Conventional

Detailed Soil Survey (Conventional method)

Detailed Soil Survey (usually at 1:4000; 1:5000; 1:8000 or 1:10,000 scales) provides the information necessary to identify homogeneous management units in the field at village level. The methodology followed in the field and laboratory is briefly indicated below.

Base Maps Used

The detailed survey of the villages can be carried out by using cadastral maps as a base. The cadastral map shows all the field boundaries with their survey numbers, location of tanks, streams and other permanent features of the area.

Remote sensing data products (1:12,500 scale) were used in conjunction with the cadastral maps to identify the landforms and surface features of the area. The Imageries helped in the delineation of the boundary between the uplands and lowlands, water bodies, forest and vegetated areas, salt affected lands, roads, habitations and other cultural features of the area.

Survey of India toposheets at 1:50,000 scale can be also used as a base for initial traversing, identification of geology and landform, drainage features, present land use and for the selection of transects at block level.

Field Investigation

Preliminary traverse can be carried out by using 1:50,000 scale toposheets. During the traverse, geological formations, drainage patterns, surface features, slope characteristics, types of land use and landforms were identified.

In the selected transect, profiles can be located at closely spaced intervals to take care of any change in the land features like break in slope, erosion, gravel, stones etc. In the selected sites, profiles (vertical cut showing the soil layers from the surface to the rock) were opened up to 200 cm or to the depth limited by rock or hard substratum and studied in detail for all their morphological and physical characteristics. The soil and site characteristics can be recorded for all profile sites on a standard proforma.

Based on the soil-site characteristics, the soils were grouped into different soil series (soil series is the most homogeneous unit having similar horizons and properties and behaves similarly for a given level of management). Soil depth, texture, colour, amount and nature of gravel present, calcareousness, presence of limestone, nature of substratum and horizon sequence were the major identifying characteristics of soil series in the area. Phases of soil series (phase is a subdivision of a soil series based mostly on surface features that affect its use and management. For example, slope, texture, erosion and stoniness) were separated and their boundaries delineated on the cadastral maps based on the variations observed in the surface texture, slope, erosion, presence of gravels, salinity, sodicity etc.

The delineated mapping units occurring in each village are shown on the soil map in the form of symbols. In arriving the phases, a combination of letters, both in upper and lower case, and numerals were used. For example, the map unit SvcB3 occurring in Illuppakudi village is a phase of Sivagangai series. In this, the first two letters indicate the name of the soil series, the third lower case letter indicates the texture of the surface soil, the fourth upper case indicates the slope of the land and the fifth numeral indicates the severity of the soil erosion.

Similarly other features like gravelliness, salinity, sodicity, etc which affects the use of the land can be indicated in phases. Wherever gravelly phase occurs, the upper case letter G follows the name of soil series in the third position

Laboratory analysis

For the soil series identified, soil samples has to be collected from representative pedons for laboratory analysis. The soil samples are to be analysed for various physio, chemical characteristics by following standard procedures.

Finalisation of Soil Maps

The soil map for each village can be finalized in the field itself after thorough checking of soil and site characteristics. The village soil maps can be generated using GIS without any generalization and loss of information and presented for each village separately.

From the village maps, the soil map of the Block can be prepared by combining all the soil maps of the villages through the use of GIS software. Since the village maps were at larger scale with phases as mapping units, they were subjected to both cartographic and categorical generalization to prepare the soil map of the block.

Generation of Thematic Maps

The soil map and other resources database can be interpreted for identifying the constraints and potentials, and evaluated for land capability, land irrigability, fertility capability and land suitability for various crops and other uses for each village separately by using GIS software.        

By using thematic information table for all the mapping units occurring in a village, required thematic maps can be generated by using GIS software at any time based on the needs of the farmers or any other users.

The major problem faced in conventional soil survey and cartography is the accurate delineation of boundary. Field observations based on conventional soil survey are tedious and time consuming. The remote sensing data in conjunction with ancillary data provide the best alternative, with a better delineation of soil mapping units (Karale 1992; Kudrat et al.,1990; 1992; Mulders & Epema 1986; Sehgal 1995).

Soil surveyors consider the topographic variation as the basis for depicting the soil variability. Even with the aerial photographs only physiographic variation in terms of slope, aspects and land forms are identified for delineating the soil boundary. Multispectral satellite data are being used for mapping soil up to family association level (1:50,000). The methodology in most of the cases involves visual interpretation (Karale et al., 1981). However, computer aided digital image processing technique has also been used for mapping soil (Epema 1986; Korolyuk & Sheherbenko 1994; Kudrat et al., 1990) and advocated to be a potential tool (Kudrat et al., 1992; Lee et al., 1988).