Table of contents

1. Introduction
   
2. Parent material and environment
   
3. Regional Distribution
   
4. Definition
   
5. Genesis
   a. Formation of smectite-rich parent material
   b. Formation of a vertic horizon
   
6. Characteristics of Vertisols
   a. Morphological characteristics
   b. Chemical characteristics
   c. Physical characteristics
   
7. Management and Use of Vertisols
   

Fig.1 The vertic structure of Vertisols

( Source: FAO, 2001.)

• The name Vertisols comes from the Latin word vertere = to turn.
  
• Vertisols belong to the Reference Soil Groups set #3 of the WRB reference system. This set includes mineral soils whose formation is conditioned by the particular properties of their parent material (World Soil Resources Report 94, 2001).
  
• Vertisols contain a high content of expanding 2:1 lattice clays which exhibit swelling and shrinking features upon wetting and drying, leading to the characteristic phenomena as slickenside formation, the formation of deep wide cracks from the surface downward upon drying and sometimes undulating microtopography ( gilgai) .
  
• They are internationally known as:
  
  1. Vertisols, black cotton soils: Soil Taxonomy, USA
     
  2. Gilgai: Australia
     
  3. Regur: India
     

Parent material:

• The type of parent material is the most important factor involved in the formation and distribution of Vertisols. The soil materials on which Vertisols form need to have the minimum clay content and the smectitic type of clay.
  
• The parent materials may originate from sediments or igneous or metamorphic origins. Most Vertisols form on weathering products of basic and/or  ultrabasic rocks or on sediments with high amounts of smectites.
  
• Examples of sedimentary origin are: loessial, fluvial, colluvial, lacustrine and marine deposits, and marl, chalk, limestone, coral and shale bedrock.
  
• Igneous and metamorphic origins refer to Vertisols developed from weathering products of volcanic ash and tuff, schist, granite, gneiss, basalt, gabbro, andesite, amphibolite, diabase or dolerite (Coulombe et al., 1996).  http://www2.volstate.edu/svinson/geo100/minerals.html
  

Envionment:

• Found in depressions and level to undulating areas.

  Fig.2 Vertisol landscape Australia

  ( Source: www.nrcs.usda.gov/.../ vertisols/vert69.htm)

  Fig.3 Eutric Vertisol, Sudan: plowed for irrigation

  ( Source: ISRIC, Nl.)

  
  
• Mainly semi-arid to sub-humid climate with an alternation of wet and dry seasons.
  
• Climax vegetation is savanna, natural grassland and /or woodland.

  Fig.4 Vertisol landscape Zimbabwe

  ( Source: ISRIC, Nl.)

  
  

Vertisols are particularly extensive on:

1. (former) sedimentary lowlands
   
   • Sedimentary lowlands with expanding smectitic clays cover large areas along the southern border of the Sahara desert where lakes and floodplains were abundant between 12,000 and 8,000 years BP (climate was more humid than at present).

     Fig.5 Smectitic clays along the southern border of the Sahara desert. Alternating dry and wet spells existed during the past. Vertisols could form in the alluvial deposits.

     ( Source: FAO, 2001.)

     
     
2. Denudation plains on Ca-, Mg- and Na-rich parent rock.

   Fig.6 Base-rich parent rock, volcanic origin

   ( Source: FAO, 2001.)

   
   
   • Most denudation plains are underlain by basic volcanic rock such as the flood basalts of the Deccan Traps in India

     Fig.7 Layers of basalt lava, called the Deccan traps. These voluminous lava flows cover over 500,000 square kilometers of India. They erupted 65 million years ago. The basaltic rock weathers to smectite rich soils on the western part of the plateau, erosion and transport has extended the material eastwards (Eswaran et al., 1999).

     or by basic basement rock (amphibolites, greenschists).
     
   • Where shallow groundwater held basic cations in solution the neo-formation of smectites could occur.
     
3. Erosive uplands with limestone, claystone, marls or shale  http://www2.volstate.edu/svinson/geo100/minerals.html
   
   • The clays originate from a marine environment or were once incorporated in limestone or marl. Uplift and renewed denudation of the landscape brought the strata to the surface again.
     
   • After limestone or marl became exposed to chemical weathering, the clastic residues were transported to lower positions in the landscape.
     
   • If the clay accumulates in wet depressions, Vertisols can form provided that there is a dry season that is long and dry enough for the clay to shrink and crack and develop vertic properties in a subsequent wet spell.
     

Fig.8 Vertisols worldwide

( Source: FAO, 2001.)

• Vertisols cover worldwide about 335 Mio. ha (~ 2-3 %) of which about half is located in the tropics. 150 Mio is potential cropland.
  
