Table of contents

1. Introduction
   
2. Parent material and environment
   
3. Regional Distribution
   
4. Definition
   
5. Genesis
   
6. Characteristics of Alisols
   a. Morphological characteristics
   b. Physical characteristics
   c. Chemical characteristics
   
7. Management and Use of Alisols
   

• Parent material: Alisols can form in a wide variety of parent materials having high-activity clay minerals such as vermiculite and smectite. Most occurrences of Alisols reported so far are on weathering products of basic rocks.
  
• Environment: most common in old land surfaces with a hilly or undulating topography, in humid (sub)-tropical and monsoon climates.
  

Fig.2 Puerto Rico: Ferralsols occupy the flat landforms, Alisols on the slopes

( Source: Beinroth et al., 1996.)

Fig.3 Eastern China: hill region of Yanshan

( Source: FAO, 2001.)

Fig.4 Alisols worldwide

( Source: FAO, 2001.)

• Worldwide, about 100 Mio. ha are used for agriculture in the tropics (Driessen and Dudal, 1991). They are mainly found on old erosion-surfaces with undulating topography and in warm-moist regions and monsoon areas. Major occurrences of Alisols are found in
  
  • Latin-America (Ecuador, Nicaragua, Venezuela, Colombia, Peru, Brazil)
    
  • Africa (West-Afrika and highlands East-Afrika, Madagascar)
    
  • SE-Asia, China, Japan, SE-USA
    
  • Mediterranean area: old fluvial terraces, relicts of ancient humid periods
    

• Must have an argic B horizon within 100 cm or within 200 cm when loamy sand is above (see chapter ( clay eluviation) ).
  
• The argic horizon has
  
  1. a  CEC > 24 cmol_c/kg (1 M NH₄OAc solution buffered to pH 7)
     
  2. and the  BS < 50 %
     
  3. Alic properties = very acid soil material with a high Al-content between 25 – 100 cm
     

• Alisols form where ongoing hydrolysis of 2:1 layered minerals (HAC) like smectites and vermiculites releases much Al. Where these minerals come to the surface (outcrop in hilly topography), intensive leaching of Si and basic cations takes place. Many Alisols occur on slopes where the erosion of the surface (truncate) exposes the Bt.

  Fig.5 Sideslope Alisol

  ( Source: University of Tennessee Forestry Arboretum – Oakridge; web.utk.edu/ ~ammonst/oakridgesideslopepit6.jpg)

  
  
• Alisol-formation requires 3 steps:
  

1. Weathering of primary minerals and moderate leaching of Si. These processes produce a ( saprolite) with little weatherable primary minerals and a dominance of HAC. The HAC are predomiantly smectitic on basic rocks (e.g. basalt, andesite), and vermiculitic on more siliceous rocks such as granite or gneiss.
   
2. The second step involves the translocation of the clay minerals to an argic B horizon: vertical (eluviation: only between pH 5 and 6.5) ( see chapter clay eluviation) or laterally.
   
3. The third step involves weathering of HAC and the release of Al. HAC are unstable in environments that are depleted of silica and alkaline earth cations. When weathering occurs in basic parent material with high amount of Fe, Alisols have reddish color, e.g. rhodic Alisols (Caribbean)

   Fig.6 Rhodic Alisol

   ( Source: wapi.isu.edu/.../ EG_module_2_part2.html)

   
   

a. Morphological characteristics

• Most Alisols have an ochric surface horizon (= light-colored, thin, low SOM of 0.6 %) see  LECTURE NOTES ON THE MAJOR SOILS OF THE WORLD

  Fig.7 Alisol with thin A horizon

  ( Source: FAO, 2001.)

  but darker umbric horizons can be expected under forest.

  Fig.8 Alisol under natural vegetation in Indonesia

  ( Source: FAO, 2001.)

  
  
• The soil structure is rather weak in the surface horizon because biological activity is hindered by the strong acidity and high Al concentration. Less humus content is present in surface soil.

  Fig.9 Alisol under sparse vegetation in Indonesia

  ( Source: ISRIC, NL.)

  
  
• The surface horizon overlies a dense argic subsurface horizon that may hinder deep percolation of water. The structure of the argic horizon is clearly more stable than that of the surface soil.

