Soil Taxonomy is the scientific system of classification of soils based on their physical, chemical, mineralogical, and morphological properties.

Developed by USDA (United States Department of Agriculture)

Published in 1975 ⭐⭐⭐

Level	Description
Order	Highest category
Suborder	Based on moisture regime
Great Group	Horizon development
Subgroup	Intergrade/Typical
Family	Physical & chemical properties
Series	Lowest category ⭐⭐⭐

Remember: Order → Suborder → Great Group → Subgroup → Family → Series

Order	Major Characteristics
Alfisols ⭐⭐⭐	Moderately leached, fertile
Andisols	Volcanic ash soils
Aridisols ⭐⭐⭐	Desert soils
Entisols ⭐⭐⭐	Young soils, little profile development
Gelisols	Permafrost soils
Histosols ⭐⭐⭐	Organic soils (Peat)
Inceptisols ⭐⭐⭐	Weakly developed soils
Mollisols ⭐⭐⭐	Grassland soils, very fertile
Oxisols	Highly weathered tropical soils
Spodosols	Acidic forest soils
Ultisols	Highly weathered acidic soils
Vertisols ⭐⭐⭐	Black cotton soils (Montmorillonite clay)

Soil Order	Major Areas
Entisols	Rajasthan, Punjab, River flood plains
Inceptisols	Indo-Gangetic Plains
Alfisols	Tamil Nadu, Karnataka
Vertisols	Maharashtra, MP, Gujarat
Aridisols	Rajasthan
Mollisols	Himalayan foothills
Ultisols	North-East India
Histosols	Kerala marshes

Question	Answer
Youngest Soil	Entisol ⭐⭐⭐
Most fertile order	Mollisol
Organic soil	Histosol
Black soil order	Vertisol
Desert soil order	Aridisol

Property	Description
Formation	River deposits
Colour	Light grey to ash grey
pH	Neutral to slightly alkaline
Fertility	Highly fertile
Rich in	Potash & Lime
Deficient	Nitrogen & Organic Carbon

Major States: Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal, Assam

Crops: Wheat, Rice, Sugarcane, Maize

Property	Description
Parent Material	Basalt
Clay Mineral	Montmorillonite ⭐⭐⭐
Colour	Black
Water Holding Capacity	Very High
pH	7.0–8.5

Major States: Maharashtra, Madhya Pradesh, Gujarat, Karnataka

Crops: Cotton ⭐⭐⭐, Soybean, Sorghum

Property	Description
Colour	Red
Cause	Ferric oxide
Fertility	Low
pH	Acidic to neutral

Major States: Tamil Nadu, Karnataka, Andhra Pradesh, Odisha

Property	Description
Rich in	Iron & Aluminium
Fertility	Poor
Rainfall	High rainfall areas

Crops: Tea, Coffee, Rubber, Cashew

Property	Description
Texture	Sandy
Organic Matter	Very low
Water Holding	Low

Major Area: Rajasthan

Characteristics: Acidic, Rich organic matter, Forest areas

Crops: Tea, Apple, Potato

Characteristics: High organic matter, Black colour, Acidic

Major Areas: Kerala, Sundarbans

Soil	Parent Material	Major Crop	Major State
Alluvial	River deposits	Wheat	UP
Black	Basalt	Cotton	Maharashtra
Red	Granite/Gneiss	Groundnut	Tamil Nadu
Laterite	Highly weathered rocks	Tea	Kerala
Desert	Sand	Bajra	Rajasthan
Mountain	Rocks	Apple	Himachal Pradesh

Soil Organic Matter (SOM) is the organic fraction of soil, consisting of plant residues, animal residues, microorganisms, and humus.

Component	Percentage
Fresh residues	10–15%
Partially decomposed matter	35–45%
Humus	40–60% ⭐⭐⭐

Element	Percentage
Carbon	58% ⭐⭐⭐
Oxygen	34%
Hydrogen	5%
Nitrogen	3%

Definition: Humus is the stable, dark-coloured organic matter formed after decomposition.

