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Soil Types

Soil reaction

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Soil reaction is one of the most important physiological characteristics of the soil solution. The presence and development of micro- organisms and higher plants depend upon the chemical environment of soil. There fore study of soil reaction is important in soil science.

There are three types of soil reactions: 1. Acidic 2. Alkaline and 3. Neutral

1. Acidic: It is common in region where precipitation is high. The high precipitation leaches appreciable amounts of exchangeable bases from the surface layers of the soils so that the exchange complex is dominated by H ions. Acid soils, therefore, occur widely in humid regions and affect the growth of plants markedly.

2. Alkaline: Alkali soils occur when there is comparatively high degree of base saturation. Salts like carbonates of calcium, magnesium and sodium also give a preponderance of OH ions over H ions in the soil solution. When salts of strong base such as sodium carbonate go into soil solution and hydrolyze, consequently they give rise to alkalinity. The reaction is as follows: 
Na2CO3 -----à 2Na + + CO3= 
2Na+ + CO3= + 2HOH -----à 2Na+ + 2OH - + H2CO3 
since sodium hydroxide dissociates to a greater degree than the carbonic acid, OH ions dominate and give rise to alkalinity. This may be as high as 9 or 10. These soils most commonly occur in arid and semi-arid regions.

3. Neutral: Neutral soils occur in regions where H ions just balance OH ions.

Soil pH: The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration of H ions (acidity) or OH ions present in it. Acidity is due to the excess of H ions over OH ions, and alkalinity is due to the excess of OH ions over H ions. A neutral reaction is produced by an equal activity of H and OH ions. According to the theory of dissociation, the activity is due to the dissociation or ionization of compounds into ions.

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Soil Texture

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Definition of Soil Texture: Soil texture refers to the relative proportion of particles or it is the relative percentage by weight of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles.

The proportion of each size group in a given soil (the texture) can not be easily altered and it is considered as a basic property of a soil.

The soil separates are defined in terms of diameter in millimeters of the particles. Soil particles less than 2 mm in diameter are excluded from soil textural determinations.

Stones and gravels may influence the use and management of land because of tillage difficulties but these larger particles make little or no contribution to soil properties such as WHC and capacity to store plant nutrients and their supply.

Gravels: 2 - 4 mm 
Pebbles: 4 - 64 mm 
Cobbles: 64 - 256 mm 
Boulders: > 256 mm

Particles less than 2 mm are called fine earth, normally considered in chemical and mechanical analysis.

The components of fine earth: Sand, Silt and Clay (Soil separates. The size limits of these fractions have been established by various organizations. There are a number of systems of naming soil separates.

  1. (a) The American system developed by USDA
  2. (b) The English system or British system ( BSI )
  3. (c) The International system (ISSS)
  4. (d) European system


Soil separates

Diameter in mm


< 0.002


0.002 - 0.05

Very Fine Sand

0.05 -  0.10

Fine Sand

0.10 - 0.25

Medium Sand

0.25 - 0.50

Coarse Sand

0.50 - 1

Very Coarse Sand

1 - 2 mm

ii) BSI

Soil separates

Diameter in mm


< 0.002

Fine Silt

0.002 - 0.01

Medium Silt

0.01 - 0.04

Coarse Silt

0.04 -  0.06

Fine Sand

0.06 - 0.20

Medium Sand

0.20 - 1

Coarse Sand

1 - 2 mm

iii) ISSS

Soil separates

Diameter in mm


< 0.002


0.002 - 0.02

Fine Sand

0.02 - 0.2

Coarse Sand

0.2 -  2

iv) European System

Soil separates

Diameter (mm)

Fine clay

< 0.0002 mm

Medium clay

0.0002 – 0.0006

Coarse clay

0.0006 – 0.002

Fine silt

0.002 - 0.006

Medium silt

0.006 - 0.02

Coarse silt

0.02 - 0.06

Fine sand

0.06 - 0.20

Medium sand

0.20 - 0.60

Coarse sand

0. 60 - 2.00


  1. Usually consists of quartz but may also contain fragments of feldspar, mica and occasionally heavy minerals viz., zircon, Tourmaline and hornblende.
  2. Has uniform dimensions
  3. Can be represented as spherical
  4. Not necessarily smooth and has jagged surface


