Systems of Soil Classification

After reading this article you will learn about the genetic and modern systems of soil classification.

Genetic System of Soil Classification:

Russian scientist V.V. Docuchaev first suggested the genetic system of soil classification, which was further developed by the American Soil Scientist C.F. Marbut.

According to this system, each soil possesses certain properties which develop as a consequence of the effects of a particular combination of factors affecting soil formation. The soils inherit their properties from the parent material in the initial stages of soil formation.

For example, soils developed from basic parent material are rich in basic elements and plant nutrients and are alkaline in reaction in the initials stages of development.

But as soils mature, the inherited character gradually disappears and the soils are dominated by the character acquired from the climate and vegetation. For example, as time passes, basic elements are gradually washed down and ultimately soils which had developed from basic parent materials become poor in basic elements, plant nutrients and acidic in reaction.

According to the genetic system, soils of the world have been classified into three orders: Zonal, Intra-zonal and Azonal.

(1) Zonal Soils:

The characteristics of soils belongings this order are mainly determined by the climate and vegetation. This means that soils that have developed from different parent material under the same climatic and vegetation conditions possess same general properties.

(2) Intra-zonal Soils:

General characteristics of soils belonging to this order are mainly determined by certain local conditions like poor drainage, soluble salts etc. The soil characteristics due to these conditions, dominate over the characteristics due to climate and vegetation.

(3) Azonal soils:

Soils belongings to this order to do not possess well developed profiles because they have not been subjected to the influence of the climate and the resultant vegetation long enough for the soil profile to develop.

Zonal soils have been divided into six suborders on the basic of wider ranges of variation in climate and the resultant vegetation. Each of the sub-orders have been subdivided into great groups on the basis of narrower ranges of variation of climate and the resultant vegetation.

Intra-zonal soils have been divided into three sub-orders:

(i) Hydromorphic soils that contain excessive amounts of water.

(ii) Holomorphic soils that contain excessive amounts of soluble salts.

(iii) Calciomorphic soils that contain excessive amounts of lime or gypsum.

Hydromorphic soils have been subdivided into great groups on the basis of increasing degrees of accumulation of water in soils.

Holomorphic soils have been subdivided into the two great groups, Saline and Alkali soils. Saline soils contain excessive amounts of neutral salts i.e. sulphates and chlorides of sodium, calcium and magnesium.

Alkali soils contain excessive amounts of sodium saturated clays and sodium carbonate.

Calciomorphic soils are divided into two great groups:

(i) Brown forest soils that are dark brown at the surface and gradually grade into gray calcareous parent material.

(ii) Black earth or Grumosols that are dark brown grassland soils developed from limestone.

The order Azonal doesn’t have any suborders, but the three following great groups have been recognized under it:

(i) Lithosols:

They are commonly found on steep slopes and consist of very shallow stony soils over bed rock.

(ii) Regosols:

They have developed from deep soft mineral deposits. They are also very young soils like Lithosols and differ from Lithosols in that they are not stony.

(iii) Alluvial soils:

The main characteristics of alluvial soils have been derived by the deposition of products of weathering of rocks by the great river systems like the Ganges, the Brahmaputra and the Indus etc. Great groups have been divided into families, each of which includes a certain number of identical soil series possessing certain common properties.

Each family has been divided into a number of series. Soils belonging to a series have originated from the same parent material, under same climate and resultant vegetation and therefore possess the same profile characteristics.

Each series has been divided into a number of soil types on the basis of the texture of the surface soil. Soil types have been divided into phases on the basis of some important deviations like slope, erosion (removal of top soil by running water or blowing wind is called soil erosion), wetness, stoniness etc.

Modern System of Soil Classification:

The Genetic system of soil classification is based entirely on soil genesis. Some soils which have been cultivated for centuries have greatly changed. Yet they are still classified on the basis of the properties presumed to be possessed by them when they were not cultivated but under natural vegetation.

For these reasons, the soil survey staffs of the United States, Department of Agriculture have proposed the new system of soil classification called the seventh Approximation because it has been modified or approximated seven times.

This system of soil classification is based on the present properties of soils. However, these properties are also the consequence of soil genesis. According to the modern system of soil classification, soils of the world have been divided into ten orders based on the variation in the major soil forming processes as indicated by the presence or absence of major diagnostic Horizons.

These ten orders are as follows:

1. Entisols:

Very young soils that do not possess a well-developed soil profile.

2. Inceptisols:

Soil in which the horizons have just started to develop. They are relatively low in organic matter or base saturation or both and contain some weatherable minerals.

3. Aridisols:

Soils of dry regions e.g. Deserts. They are relatively low in organic matter and possess a few pedogenic horizons.

4. Vertisols:

Soils those are very rich in the expanding type of clay minerals and therefore have swollen and shrunk a number of times. As a consequence, they possess deep wide cracks at some stage in most of the years.

5. Mollisols:

Grassland soils that possess a thick soft dark coloured surface horizon, relatively high in organic matter and high base saturation in all horizons.

6. Spodosols:

Soils that possess ash grey horizons near the surface due to eluviation of clay and sequioxide and the subsurface illuvial horizon called the spodic horizon, rich in sequioxide and humus.

7. Alfisols:

Cool humid region soils containing appreciable amounts of primary minerals from which basic elements are being slowly leached. They have gray to brown surface horizon and are medium to high in base saturation and possess an illuvial horizon of silicate clays that is more 35 per cent base saturated.

8. Ultisols:

Warm humid region, highly leached acidic soils containing low amounts of basic elements and plant nutrients. So the base saturation is low. They possess an illuvial horizon of silicate clays with base saturation lower than 35 per cent.

9. Oxisols:

Hot humid climate soils that possess an oxic horizon (this horizon contains high amounts of low cation exchange capacity clays, which are highly resistant to dispersion in water). These clays are very inactive and do not possess an illuvial horizon of silicate clays.

10. Histosols:

Organic soils that consist of plant materials. Orders (1 to 9) have been divided into sub-orders on the basis of properties associated with wetness or dryness, major parent material and vegetation, degree of decomposition of organic matter is the basis of classification of the order Histosols.

The sub orders have been divided into great groups on the basis of differentiating soil horizons and soil features. Differentiating soil horizons include those that have accumulated clay, sesquioxide and/or humus and those that have pans (hard layers). The differentiating soil features include expansion and contraction properties of clays, soil temperature and the presence of basic elements and soluble salts in the soil.

Great groups have been divided into not more than the following three sub­groups:

(i) The first sub-group possesses only the properties of the concerned great group.

(ii) The second sub-group possesses some of the properties of other sub­groups, great groups, sub-orders and orders in addition to the properties of the concerned great groups.

(iii) The third sub-group possesses some properties that have not been found in any other great groups, sub-orders and orders in addition to the properties of the concerned great group.

Sub-groups have been divided into families on the basis of these properties important to plant growth, including soil texture, clay minerals, and soil reaction, mean annual soil temperature and mean rainfall.

Families have been divided into series on the basis of profile characteristics which means that soils belonging to a series have developed from the same parent material under the same climate and resultant vegetation and therefore possess identical profile characteristics. Soil series have been divided into phases which describe soil texture, percentage slope, stoniness, wetness, erosion, soluble salts etc.