Characteristics: Dark colour, Colloidal, High CEC, High water holding, Improves fertility.

Physical: Improves soil structure, Increases aggregation, Increases water holding capacity, Reduces erosion, Lowers bulk density.

Chemical: Increases CEC ⭐⭐⭐, Stores nutrients, Buffers soil pH, Chelates micronutrients.

Biological: Food for microbes, Increases microbial activity, Improves nutrient cycling.

Material	C:N Ratio
Soil Humus	10:1 ⭐⭐⭐
FYM	20:1
Legumes	20–30:1
Straw	80:1
Sawdust	400:1

Humification: Conversion of organic residues into Humus.

Mineralization: Conversion of organic nutrients into plant-available inorganic forms (e.g., Organic N → NH₄⁺ → NO₃⁻).

Immobilization: Conversion of inorganic nutrients into microbial biomass, making them temporarily unavailable to plants.

Factor	Effect
Temperature	High → Faster decomposition
Moisture	Optimum → Faster
Aeration	Good aeration → Faster
C:N Ratio	Narrow ratio → Faster
Microorganisms	Increase decomposition

Improves fertility, Improves water retention, Reduces nutrient loss, Enhances microbial activity, Improves soil aggregation, Increases CEC, Reduces erosion.

Question	Answer
USDA Soil Taxonomy published	1975
Lowest category in taxonomy	Series
Youngest soil order	Entisol
Black soil order	Vertisol
Organic soil order	Histosol
Black soil clay mineral	Montmorillonite
Organic matter carbon content	58%
C:N ratio of humus	10:1
Stable organic matter	Humus
Dark colour of soil	Humus

#	Fact / Concept
1	USDA Soil Taxonomy was published in 1975.
2	There are 12 soil orders.
3	Series is the lowest category in soil taxonomy.
4	Entisols are the youngest soils.
5	Vertisols are black cotton soils.
6	Histosols are organic soils.
7	Mollisols are highly fertile grassland soils.
8	Aridisols occur in deserts.
9	Alluvial soils are formed by river deposits.
10	Black soils originate from basalt.
11	Montmorillonite is the dominant clay mineral in black soil.
12	Black soils have high water-holding capacity.
13	Red soil gets its colour from ferric oxide.
14	Laterite soils are rich in iron and aluminium.
15	Desert soils are sandy.
16	Mountain soils are acidic.
17	Peaty soils are rich in organic matter.
18	Wheat grows well in alluvial soil.
19	Cotton grows best in black soil.
20	Tea grows well in lateritic soil.
21	Apple grows well in mountain soil.
22	Rajasthan has extensive desert soils.
23	Maharashtra has extensive black soils.
24	UP has extensive alluvial soils.
25	SOM means Soil Organic Matter.
26	Humus is stable organic matter.
27	Humus is dark coloured.
28	Humus has high CEC.
29	Humus improves soil structure.
30	Organic matter contains about 58% carbon.
31	C:N ratio of humus is 10:1.
32	Straw has a wide C:N ratio.
33	Sawdust has a very wide C:N ratio.
34	Mineralization releases plant nutrients.
35	Humification forms humus.
36	Immobilization temporarily ties up nutrients.
37	High temperature speeds decomposition.
38	Good aeration increases decomposition.
39	Adequate moisture increases decomposition.
40	Narrow C:N ratio decomposes faster.
41	Humus increases water-holding capacity.
42	Humus reduces bulk density.
43	Organic matter improves aggregation.
44	Organic matter supports soil microorganisms.
45	Organic matter helps reduce erosion.
46	Organic matter buffers soil pH.
47	Vertisols shrink and swell because of montmorillonite clay.
48	Laterite soils are highly weathered.
49	Alluvial soils are among the most productive agricultural soils.
50	Soil organic matter is essential for maintaining long-term soil fertility.