Agriculture for Engineers 4 (3+1)

Lesson 4. Classification of Soil - Taxonomy Orders

4.2 CHARACTERISTICS OF SOIL CLASSIFICATION-

      7^th 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 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.

4.3 CATEGORIES

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.

(i) Order: The order is based on soil forming process. In a given order, soil properties are similar in their genesis. There are following eleven soil orders in soil taxonomy (Table 4.1).

Table 4.1: Soil orders, its formative elements and their major characteristics

	Soil order	Formative element	Major characteristics
1	Entisols	ent	Little profile development, Ochric epipedon common
2	Inceptisols	ept	Embryonic soils with few diagnostic features, Ochric or umbric epipedon; Cambic horizon
3	Mollisols	oll	Mollic epipedon, high base saturation, dark soils, some with argillic or nitric horizons
4	Alfisols	alf	Argillic or nitric horizon; high to mediumbase saturation
5	Ultisols	ult	Argillic (clay) horizon; low basesaturation.
6	Oxisols	ox	Oxic horizon, no argillic horizon, highlyweathered.
7	Vertisols	ert	High in swelling clays, deep cracks whensoil dry, dark colour.
8	Aridisols	id	Dry soil, ochric epipedon, sometimes argillic or nitric horizon.
9	Spodosols	od	Spodic horizon commonly with Fe, Al, and humus accumulation, forest.
10	Histosols	ist	Peat or bog; more than 30% organic matter(organic soil).
11	Andisols*	and	From volcanic ejects, dominated by allophane or Al-humic complexes.
12	Gelisols	el	Permafrost often with cryoturbation (frostchurning)

*Recently added as a soil order.

(ii) 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 recognized (Table 4.2).

Table 4.2:  Formative elements in names of suborders.

Formative Element	Derivatives	Connotation of formative element
alb	L. albus, white	Presence of albic horizon (a bleached eluvial horizon)
anthr	Gk.anthropos, human	Presence of anthropic or plaggen epipedon
aqu	L. aqua, water	Characteristics associated with wetness
ar	L. arare, to plow	Mixed horizon
arg	L. argilla, white clay	Presence of argillic horizon (with illuvial clay)
calc	L. calcis lime	Presence of calcic horizon
camb	L. cambriar, to change	Presence of cambric horizon
cry	Gk. kryos, icy cold	Cold
dur	L. durus, hard	Presence of a duripan
fibr	L. fibra, fibre	Least decomposed stage
fluv	L. fluvius, river	Floodplain
fol	L. folia, leaf	Mass of leaves
gyps	L. gypsum, gypsum	Presence of gypsic horizon
hem	Gk. hemi, half	Intermediate stage of decomposition
hist	Gk. histos, tissue	Presence of histic epipedon
hum	L. humas, earth	Presence of organic matter
orth	Gk. orhos, true	The common ones
per	L. per, throughout time	Of year-round humid climates, perudic moisture regime
psamm	Gk. psammos, sand	Sand textures
rend	Modified from rendzina	Rendzinalike-high in carbonates
sal	L. sal, salt	Presence of salic · (saline) horizon
sapr	Gk. sapros, rotten	Most decomposed stage
torr	L. torridus, hot and dry	Usually dry
turb	L. turbidus, disturbed	Cryoturbation
ud	L. udus, humid	Of humid climate
ust	L. ustus, burnt	Of dry climates, usually hot in summer
vitr	L. vitreus, glass	Resembling glass
xer	Gk. xeros, dry	Dry summers, moist winters

 

(iii) Great group: Diagnostic horizons are the primary bases for differentiating the great group in a given sub-order. Nearly 230 great groups are recognized.

(iv) Sub-group: The sub-groups are sub-divisions of the great groups. There are more than 1200 sub-groups.

(v) Family: The family is differentiated on the basis of texture, mineralogy, and temperature and soil depth. Some 6600 families are recognized.

(vi) 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 arrangement of horizons. About 16,800 soils series are recognized.

 4.4 Nomenclature in soil taxonomy:

1.   Order: ends with - Soil e.g. arid soil.

2. Sub-order: It is composed of formative element of order + any differentiating character like temperature, moisture, drainage, diagnostic horizon etc. e.g. Aquolls wet soil-Mollisols (oil), Aquents wet soil- Entisols  (ent), Argid aridi soil (id).

3. Great group: It is composed of order + sub-order + one or more specific characters which modify the sub-order e.g. Argiaqoll Molli soil –order, aqoll - sub-order wetness, Arg -argillic horizon.

4. Sub-group: It is identified with a binomial nomenclature. e.g. A 'Typic' adjective is used when sub-group represent a 'Central concept' of the great group. e.g. Typic Argiaqoll. There are integrated between different sub-groups or between great groups in the same order.

5. Family: In nomenclature of family- we have textural class, mineralogy, temperature regime and some time reaction, drainage class and topography. e.g. Fine-clay/Very fine clayey, mixed montmorillonitic, calcareous, hyperthermic, Typic Chromustert.

6. Series: Name of the series is given on the basis where it is described. e.g.

Soil Series         Sub-group               Great group           Sub-order         Order

Bodali         VerticUstorthents          Ustorthents             Orthents          Entisols

Eru              TypicChromusterts        Chromusterts          Usterts           Vertisols

Ilav              TypicUstorthents           Ustochrepts            Orthents        Inseptisol

Dandi          TypicHalaquepts            Halaquepts             Aquepts        Inseptisol

 4.5 SOILS OF INDIA

Soils of India have been divided into the following eleven major groups:

(1) Red soils (2) Lateritic soils (3) Black soil (4) Alluvial soils (5) Desert soils (6) saline and alkaline soils (7) Peaty and marshy soils (8) Tarai soils (9) Brown hill soils (10) Sub-mountain soils (11) Mountain meadow soils.