  1. Particle size intermediate between sand and clay
  2. Since the size is smaller, the surface area is more
  3. Coated with clay
  4. Has the physico- chemical properties as that of clay to a limited extent
  5. Sand and Silt forms the SKELETON


  1. Particle size less than 0.002 mm
  2. Plate like or needle like in shape
  3. Belong to alumino silicate group of minerals
  4. Some times considerable concentration of fine particles which does not belong to alumino silicates. (e.g.) iron oxide and CaCO3
  5. These are secondary minerals derived from primary minerals in the rock
  6. Flesh of the soil

Knowledge on Texture is important. It is a guide to the value of the land. Land use capability and methods of soil management depends on texture.

Particle size distribution/ determination

The determination of relative distribution of the ultimate or individual soil particles below 2 mm diameter is called as Particle size analysis or Mechanical analysis

Two steps are involved

i) Separation of all the particles from each other ie. Complete dispersion into ultimate particles

ii) Measuring the amount of each group


Sr. No.

Aggregating agents

Dispersion method


Lime and Oxides of Fe & Al

Dissolving in HCl


Organic matter

Oxidises with H2O2


High conc. of electrolytes 
( soluble salts)

Precipitate and decant or filter with suction


Surface tension

Elimination of air by stirring with water or boiling

After removing the cementing agents, disperse by adding NaOH


Once the soil particles are dispersed into ultimate particles, measurement can be done

i) Coarser fractions - sieving - sieves used in the mechanical analysis corresponds to the desired particle size separation for 2 mm, 1 mm and 0.5 mm – sieves with circular holes, for smaller sizes, wire mesh screens are used (screening)

ii) Finer fractions - by settling in a medium the settling or the velocity of the fall of particles is influenced by viscosity of the medium. Difference in density between the medium and falling particles, size and shape of object

Stokes' Law:

Particle size analysis is based on a simple principle i.e. "when soil particles are suspended in water they tend to sink. Because there is little variation in the density of most soil particles, their velocity (V) of settling is proportional to the square of the radius 'r' of each particle. 
Thus V = kr2, where k is a constant. This equation is referred to as Stokes' law.

Stokes (1851) was the first to suggest the relationship between the radius of the particles and its rate of fall in a liquid. He stated that "the velocity of a falling particle is proportional to the square of the radius and not to its surface. The relation between the diameter of a particle and its settling velocity is governed by Stokes' Law:

V = 2/9 gr^2 (ds – dw) / n

V - Velocity of settling particle (cm/sec.) 
g - Acceleration due to gravity cm/ sec2 (981) 
ds - Density of soil particle (2.65) 
dw - Density of water (1) 
n - Coefficient of viscosity of water (0.0015 at 4oC) 
r - Radius of spherical particles (cm).

Assumptions and Limitations of Stokes' Law

Particles are rigid and spherical / smooth. This requirement is very difficult to fulfill, because the particles are not completely smooth over the surface and spherical. It is established that the particles are not spherical and irregularly shaped such as plate and other shapes. 
The particles are large in comparison with the molecules of the liquid so that in comparison with the particle the medium can be considered as homogenous i.e. the particles must be big enough to avoid Brownian movement. The particles less than 0.0002 mm exhibit this movement so that the rate of falling is varied.

The fall of the particles is not hindered or affected by the proximity (very near) of the wall of the vessel or of the adjacent particles. Many fast falling particles may drag finer particles down along with them.

The density of the particles and water and as well as the viscosity of the medium remain constant. But this is usually not so because of their different chemical and mineralogical composition.

The suspension must be still. Any movement in the suspension will alter the velocity of fall and such movement is brought by the sedimentation of larger particles (> 0.08 mm). They settle so fast and create turbulence in the medium.

The temperature should be kept constant so that convection currents are not set up.

