Soil Survey:
Soil survey is the study and mapping of soils in the fields. It is starting point for all soil researches.
The following are the main objectives of soil survey:
(1) To describe and classify soils giving uniform system of classification with uniform nomenclature in order to correlate the soils of different area.
(2) To show distribution of different soils in the field (soil mapping).
(3) To provide data for making interpretations as to the adaptability of particular soils for agricultural purpose and also for many other purposes (as in soil management).
Soil surveys are of the following three types:
(1) Detailed
(2) Reconnaissance
(3) Detailed reconnaissance
1. Detailed Soil Survey:
In this, soil boundaries are plotted accurately on maps on the basis of observations made throughout the surveyed area. In this, geographical distribution of soil is also described. Detailed soil surveys are important in the sense that they provide information’s needed for planning land use and management and formulating agricultural research and extension programmes.
2. Reconnaissance Survey:
In this, soil boundaries are plotted from the observations made at intervals.
3. Detailed Reconnaissance Survey:
In this, a part of surveyed area is plotted on the map by detailed method and remainder by reconnaissance method. In mapping of soils either aerial photographs are taken or good topographic maps are made. In mapping of cultivated areas, generally a scale of four inches to a mile is used but detailed or special maps are made on scales of six inches, eight inches or twelve inches to a mile.
In India, soil surveys are being carried out by central and provincial agencies. Some surveys of limited areas have been conducted for specific objectives, such as fertility surveys, surveys for soil classification, survey from geological point of view, for physicochemical properties of surface sample and surveys for genetic classification.
Pre and post-irrigation surveys have also been used for limited areas under command of big irrigation projects to determine the nature, quality and the concentration of soluble salts in different horizons of soil. In India, a few generalized soil maps have been prepared under the auspices of International Society of Soil Science, Geological Survey of India and I.A.R.I. (Indian Agricultural Research Institute), New Delhi. The latest soil map of India is given in Fig. 24.1.
Aerial Photo-interpretation for Soil Surveys:
Normally the scales recommended for detailed survey are 1: 10,000 or 1 : 25,000. In such surveys, the air photos serve as an ideal base for plotting soil boundaries. This is on account of the wealth of pictorial detail available in the air photo, thus rendering orientation and navigation very easy. Normally a limited use of pocket stereoscope is made for delineating boundaries based on relief and other features. The final soil map in such surveys as well as the base photo are prepared on large scale.
Medium and Small Scale Map:
Aerial photo-interpretation is of greater utility in this case. The degree of utility increases from medium to small scale. The procedure for photo-interpretation is based mainly on the principle that physiographic units have unique soil patterns. The scale recommended for semi- detailed reconnaissance surveys are 1: 25,000 to 1: 1, 00,000 and 1: 2, 50,000. In similarity to the techniques used by geologists and foresters, the soil scientists also make use of such elements as relief, slope, geological features, vegetation, tone, textures, pattern etc.
Systematic Photo-interpretation:
The most favored method adopted by soil scientists involves delineation of the soil boundaries by air photos which are sub-divisions of physiographic units. In the next stage, they select sample areas that represent all the delineations of the photo-interpretation.
These consist of long, narrow areas which cut across as many sub-divisions as possible. Detailed soil observations are taken up in each of the delineation of the interpretation. Based upon the number of such observations, the soils of each unit are classified and related to the photo-interpretation unit. Thus soil composition of each unit is established.
With the finding of the sample areas as basic, the remaining aerial photographs are interpreted by the principle of extrapolation of knowledge gained in the sample area studies. It may be mentioned that this “adjusted photo-interpretation” is very often a partial revision of the pre-field interpretation done in the office. Final phase comprises a “selective ground check”. This achieves the testing of the validity of the adjusted photo-interpretation. The resulting field sheet namely air photos contain the correct soil boundaries.