• The large contiguous areas of Vertisols are in
  
  • the Gezira Plains of the Sudan (~ 50 Mio ha or 16 %)
    
  • the Deccan Plateau of India (~ 80 Mio ha or 25 %)
    
  • the Murray-Darling basin of South Eastern Australia (~ 70 Mio ha) or 22 %
    
  • The Blacklands of Texas (~ 18 Mio ha or 6 %) and the Southern Uruguay (~ 1 Mio ha)
    
  • And the Northern Argentina Rio Plata basin (~ 6 Mio ha)
    
• Typically, most of these are derived from alluvium from basic or ultrabasic rocks.
  
• In Africa, alluvial areas with Vertisols occur in
  
  • Burundi, in the Kafue flats of Zambia (~ 5 Mio ha)
    
  • The Springbok flats of South Africa (~ 2 Mio ha)
    
  • In the rift valley and the plateau of Ethiopia (~ 13 Mio ha)
    
• In situ Vertisols formed directly on the weathering products of rocks are local and occur in the Pacific islands and other areas affected by volcanism (Dudal and Eswaran, 1988).
  

1. Has a vertic horizon within 100 cm from the surface. The diagnostic criteria of vertic are:
   a. contain 30 % or more clay throughout; and
   b. have wedge-shaped parallel epipeds or  parallelepiped structural aggregates with the longitudinal axis tilted between 10^o and 60^o from the horizontal;

   Fig.9 Vertisol with strong angular structure elements

   ( Source: FAO, 2001.)

   Fig.10 Eutric Vertisol, South Khartoum, Sudan

   ( Source: ISRIC, NL.)

   and
   c. Have intersecting  slickensides; and

   Fig.11 Slickensides of a vertisol on Cuba

   ( Source: FAO, 2001.)

   Fig.12 Slickensides and shearing

   ( Source: www.nrcs.usda.gov)

   
   Have a thickness of 25 cm or more.
   
2. After the upper 20 cm have been mixed:
   
   1. 30 % or more clay in all horizons to a depth of 100 cm or more.
      
   2. Or to a contrasting layer between 50 and 100 cm (e.g. petrocalcic -> see  http://www.fao.org/DOCREP/003/Y1899E/Y1899E00.HTM)
      
   3. Or a sedimentary discontinuity.
      
3. Cracks, which open and close periodically.

   Fig.13 Deep cracks which open and close periodically

   ( Source: FAO, 2001)

   Fig.14 When organic matter content is high, the surface cracks are not so evident and appear merged as in this Vertisol from Australia.

   ( Source: www.nrcs.usda.gov)

   
   

• Commonly associated with some Vertisols are the  diapiric structure and the  gilgai micro-relief. The ( gilgai micro-relief) is lost on plowing and so its presence is not a requirement for the definition of Vertisols.
  

a. Formation of smectite-rich parent material

b. Formation of a vertic horizon

- classical „self-swallowing model“

- shear failure model

• The optimal condition for Vertisol formation is an environment that has high bases or promotes the accumulation of basic cations. A period during the year when evapotranspiration exceeds precipitation helps in maintaining the high pH of the system, which is critical for smectite formation. The role of external climate is secondary. Landscape position and landform are the major controls of Vertisol formation (Eswaran et al., 1999).
  

a. formation of smectite-rich parent material

1. Rainfall must be sufficient to enable weathering but not so high that
   leaching of basic cations and Si occurs
   No ultimate weathering to 1:1 ( clay minerals)
   
2. Dry periods must allow crystallization of clay minerals that form upon rock or sediment weathering. Smectite is the first secondary clay minerals to form upon rock weathering in the semi-arid to sub-humid tropics (see ( clay minerals) : smectite).
   
3. Drainage must be impeded so that leaching and loss of weathering products are curbed.
   
4. High temperatures promote weathering processes. Under such conditions smectite clays can be formed in the presence of Si and basic cations (Mg, Ca), if soil pH is above neutral.
   

b. Formation of a vertic horizon is the principal genetic process in Vertisols. A minimum amount of clay composed dominantly of smectitic mineralogy is essential for a soil to express vertic properties.The typical structure may occur in most of the solum but has its strongest expression in the vertic horizon. The changes in microstructure and porosity upon changing soil moisture conditions are believed to induce soil movement and is expressed by 2 common models:

Self-swallowing model (see Buol et al., 2003):

• A common initial situation in environments with distinct dry and wet season are:
  
  • the clay plain is flooded at the end of the rainy season but most of the standing water evaporates eventually.
    
  • When the saturated surface soil starts to dry out -> shrinkage of the clayey topsoil is initially one-dimensional and the soil surface subsides without cracking.
    