Methods of Textural determination

Numerous methods for lab and field use have been developed 
i) Elutriation method – Water & Air
ii)Pipette method
iii) Decantation/ beaker method
iv) Test tube shaking method
v) Feel method - Applicable to the field - quick method - by feeling the soil between thumb and fingers

Feel Method

Evaluated by attempting to squeeze the moistened soil into a thin ribbon as it is pressed with rolling motion between thumb and pre finger or alternately to roll the soil into a thin wire
η four aspects to be seen - i) Feel by fingers, ii) Ball formation, iii) Stickiness and iv) Ribbon formation




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In general, the soils of Kerala are acidic, kaolintic and gravelly with low CEC(Cation Exchange Capacity), low water holding capacity and high phosphate fixing capacity. Climate topography, vegetation and hydrological conditions are the dominant factors of soil formation. On the basis of the morphological features and physico-chemical properties, the soils of the State have been classified into red loam, laterite coastal alluvium, riverine alluvium, Onattukara alluvium, brown hydromorphic, saline hydromorphic, Kuttanad alluvium, black soil and forest loam.

Red Loam

Location :- Red loams of Kerala are localized in occurrence and are found mostly in the southern parts of Thiruvananthapuram district.

Formation and Occurance:-These soils occur in catenary sequence along with laterites and are found mainly as deposits by colluviation in foothills and small hillocks.


    1. Colour and texture:- The soils have red colour, which has been attributed to the presence of hematite or anhydrous ferric oxides.

    1. Permeability:- The rapid permeability of the surface soils also has been responsible for the characteristic development of these loamy oils, which are very deep and homogeneous without much expression of horizons.

    1. Fertile Nature:- They are not fertile due to low organic matter content as well as low essential plant nutrients.

    1. Other Features:- These soils are essentially kaolinitic in nature, acidic in reaction and friable.


Location :- This is the most abundant soil seen in Kerala. They cover about 65 per cent of the total area of the State, occupying a major portion of the midland and mid-upland regions and are the most extensive of the soil groups found in Kerala. Laterite soil is predominantly seen in northern parts of Kerala like Kozhikode, Kannur,Mallapuram and Kasaragod.

Formation and Occurance:-Laterite of Kerala is typical kaolinitic weathering products of gneissic and granitic rocks developed under humid tropical conditions. Heavy rainfall and high temperature prevalent in the State are conductive to the process of laterisation.


  1. Colour and texture:- The surface soil, which is reddish brown to yellowish red, is mostly gravelly loam to gravelly clay loam in texture. It shows the development of AB(c) profiles. The profiles have well-developed B-horizon with abundant ferruginous and quartz gravels. The plinthite is characterized by a compact vesicular mass below the B-horizon, composed essentially of a mixture of hydrated oxides of iron and aluminum. The plinthite includes quarriable type that breaks into blocks and also non-quarriable type that breaks into irregular lumps.

  1. Permeability:-They have poor water-holding capacity

  1. Fertile Nature:- Tough this soil, in general, is acidic, poor in available nitrogen, phosphorus and potassium and are low in the bases,it is well drained, widely cultivated and respond to management practices. A variety of crops like coconut, tapioca, rubber, areca nut, pepper, cashew etc can be successfully grown by the proper application of fertilizers and irrigation.

  1. Other Features:- They are generally acidic with pH ranging from 4.5 to 6.2.They have CEC and high P fixing capacity with low organic matter content.

Coastal Alluvium

Location :- These soils are seen in the coastal tracts along the West as a narrow belt with an average width of about 10 km.

Formation and Occurrance:- As it has been developed from recent marine deposits Sand is the chief constituent of this type of soil. Soil profile surface is not well developed. They show incipient development.


  1. Colour and texture:- The texture is dominated by sand fraction The A horizon is usually thin and the surface textures observed are loamy sand and sandy loam.

  1. Permeability:-It has very rapid permeability.

  1. Fertile Nature:- These soil are of low fertility level

  1. Other Features:- These soils are acidic (pH value less than 6.5 in most of the areas).They are also low in organic matter, clay and CEC.

Riverine Alluvium

Location :- This soil occurs throughout the state cutting across the extensive laterite soils. They are important in Kollam taluk.

