetic, characteristics are being examined. Since the topsoil is generally that part of the profile most subject to variation over short distances, it is recommended that composite samples be obtained from this source as a matter of routine.

Care should be taken, however, to avoid collecting sub samples from locations having a different history or land use or recent fertilizer application.

Correlatory Samples:

The remarks made above refer specifically to those samples collected for detailed laboratory analysis. In addition to these, large numbers of correlatory samples may be collected, on which only a very limited number of tests are made such as pH and electrical conductivity.

These tests are usually carried out on site or in a field laboratory. Undisturbed samples are usually taken sometimes for soil physical tests viz. pF and bulk density.

Samples for Namatode Analysis:

Whenever coarse-textured soils are being surveyed, consideration should be given to the desirability of sampling for nematode analysis. This is particularly the case where the proposed agricultural development involves high-value crops susceptible to nematode attack, such as sugarcane, sugar beet, tobacco, citrus, bananas, tomatoes and onions.

The sampling procedure differs from that normally used in that the soil must be collected in a moist (but not wet) condition, and must reach the test laboratory not more than three days after sampling.

Sample preparation, recording and dispatch:

The following points should be borne in mind when sending samples for analysis:

i. If possible, the samples should be air dried before being sent to the laboratory.

ii. Each sample should be labelled twice; once with a tie on label and once with a label inside the sample bag.

iii. Labels should be short and simple, since soil laboratories occasionally make mistakes in transcribing numbers. The project name should be clearly marked on each label.

iv. The laboratory should be requested to keep any unused samples, carefully labelled, until about six months after completion of the project in case of any queries.

v. About 1 kg of air-dried soil is sufficient for complete analysis; for a restricted range of chemical analyses only 150 g may be needed.

Field Laboratories:

On many surveys, particularly at the semi-detailed and development level, it is advantageous to set up a field laboratory in regions having salinity hazards, in order to carry out routine tests whilst the work is in progress. Normally the tests are restricted to EC, pH, bulk density and field capacity. The amount of apparatus will clearly depend on the size of the project and the number of samples to be tested.

Air photo interpretation and remote sensing:

Remote sensing techniques are divided on a technical basis into the use of photographic and non-photographic sensors, and into imagery taken from aircraft and from satellites. From a viewpoint of utility in soil survey a more pragmatic division is as follows.

(i) Air photograph interpretation (API), using photographs taken from aircraft (both black and white and true colour).

(ii) Other remote sensing techniques, comprising the use of other types of sensors from aircraft together with satellite imagery.

Air Photo Interpretation:

Recent air photographs are a better guide to roads and tracks than are maps. Secondly, the position of a soil observation site can be located on a photograph e.g. by reference to a group of huts, clump of vegetation or a single large tree, in a way that it is impossible with even the best of maps.

Thus, besides their use for interpretation, photographs are of considerable value in planning and following traverse routes, and locating and subsequently plotting soil observation.

The method used in photo interpretation for soil survey is similar irrespective of scale. Pre-field work of interpretation is the main operation. The first stage is to gain an appreciation of the primary land units, e.g. hill masses, plans, alluvial areas, from non-stereoscopic viewing of the survey area as a whole.

This can be done from a print lay-down or photo-mosaic, the latter having the advantage of eliminating boundaries between photographs although not differences in tone. Satellite imagery, where available, is superior for this purpose. The next stage consists of detailed delineation of the primary units by stereoscopic examination of successive photo-pairs.

This is followed by subdivision into secondary units, e.g. level crest areas, pediments, valley floors. At all stages the recognition and delineation of mapping units is largely on the basis of landform differences, and to a subsidiary extent on vegetation; direct interpretation of soils is neither necessary nor possible.

It is also not necessary to be able to interpret all the phenomena employed in map unit differentiation; and photographic difference of tone, texture or pattern can be employed. It is best to follow the rule of subdividing areas wherever any consistent difference can be detected; the additional time taken is small, and boundaries which prove superfluous can be removed subsequently.

The mapping units distinguished in pre-fieldwork interpretation will not correspond exactly to those of the final soil map, and may be called API mapping units. An interim map legend is drawn up, giving for each unit the landforms, vegetation and land use, and the photo-diagnostic features by which it has been identified.

Post-field work interpretation needs considerably less time. A high proportion of the boundaries between API mapping units survive, notably boundaries of hill areas and poorly- drained valley floors.

It is at this stage that the relations between API mapping unit and soil types, investigated during field work, are finally collated, with the following possible results:

(i) One API mapping unit corresponds with one soil type;

(ii) Two or more API mapping units possess the same soil type;

(iii) One API mapping unit contains two or more soil types.

Interpretation of Soil Survey and Soil Survey Report:

The most important and specific objective of soil survey is to interpret the soils based on their properties and characteristics for various uses. Interpretation of soil survey is aimed at predicting the use potential of a soil for a specific purpose, such as, productivity of soils, capability of soils for raising specific kinds of crops, waste recycling, water supply, urban and industrial structures and many more specialized activities.

Interpretation of soils for land capability classification is based on inherent soil characteristics, external land features, erosion hazards and environmental factors that limit the use of the land.

The soil survey report and soil map are the end-product of any survey. It contains general description of the area (location, extent, physiography and relief, rivers, drainage, geology, climate, vegetation), socio-economic condition, agriculture and present land use, detailed description of soils (morphology, characteristics) land capability classification and other general problems of the area.

The soil survey reports have been found to be extensively used especially in designing drainage and irrigation networks, planning the ameliorative measures for salt affected and eroded areas, for sanctioning loans etc. This report is also used for developing rational land use planning for growing crops.

Importance of Soil Survey:

Soil surveys are land inventories composed of soil maps, soil descriptions, some physical and chemical analyses, engineering properties and limitations for major land uses. The utilization of soils in a better way has also been reported possible through soil survey.

Soil surveys are necessary to plan, develop and apply effectively drainage and irrigation practices on farm lands, conduct agricultural research on mapped soils if the research findings are to be transferred to other areas having comparable soil site characteristics.