  Fig.10 Strong developed structure of B horizon

  ( Source: FAO, 2001.)

  
  

b. Physical characteristics

• The physical characteristics of Alisols are directly related to the relative contents of HAC, LAC and Fe-oxides. Where shrink-swell minerals dominate, the formation of vertic properties/horizons is favored. These Alisols have BS > 50 %.
  
• In many Alisols, textural differentiation between surface and subsurface horizons imparts different physical properties. Surface horizons tend to have an unstable structure (slaking) and reduced permeability, in particular where the subsoil is consolidated. This delays internal drainage and intensifies erosion at slopes.

  Fig.11 Alisol with typical angular blocky soil structure

  ( Source: FAO, 2001.)

  
  

c. Chemical characteristics

• In many Alisols, toxic quantities of Al are present whereas other plant nutrients are low and unbalanced. Exchangeable Al often is between 7-10 cmol_c/kg. By comparison Ferralsols have 1 cmol_c/kg of exchangeable Al.
  
• However, some Alisols have good nutrient availability (favorable cation exchange properties (when high HAC content). The mineral reserves of Alisols are conditioned by the clay fraction and depend largely on the composition of HAC that act as weatherable minerals in the system. ( see chapter clay minerals)
  

• Generally, Alisols are unproductive soils. Alisols occur predominantly on old land surfaces with hilly or undulating topography. The generally unstable surface soil of Alisols makes them susceptible to erosion. Toxic levels of Al at shallow depth and poor natural soil fertility are added constraints. Some cropping systems are shown here:
  

  Fig.12 Alisols on hill slopes in Eastern China are terreced and use to grow irrigated rice

  ( Source: ISRIC, NL.)

  Fig.13 Lose of organic matter

  ( Source: ISRIC, NL.)

  Fig.14 Other common crops on these soils are buckwheat, millet, peanuts and tea

  ( Source: ISRIC, NL.)

  Fig.15 A well-studied locality with Alisols in the Amazon region of Peru

  ( Source: ISRIC, NL.)

• There use is restricted to acid-tolerant crops as tea, rubber, oil palm, cassava, cowpea, potato and traditional adapted rice cultivars (unless fertilization and liming). Aluminum-sensitive plants are maize, wheat and soybean.

  Fig.16 Relationship between aluminium, soil pH and yield of four root crops

  ( Source: Marschner, 1995.)

  
  
• Management strategies for the amelioration of Al-toxicity include the application of mulch

  Fig.17 Pueraria phaseoloides is used as cover crop

  or green manure to increase SOM. Soil organic matter (fulvic acid, organic acids) complexes Al in non-exchangeable forms and ameliorates the phytotoxicity of Al-species on root growth.
  
• Liming is another strategy to overcome or eliminate Al-toxicity for plants (Al impairs root development). The application of liming materials such as CaCO₃, Ca(OH)₂, CaO und MgCO₃ raise the pH of soils [Colloid-H⁺ + CaCO₃ -> Colloid-Ca + H₂O + CO₂] above 5.4 so that no free Al is existent. ( see chapter Al toxicity)
  
• However, in many cases lime must be added in large enough quantities

  Fig.18 Limestone requirement of soil with several different texture (and therefore with different buffering capacities) to raise pH to 6.5. Note: in warmer regions the SOM levels are lower and kaolinite often predominates, hence the amount of lime would be ½ or 1/3 of those indicated here (Brady and Weil, 2002).

  ( Source: Brady and Weil, 2002.)

  and many small-scale farmers cannot afford the costs of liming.
  
• For most purposes, amelioration will be achieved if the pH is raised just enough to eliminate exchangeable Al. This attempt (rather than achieve a certain pH) is appropriate for highly weathered soils (Ferralsols, Alisols). Using this approach, the amount of lime can be calculated using
  
  1. values for the initial CEC
     
  2. the Al saturation (%)
     
• An often used formula to determine lime requirement (Cochrane et al. 1980) is: tons CaCO₃/ha = 1.8 [Al – RAS (Al+Ca+Mg)] / 100, where
  
  1. RAS = desired % Al saturation
     
  2. Al, Mg, Ca = cmol_c/kg of each exchangeable cation (from van Wambeke, 1992).