Formation and Occurrance:- This type of soil, developed along river valley, occurs mostly along the banks of rivers and their tributaries.


  1. Colour and texture:- They are very deep soils which surface texture ranging from sandy loam to clay loam. Horizon differentiation is not well expressed.

  1. Other Features:- They are moderately supplied with organic matter, nitrogen and potassium. They are acidic and poor in phosphorus and lime. They show wide variation in their physico-chemical properties depending obviously on the nature of alluvium that is deposited and the characteristics of the catchment area through which the river flows.

Onattukara alluvium

Location :- These soils are confined to the Onattukara region comprising the Karunagapally, Karthikapally and Mavelikara taluks of Kollam and Alappuzha districts.

Formation and Occurrance:- They occur as marine deposits extending to the interior up to the lateritic belt.


  1. Colour and texture:- The soils are, in general, coarse textured with immature profiles.

  1. Permeability:-These soils have very rapid permeability. In low-lying areas, the water table is high and drainage is a problem.

  1. Fertile Nature:- Addition of sufficient organic matter and irrigation facilities improve the water holding capacity. Coconut, Paddy and Tapioca are the major products derived from these soils.

  1. Other Features:- They are acidic in reaction and are extremely deficient in all the major plant nutrients.

Brown hydromorphic

Location :- Hydromorphic soils, as a group, occur extensively in the State. These soils are mostly confined to valley bottoms of undulating topography in the midland and to low lying areas of coastal strip. These are also found in areas of wetland.

Formation and Occurrance:- They have been formed as a result of transportation and sedimentation of material from adjacent hill slopes and also through deposition by rivers. Presence of Lateritic and gravel suggest that these are formed by the action of gravity.


  1. Colour and texture:- As the name implies the soil is deep brown in colour. Compositionally it ranges between sandy loam to clay. Clay is of pottery type.

  1. Permeability:-Drainage is the major problem of this kind of soil.

  1. Other Features:- They exhibit wide variation in physico-chemical properties and morphological features.The development of the soil profiles has occurred under impeded drainage conditions. These soils,therefore, exhibit characteristic hydromorphic features like grey horizons, mottling streaks, hard pans, organic matter depositions, iron and manganese concretion, etc. They are moderately supplied with organic matter, nitrogen and potassium and are deficient in lime and phosphorus. Acidity is a problem in some areas.

Saline hydromorphic

Location :- These soils are usually seen within the coastal tracts of the districts of Ernakulam, Alappuzha, Trissur and Kannur.

Formation and Occurrance:- The origin, genesis and development of these soils have been under peculiar physiographic conditions. They are, therefore, not comparable with the saline soils occurring in the other parts of the Country. The network of backwaters and estuaries bordering the coast serves as an inlet of tidal waters to flow into these areas, causing Salinity. During the period of March – April the soil is characterised by high salinity, but in June – July they are devoid of salts. Wide fluctuation in the intensity of salinity has been observed. During the rainy season, the fields are flooded and most of the salt is leached out, leaving the area almost free of the salts.


  1. Colour and texture:- These soils are in general brownish and deep. The profile show wide variation in texture, as is common in most of the alluvial soils.

  1. Permeability:- This soil is imperfectly drained.

  1. Fertile Nature:- In these soils one crop of Paddy is cultivated during Auguast – December.

  1. Other Features: - Electrical conductivity of the soil during June – July season ranges from 0.1 to 2.0 m mhos/cm2. The maximum accumulation of toxic salts is observed during the summer months from March to April when electrical conductivity rises to the range of 10 - 15m mhos/cm2. Being developed in areas with relatively high ground water table, these soils show aquic properties. In some areas,undecomposed organic matter is observed in the lower layers, causing problems of acidity. The Pokkali (Ernakulam district) and kaipad (Kannur district) soils come under this category.