  • Upon further drying, the soil loses its plasticity and tension builds up until the tensile strength of the soil material is locally exceeded, and the soil cracks.
    

  Fig.15 Self-swallowing model

  ( Source: Buol et al., 2003.)

• Granules or crumbs of the surface material fall into the cracks.
  
• Upon re-wetting, part of the space that the soil requires for its increased volume is occupied by mulch material. Continued water uptake generates pressures that result in  shearing.
  
• Shearing occurs as soon as the  shear stress that acts upon a given volume of soil exceeds its  shear strength or tensile strength.
  
• The swelling pressure acts in all directions. Mass movement along oblique shear planes at an angle of 10^o to 60^o with the horizontal plane resolves this pressure. The resulting wedge-shaped structural aggregates result from the intersection of  slickensides.
  
• The swelling depends on water content and expansibility of the clay and the pressure depends on swelling and amount of material in the cracks.
  
• Although the structure conforms to the definition of an angular blocky structure, the specific shape of the peds has prompted authors to coin special names such as lentils, wedge-shape peds, tilted wedges, parallelepipeds or bicuneate peds (World Soil Resources Report 94, 2001).
  

The sliding of crumb surface soil into cracks and the resultant shearing have important consequences

1. Mixing of surface soil and subsurface soil or self-mulching effect (= inverting, churning effect)

   Fig.16 Grumic Vertisol, refers to the crumb structure at the surface

   ( Source: FAO, 2001.)

   leading to a granular structure in the upper 10 cm.
   
2. In churning Vertisols, coarse fragments (quartz gravel and hard rounded carbonatic nodules) are concentrated at the surface, leaving the solum gravel free. They remain on surface as most of the cracks are to narrow to let them fall back.

   Fig.17 Collected stones in landscape

   ( Source: FAO, 2001.)

   
   

• However, this model does not explain why
  
  • not all Vertisols develop a surface mulch. Some develop a hard surface crust (sharp-edged, remain open through the dry season and little surface soil falls into them).
    
  • not all Vertisols show a uniform soil profile (strong homogenization) but have albic or Bt horizons.
    
  • also, recent radiocarbon dating have shown that many Vertisols have a increasing mean residence time of organic matter with depth.
    
  • Slickensides are also found below the depths of normal field cracking.
    

Shear failure model:

• The shear failure model was proposed by Wilding and Tessier (1988; see Coulombe et al., 1996) and integrates the Coulomb-Mohr theory of shear failure to explain the formation of vertic properties in Vertisols:
  
• In a soil profile, stresses act vertically (vertical stress) or horizontally (lateral stress).

  Fig.18 Slickenside formation according to the theory of shear failure when stresses exceed shear strength in a confined system: (A) vertical and horizontal stresses acting on soil ped, and (B) orientation of shear plane at 45o to the principal stress.

  ( Source: Adopted from Coulombe et al., 1996.)

  The failure plane (see  structural failure) occurs when vertical forces are confined and lateral stresses exceed the  shear strength of the soil.
  
• The load of the soil material at any depth provides the vertical downward force. The vertical upward force is the swelling pressure and the net result is the shearing force whereby the direction of this net force is at any angle up to 60^o form the horizontal.
  
  • In the surface soil (top 20 to 30 cm), only little pressure exists so that pressure equalization may occur through upward movement. The swell/shrink is reflected in the angular to subangular pedal elements.
    
  • In the subsurface soil (60 to 100 cm): increasing overburden and the pressure of the overlying surface horizon curbs vertical movement, i.e. the resultant force causes the soil material to shear. This shearing can only take place between critical soil moisture limits when the soil material is plastic. The pressure equalization is inclined (angular) and results in tilted slickensides. In practice slickensides are observed from 20 to 60^o and are generally arranged in sub-parallel sets and they never intersect (Eswaran et al., 1999).
    
• However, this model does not explain the mixing of A and Bss or BC materials that can be found in soils within depths of cracking and adjacent to slickenside planes which support the pedoturbation or self-swallowing model (Coulombe et al., 1996).
  

• Is not always clear where A-horizon ends and B-horizon begins.
  
• Vertisols may have variable colors, e.g. black, gray, brown or red.
  
• The dark color is not a reflection of the amount of organic carbon, which in comparison to other soils with similar color, is low.
  
• A more brownish or reddish color in Vertisols is attributed to the presence of Fe-oxides or oxyhydroxides -> for example:
  
  • at higher topographic positions that promote leaching and oxidation.
    
  • or a higher Fe content in the parent material.
    
  • the dissolution of Fe-rich smectites in slightly acidic environments, and coatings of Fe oxides on minerals and ped surfaces inherited from the parent material.
    

