Methods of soil survey

To carry out the survey work the following base maps are required

Methods of soil survey:

Soil surveys are being conducted for diagnosing the nature and extent of distribution of different soils, including normal and problem soils. There are two methods of survey.

         1. Grid survey

         2. Free survey

Soil and Landform attributes:

         Many soil and landform attributes such as soil texture, depth, profile development, drainage, soil moisture and temperature regimes can be observed and mapped.

General Traversing:

         The first step in soil survey begins with the general traversing of the area. Recent use of aerial or satellite imageries has become common as they have more details than cadastral or topographical maps. The surveyor studies the topography, geology, climate and vegetation – the factors of soil formation. After the preliminary studies, the surveyor goes to the field to study the soils and prepares a field mapping legend, based on different attributes that affect the plant growth or based on soil genesis. The preliminary legend provides the basis for soil mapping.

  For carrying out the field work, the surveyor walks briskly across the fields with a base map and auger in his hands. The kind of auger used depends upon the terrain or area being surveyed. After few minutes, the surveyor stops to bore a hole in the soil in order to note the sub soil variations in different attributes / characteristics such as texture, depth, colour, moisture status, mottling, concretions, calcareousness etc., Thus he identifies soil units. He thus determines the depth of horizons and also the illuvial and eluvial horizons. He makes observation on land use, relief, erosion and then draws the same on the base map . He collects soil samples at intervals depending upon the variability in soils and intensity of mapping.

Observations recorded during traversing:

       Soil is composed of distinct layers called horizons. The intensity of colour in a layer denotes the amount of organic matter, lime , iron or reduction and oxidation status. The lower layers have streaks or spots of grey, yellow or red colours called mottles, which indicate impeded drainage and restricted aeration. Texture which is the relative proportion of sand, silt and clay is determined by feel method and is confirmed by mechanical analysis in the laboratory. The depth of bed rock or compact layer is determined. External features such as quantity of gravel, stones or rocks , slope, erosion, salinity are also recorded.

    Grid survey

       The procedure is adopted for mapping small areas, such as a micro watershed or agricultural research station. The traverse lines are located on a grid pattern and the density of mapping is adjusted according to the area surveyed so that the number of observations per cm2 of the final map is independent of the scale. Generally four or five observations per ha are recommended. Points of comparable observations are drawn. In this computer age, grid survey at georeferenced points (with latitude and longitude) have been found to be of great value in digitizing the database and generating several thematic maps of practical value. Such surveys are expensive and recommended for experimental stations from where the agro technology generated is transferred to other areas having comparable soil-site characteristics. The most important limitation is the validity of interpolated boundaries. Further that some of the grid points may fall in sites such as roads, building, field boundaries, which the survey is intended to exclude.

 Grid survey for traversing: (Boundaries separate soils with varying salinity levels)

Free survey:

    In free survey, the surveyor chooses observation points on the assumption that changes in physiography, as such observed by aerial photo or satellite imagery interpretation and other surface features, such as colour, vegetation and land use are indicative of differences in soil characteristics. The density of observations can be varied as the mapper concentrates on confirming the inferred boundaries and checking the uniformity of the soil within each boundary. On small scale, the inferred boundaries are often accepted as soil boundaries. On large scale, several new boundaries within the physiographic boundaries are recognized depending upon the scale of mapping.

On small scale (1:2,50,000 or smaller), the inferred boundaries are accepted as soil boundaries and limited efforts are made to find variations within the physiographic boundaries. On large scale (1:50,000 or larger), several new boundaries within the physiographic boundaries are recognized, depending upon the scale of mapping. Based on the differences in various units such as physiography, parent material, drainage class and profile development , the soils are grouped into defined soil units.

     For large areas such as a state or country, the mapping is generally undertaken on small scale (1:250,000 or 1:1 M) depending on the terrain complexity, purpose and time available for mapping.  Ex. Soil resource map of India.  A three tier approach comprising of land sat imagery interpretation, field and laboratory investigations, cartography and printing was adopted.

        These maps provide information and database on various attributes used for mapping and laboratory investigations. These database stored in computer are potentially useful for extension staff, planners, scientists, administrators and decision – makers. By using these database and digitizing the soil map, several thematic maps for practical applications were generated. Such maps are not only easily understood by the land users but also help in determining fertilizers, amendments and other needs for optimizing land use.

       And in most modern soil survey involving 5000 ha and above, this is usually the type of survey methodology adopted. The advantage is that it is cheaper because there is less number of observation points. However, because it depends largely on the experience of the surveyor, it can be very inaccurate ,since boundary placement is more difficult.