Kuttanad alluvium/Acid saline soil

Location:- Kuttanad

  1. Characteristics:- The Kuttanad region covering about 875 km2 is a unique agricultural area in the world. A good portion of this area lies 1-2 m below MSL and is submerged for major parts of the year. The area is susceptible to seasonal ingress of saline water as a result of tidal inflow from the sea. During the monsoons, the rivers and rivulets pour fresh water into the area. As the North East monsoon recedes, seawater again enters the Vembanad Lake and the whole area becomes saline. Hence, the soils of Kuttanad area are faced with the serious problems of hydrology floods, acidity and salinity. Consequent on the construction of the Thanneermukkam bund, salinity hazards have been considerably reduced. The soils of Kuttanad form the typical waterlogged soils and are entirely different from normal well-drained soils in their morphological, chemical and physical characteristics.

They can be grouped into three categories which are dealt with in the zone of Problem Areas.

      1. The Kayal soils

Location :- This type of soil is seen in the reformed lake bed of Kottayam and Alappuzha districts.


  • Colour and texture:- This soil is dark brown in colour but a white colour seen on the surface due to presence of salt. They contain Alluvial silt and Clay Loams some lime shells, organic matter,calcium, little nutrients and some salts on the surface.

  • Permeability:- They are poorly drained soils.

  • Other Features: -They are slightly acidic in nature and are deeply situated. These soils show some aquic features like grey colour, red and brown streak, as the clay content decreases with depth.

      1. The karappadam soils

Location :- These occur in the flat lands of large part of upper Kuttanad and also along the inland stream and rivers.

Formation and Occurrance:- They are formed by the transportation activity of rivers. These are situated 1 – 2m below sea level.


  1. Colour and texture:- This soil contain Clay Loams, Silty clay sub soils, some sands, decayed organic matter, and little amount of nutrients Phosphorus and Calcium.

  1. Permeability:- They are poorly drained soils.

  1. Other Features: -The important features of these soils are high salt content and presence of decomposed organic materials

      1. The Kari soils

    1. Location :- This soil is found insome parts of Alappuzha, Kottayam and Ernakulam districts.

Formation and Occurrance:- These are situated 1–2m below sea level.

Black soils

Location:- Black soils are restricted in their occurrence to Chittoor taluk of Palakkad district. They are found to occur in patches and are considered as extension of the black cotton soils observed in the adjacent Coimbatore district of Tamil Nadu.

Characteristics:- These soils are dark, low in organic matter, calcareous, neutral to moderately alkaline (PH 7.0 to 8.5) and high in clay content and CEC. Hence they exhibit the characteristic cracking during dry periods. They are usually located in gently sloping to nearly level lands. The levels of potassium and calcium are moderate and those of nitrogen and phosphorus, low. In a relatively small area of 1000 ha. in Chittoor block, a highly dispersed soil termed as "Poonthalpadam" soil, is seen. This soil occurs as a slushy layer to a depth of about 0.5m to 1.5m. A bed of limestone is seen beneath the slushy layer. The physical properties like plasticity, cohesion, expansion and shrinkage are similar to those of the regular soils of the Deccan.


Forest loam

Location:- They are found in the eastern parts of the State.

Formation and Occurrance:- They are the products of weathering of crystalline rocks under forest cover.


  1. Colour and texture:- They generally show vide variation in depth and are dark reddish brown to black, with loam to silty loam texture.They have immature profiles with shallow soils, followed by gneissic parent material in various stages of weathering. In areas with lesser canopy cover, signs of laterisation have been observed in the profiles. In denuded areas, leaching and deposition of humus in the lower layers are observed. The B-horizon usually contains gneissic gravels and boulders.


  1. Fertile Nature:-The soil is quite fertile under forest cover and promotes prolific undergrowth. This soil is often found under vegetation


  1. Other Features: - These soils are generally acidic with PH ranging from 5.5 to 6.3. They are rich in nitrogen; but poor in the bases because of heavy leaching.

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Know Your Soil

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What is soil? It is difficult to explain in a single sentence. It depends on what you use for. For an earth mining engineer soil is a useless thing and removes it. A highway engineer looks for gravel and rocks. For a farmer soil is everything for his life and precious. At first sight it may be sold substance. But soil contains water, air and organic matter in a certain ratio. There may be slight variations in cultivable soil. But major variations will affect the growth of plants seriously. Plants growing in water logged land may become yellowish. That is because lack of proper air circulation in the soil and it becomes difficult for the plant to breath and will die slowly. Flushing out water will help the plant to grow well.

