Classification System of Soil

After reading this article you will learn about the classification system of soil.

A. Old Classification:

Geological Approach:

The earliest was the geographical system according to which the soils were divided into groups depending upon the geological materials or rocks from which the soil was derived.

According to this system, soils were divided into two main groups:

(1) Sedentary and

(2) Transported.

The classification was further improved and soils were grouped into red soils, black soils (regurs), laterite and lateritic soils, delta soils, desert and tarai soils.

B. Modern Classification System:

1. Physical Classification:

According to this system, soil is divided into a number of groups or classes such as sand, clay, loam etc. It takes into account the mechanical composition of the upper 6 or 7 inch layer of soil only. This system is good enough for agricultural purposes but does not meet the requirements of soil science. As it does not take lowers of soil into consideration it has very limited use.

2. Genetical Classification:

The basis of this classification depends on the genesis or origin of the soil and its subsequent development. These classification systems are based upon the study of the soil profile which is the ultimate product and reflection of all the soil-forming factors and processes.

The soils are classified into well-defined categories, the higher categories are order, sub-order, great soil group and sub-group. They give a general understanding of the soils over large areas. The lower categories are families, series, types and phases. They are more important in recognising local differences and assessing productive capacities of soil.

At the order level, the soils are classified into zonal, azonal, and intra-zonal groups. Zonal soils are more or less mature i.e., well-developed horizon differentiation, colour and structure etc. Zonal soils are developed under the influence of climate and vegetation. Azonal soils are still immature and have not yet developed their profile characteristics.

Such soils are found on steep rocky slope, on fresh alluvial deposits. Azonal soils may be found in any climatic region. Intra-zonal soils have a distinct profile and their characteristics are more influenced by local conditions of relief (topography) or parent material than by the normal effects of climate and vegetation.

The soils are further sub-divided into great soil group (Table 21.1)

Marbut proposed a modification and divided zonal soils into two main orders:

(1) Padalfers, and

(2) Pedocals.

Pedalfers are soils developed in humid regions under conditions of free drainage where calcium and other are leached out. Pedocals are soils where calcium carbonate and other bases accumulate at some depth in soil profile due to restricted drainage (Table 21.2).

Soil Classification-7th Approximation:

The soil survey staff of the United States Department of Agriculture has proposed a new natural soil classification system called. “Soil classification, A comprehensive system—7th Approximation, 1960.” The 7th approximation lays more stress on the morphology of soils themselves rather than on the environmental factors. The system classifies the soils into several categories as orders, sub-orders, great groups, sub-groups, families, series and phase (type).

Example of the Classification of a Miami Silt Loam Soil:

Order -Alfisol

Sub-order -Udalf

Great group -Hapludalf

Sub-group -TypicHaludalf

Family – Fine loamy, mixed

Series – Miami

Phase – Miami, eroded phase

Characteristics of Soil Classification-7th approximation:

1. It is a natural classification of soil.

2. The classification is based on properties of the soils.

3. The properties selected should be observable or measurable. Properties which can be measured quantitatively should be preferred.

4. The properties selected should be those that either affect soil genesis or result from soil genesis.

5. The properties with the greater significance to plant growth should be selected for the higher category.

6. The classification system is flexible.

The various soil orders in which world soils have been divided into are given in Table 21.3.

C. New Classification System—Soil Taxonomy:

The comprehensive soil classification, called Soil Taxonomy maintains the natural body concept.

This system has two important features:

(a) The system is based on soil properties that are easily verified, and

(b) The unique nomenclature system.

Categories of the System:

There are six categories of classification in Soil Taxonomy:

(i) Order

(ii) Sub-order

(iii) Great group

(iv) Sub-group

(v) Family, and

(vi) Series.

These categories may be compared with those used for the classification of plants.

Comparison of the classification of common cultivated plant, white clover (Trifoliumrepens) and a soil, Miami Series.

(i) Order:

The order is based on soil forming process. In a given order, soil properties are similar in their genesis. For example, soils that developed under grassland vegetation and are characterised by a thick, dark surface horizon with high metallic cations. Soils with these properties are included in the order, Mollisols.

There are following eleven soil orders in soil taxonomy:

(i) Sub-order:

The sub-orders are sub-divisions of orders. The sub-order indicates genetic homogeneity. Climatic environment, vegetation and wetness help in determining the genetic processes. Forty seven sub-orders have been recognised.

(ii) Great group:

Diagnostic horizons are the primary bases for differentiating the great group in a given sub-order. Nearly 230 great groups are recognised.

(iii) Sub-group:

The sub-groups are sub-divisions of the great groups. There are-more than 1200 sub-groups.

(iv) Family:

The family is differentiated on the basis of texture, mineralogy, temperature and soil depth. Some 6600 families are recognised.

(v) Series:

The series is a sub-division of the family and is the most specific unit of classification. Differentiating characteristics are primarily based on the kind and arrangement of horizons. About 16,800 soils series are recognised.