• The chemistry of Vertisols is to a great extent controlled by the kind and amount of clay, the landscape position, the nature of the parent material, and climatic conditions. Consequently, there is considerable variability in chemical parameters.
  
• The majority of Vertisols is neutral or alkaline (pH) because they are mostly derived from base-rich parent materials.
  
• The organic carbon content (OC) may vary from as low as 0.3 % (or 3 g kg^-1) to 6 % (or 60 g kg^-1).
  
• Generally have high ( CEC) (30 – 80 cmol_c kg^-1) and high ( BS) (> 50, often close to 100, with Ca²⁺ and Mg²⁺ occupying more than 90 %). The amount and type of clay, in particular the smectitic content, and the OC content are the determinant factors.
  

• The physical properties of Vertisols are major constraints to their optimal utilization.
  
• Several of the physical properties vary with moisture content and associated shrink-swell phenomena. The consistency of Vertisols varies form plastic and sticky when wet, friable when moist to hard and a coarse prismatic structure in the topsoil when dry.
  
• Shrinkage is a fundamental process of Vertisols and results from changes in water potential and moisture content. The soil shrinkage potential is derived from the coefficient of linear extensibility ( COLE).
  

• Worldwide the largest acreages are used for pasture.
  
• The high clay content and associated slow permeability of these soils when wet make them recommendable for paddy rice cultivation that require retention of surface water.

  Fig.19 Rice cultivation

  ( Source: www.nrcs.usda.gov)

  
  
• Vertisols are prone to water erosion due to their slow infiltration. Once the soil is thoroughly wetted and the cracks are closed the rate of water infiltration becomes almost zero.

  Fig.20 Soil losses from Vertisols

  ( Source: Van Wambeke, 1997.)

  
  
• Therefore the management of Vertisols comprises to a great part the control of water.

  Fig.21 Rill erosion

  ( Source: www.nrcs.usda.gov)

  Fig.22 Soil erosion in Southern Ethiopia

  ( Source: FAO, 2001.)

  Fig.23 Gully erosion

  ( Source: FAO, 2001.)

  Fig.24 Gully erosion

  ( Source: FAO, 2001.)

  Fig.25 Gully erosion, erosive uplands Ethiopia

  ( Source: FAO, 2001.)

  
  

Management practices to improve water regime

1. Rain-fed post-rainy season cropping
   
   • Is practiced by many small-scale farmers in the tropics and subtropics who do not have the resources to build drainage systeme to evacuate surplus water from the land during the rainy season.
     
   • e.g. India: the land is left fallow during the ( monsoon) to store the water for the crops of the post-rainy season cropping.
     
   • Here, the water storage capacity and water-acceptance rates of the soil and the risk of erosion during the fallow are critical parameters and/or disadvantages of this type of land use.
     
2. Rainy season cropping; important is:
   
   • Prevention of ponding, runoff and erosion.
     
   • Protection of surface soil form direct rainfall.
     
   • To capture as much precipitation as possible and keep it available for plant growth.
     
   • One management systems to control the water during rainy season cropping (surface drainage) is the broad bed and furrow (BBF) system which uses alternating broad beds and furrows.

     Fig.26 Broad bed and furrow system; was developed by ICRISAT in India and is recommended for areas receiving 750 to 1250 mm per year

     ( Source: Van Wambeke, 1997.)

     Fig.27 Bed + furrow system in Ethiopian Highlands

     ( Source: FAO, 2001.)

     Fig.28 The ILRI devised an oxen-drawn broadbed-and-furrow maker

     ( Source: FAO, 2001.)

     Fig.29 Broad bad + furrow system on an eutric Vertisol, Sudan

     ( Source: ISRIC, Nl.)

     Fig.30 Intercropping on broad bed and furrow system

     ( Source: www.nrcs.usda.gov)

     
     
   • The high-cambered beds with deep drains evacuate the water rapidly. The beds place plant roots above the level of occasional flooding and the furrow either serves as drainage way or increase the infiltration of water.
     
   • The drained water may be stored in small ponds (water for cattle, e.g. in the Ethiopian highlands).

     Fig.31 Collecting of excess water in tanks

     ( Source: www.nrcs.usda.gov)

     Fig.32 Storage of excess water for cattle during the dry season

     ( Source: FAO, 2001.)

     
     
   • The BBF may promote soil erosion by concentrating water flow in the furrows -> the runoff water must be brought down safely in the lowest part of the landscape, e.g. along grassed waterways.

     Fig.33 Broad bed and furrow system for sugar cane in Guyana

     ( Source: www.nrcs.usda.gov)

     Fig.34 Bed and grassy furrows on sloping land

     ( Source: www.nrcs.usda.gov)