The Procedures of Soil Survey (Soil surveying)

The processes of surveying and mapping enable us to recognize and correlate different soil categories. Soil surveying is the process of studying and mapping the earth's surface in terms of soil types. A soil survey report consists of two parts:

- The soil map which is accompanied by

- A description of the area shown on the map. Two main conventional methods of soil survey are (i) the rigid grid procedure and the (ii) the free survey method.

1. The Rigid Grid Procedure

Traverses are used here. A traverse is a straight path opened along a baseline in the area to be surveyed. Traverses are cut at regular intervals which depends on the kind of survey. Soils along the traverses are dug and studied. Tools such as the mud auger, bucket auger, shovel, and spade are used.

The surveyor digs holes and profiles, examines pedons, and construct legend containing names of the soil map units that the soil surveyor wants to use in the survey. There may be for example 50 types of soil in the area. The intensity of observation could be increased by digging more holes.

Along each of the traverses, soils are described at fixed intervals, such that the traverses and points of observation with the soil auger are fixed in a grid pattern. This method of survey gives all soils present in the area a statistically good chance of being represented on the final soil map.

The Rigid Grid procedure is particularly useful in carrying out soil surveys in landscapes covered with thick forests where visibility is severely limited.

Under such conditions, position location is difficult by other means including aerial photography, and it would be difficult to use the morphological features of the landscape as a guide to the positions of soil boundaries. Instead, boundaries have to be interpolated between points of contrasting soil mapping units.

Rigid Grid surveys are best suited for high to very high-intensity surveys at scales large than 1:10,000. Examination of the soil at fixed points throughout the survey area eliminates the subjective element of interpretation and soil boundaries are interpolated between differing observations.

Read Also: Physical Properties of Soils (Soil Physical Properties)

2. Free Survey

In this procedure, observations are also made along traverses, but the soil surveyor uses his judgment in siting soil observations in relation to landforms and other environmental features. In other words, observations are not made at fixed intervals.

This method of survey is only convenient in open countries where both access and visibility are almost unrestricted.

Although this is an effective and relatively rapid method of soil surveying, it can lead to a concentration of observations around the edge of soil mapping units within which “impurities” of other soils could occur.

The great advantage of the method is that the surveyor is free to vary the intensity of his observations according to the intricacy of the soil pattern. This results in greater accuracy when soil patterns are complex and do not waste time and energy when conditions are uniform.

The use of the free survey method necessitates a good base map or air photographs upon which the surveyor can work without any problems of location so that observations and boundaries are correctly placed.

Cartographic Production of Soil Map

The principal result of a soil survey is a soil map upon which is shown the distribution of the soil mapping units.

Once the field surveyors have compiled a “field” copy of their map, it is passed on to the cartographers who have the responsibility of preparing it for publication.

One of the most critical features of a map is its scale, for this determines what can be shown, especially in terms of the smallest area capable of being represented. On a soil map, this is limited by practical considerations to an area of about 0.25cm².

Thus, any soil area which occupies less than 0.25cm² when represented upon the map must be combined with adjacent soil mapping units.

It is pertinent to add that in virtually all soil surveys, the published map is produced at a smaller scale than that at which the fieldwork took place. For example, the fieldwork of a map published at 1:25,000 could be carried out at base maps of a scale of 1:10,000. The use of a larger-scale map for fieldwork has the advantage that the surveyor has.

The Soil Survey Report

A soil survey report is a written report which accompanies the soil map when a soil survey project is undertaken. Such a report explains in detail the mapping units, their properties, and relationships. According to the Soil Survey Manual (Soil Survey Staff, 1951), every soil survey report should contain.

- An explanation of how to use the soil map and report;

- A general description of the area;

- Description of the individual mapping units shown on the map, tables showing their characteristics

- Prediction of the yields of common crops under specifically defined sets of management practices for all the soils mapped

- Explanations of the management problems of each soil with special emphasis on how the characteristics of the soil influence the problems and their solutions.

In summary, for efficient and effective land uses, soil scientists survey land and prepare a soil map on which the soils in the area are classified based on observable and unchangeable soil properties.

Read Also: Concept, Definition, and Types of Soil Survey