Soil survey intrepretation

INTERPRETATIVE GROUPINGS OF SOILS: LAND CAPABILITY CLASSIFICATION AND

FERTILITY CAPABILITY CLASSIFICATION

The term soil survey interpretation comprises two parts namely soil survey and interpretation. The meaning of the word interpretation is to explain meaning, to show the purpose, or to translate into familiar terms something, which is under consideration. Soil survey comprises collection of wide variety of scientific information about the soils of the area, classification of soils into different categories and showing their distribution on suitable scaled base maps. Any data without interpretation are of limited value to land users, planners, and decision makers.

The most widely adopted land evaluation methods (Interpretations) are:

1. Thematic maps - single theme interpretative

2. Land capability classification

3. Land irrigability classification

4. Land suitability classification

5. Productivity potential classification

I. Thematic interpretative classification

Interpretations of soil depth, soil surface texture, soil drainage, soil available water capacity, soil erosion, soil gravelliness and soil calcareousness based on site characteristics recorded during soil survey mapping, help in identifying areas that have specific problems or have high potential for agriculture.

Soil depth

Depth of soil determines the effective rooting depth for plants and in soils of the same textural class, the capacity of the soil column to hold water

Class     Solum depth (cm)      Description

d1            0-25                            Very shallow

d2            25-50                           Shallow

d3           50-100                       Medium Deep

d4         100-150                         Deep

d5          150-200                       Very deep

d6            > 200                         Extremely deep

Surface soil texture

The surface layer of soils to a depth of about 25 cm is the portion of the soil column most used by crop plants. The texture of the surface soil is an important factor in evaluating the suitability of soils for the production of specific crops.

Class       Surface texture      Description

1.                   s/ ls                    Sandy

2.                  sl/ scl                   Loam

3.               sc/ cl/ sicl/ c           Clayey (clay 35-60 %)

4.                 c/sic                      Fine clayey

                                                 (Clay 60 % and above)

Soil drainage

Soil drainage refers to the rate of removal of added water. The class of soil drainage is indicated by the morphology of the soil in relation to slope and texture.

Class                  Description

1.                          Very poorly drained

2.                           Poorly drained

3.                          Imperfectly drained

4.                         Moderately well drained

5.                         Well drained

6.                        Some what excessively drained

7.                      Excessively drained

Soil available water capacity (AWC)

Classes of AWC are based on the ability of the soil column to retain water between the tensions of 0.33 and 15 bar in a depth of 100 cm. The available water capacity of the soils composing the mapping unit was estimated from a consideration of texture, mineralogy, depth, and gravel content.

Class                 Water (mm)               Description

1.                            < 50                           Very low

2.                           50-100                         Low

3.                      100-150                           Medium

4.                      150-200                           High

5.                       > 200                            Very high

Soil erosion

Erosion broadly means removal of earth’s surface layer by water and wind. In agriculture, the term soil erosion is commonly used for accelerated erosion caused by disturbance of the landscape, usually by man. The erosion classes showing an estimate of the current erosion status is judged from the field observations.

Class              Description

e1                   Slight erosion

e2                    Medium Erosion

e3                    Severe erosion

e4                   Very severe erosion

Soil gravelliness

Gravel is the name given to soil particles of diameter larger than 2 mm. The materials that constitute gravel are usually rock fragments both fresh and partly weathered. The presence of gravel in the soil hinders plant growth directly by impeding root development and seedling emergence and indirectly through reducing the volume of soil that can be drawn upon by plants for water and nutrients.

Class        % Gravel in profile       Description

1.               < 15                                    Non -gravelly

2.              15-35                                   Gravelly

3.                35-80                                  Skeletal

Soil calcareousness

Soil calcareousness is the term used to indicate the content of calcium carbonate in soil.

Class          Description

1.                 Slight effervescence

2.                 Medium effervescence

3.                 Strong effervescence

     CaCO3 content is estimated in the field by observation of the degree of effervescence given by the soil when it is moistened with dilute HCl.