There are different types of soil. Soil contains more sand, clay, sticky soil etc. Presence of organic matter in soil is very important. That is by the activities of microorganisms like bacteria. Earthworms also plays big role. The life of soil is the microorganisms which digests garbages, wastes, excretions etc. The base of existence of farming is the ability of soil to avail required nutrients. All the required nutrient elements should be there in soil for the plants to grow well. The ability to absorb nutrient elements and providing them to roots of plants is another important factor. Quality of soil increases when ability of absorbing elements presented in the form of ion increases. Fore more details please contact 'Krishy Bhavan' Agricultural offices near to you.

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The Soils in India for Rice Cultivation can be classified into the following categories :

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  • Sub-montane soils
  • Hill Soils
  • Tarai Soils
  • Calcareous Alluvial soils
  • Riverine Alluvium Soil
  • Laterite Soils
  • Saline and Alkaline soils
  • Red Yellow loamy soil
  • Red Soil
  • Black soils
  • Mixed Red and Black Soil
  • Deltaic Alluvium Soil
  • Coastal Alluvium Soil

1) Sub-montane soils
These soils are formed from the alluvium deposited in the valley floor by the Jhelum and the Indus rivers. They are silty loam to clay loam and are neutral to alkaline.

2) Hill Soils

These soils are shallow with fragments of rock occurring according to the elevations and have been categorized as red loam, brown forest soil, meadow soil and podzolic soil.

3) Tarai Soils

These soils are always saturated because of sufficient rainfall and high ground water table. These soils have been formed from transported materials by different rivers originating from the Himalayas. The tarai soils are very productive and responding well to fertilizer application.

4) Calcareous Alluvial soils

The alluvial soils are rich in potash and calcium but are deficient in organic matter, nitrogen and phosphorus. Alluvial soils cover about 24% of the total land and occur in the great Indo-Gangetic Plains, in the valleys of Narmada and Tapti in Madhya Pradesh and the Cauvery in Tamil Nadu.

5) Riverine Alluvium Soil

Seen along the banks of rivers. Shows wide variation in physico-chemical properties depending on the nature of alluvium and the characteristic of the catchment area through which the river flows. Organic Matter, N and K are moderate.

6) Laterite Soils

These soils are red but they differ from red soils. Such soils are found in heavy rainfall and high temperature areas. These soils are acidic, pH ranging from 4.0 to 5.0.

7) Saline and Alkaline soils

The soils are highly alkaline and have below hard pan which obstruct the downward movement of water.

8) Red Yellow loamy soil

These soils are encountered over extensive nonalluvial tracts of peninsular India. They develop in areas in which rainfall leaches soluble minerals out of the ground and results in a loss of chemically basic constituents.

9) Red Soil

The red color in soil usually indicates a high amount of iron, in the form of iron oxide, that coats the particles of the soil. These soil are acidic. These soils are rich in potassium and poor in phosphorus.

10) Black soils

Black soils are spread mostly across the Deccan Lava Plateau, the Malwa Plateau, and interior Gujarat, where there is both moderate rainfall and underlying basaltic rock. These soils contain sufficient lime and pH ranges from 7.0 to 8.5 . These soils are also deficient in phosphorus and low in organic matter and nitrogen. Black Soil can be classified as : Medium black soil, Shallow Black Soil and Deep Black Soil.

11) Mixed Red and Black Soil

Sometimes black soils are also found in isolated pockets along with the red soils.

12) Deltaic Alluvium Soil

The coastal belt is rich with highly fertile deltaic soil, giving it the reputation of being the rice granary of the country. The alluvial soils of the deltas are very deep and well drained. These soils are very fertile.

13) Coastal Alluvium Soil

Seen in the coastal tracts along the west. They have been developed from recent marine deposits. Permeability is more. Low organic matter content. Low CEC. Water Holding Capacity is less.

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