Table of contents

1. Introduction
   
2. Parent material and environment
   
3. Regional Distribution
   
4. Definition
   
5. Genesis
   a. Genesis under cool-humid and acid conditions
   b. Genesis under warm-humid, less acid soil conditions and lower organic matter production
   c. Characteristic features to consider
   
6. Characteristics of Andosols
   a. Morphological characteristics
   b. Physical characteristics
   c. Chemical characteristics
   
7. Management and Use of Andosols
   

Fig.1 Andosol landscape. Here: mollic Andosols, Rwanda

( Source: ISRIC, NL.)

• The name Andosols refers to the Japanese words an = black and do = soils, i.e. black soils of volcanic landscapes.
  
• Andosols 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).
  
• Andosols developed in volcanic material and are strongly influenced by pyroclastic material (= mixture of volcanic ashes, stones and gases).
  
• They are internationally known as:
  
  1. Andisols = Soil Taxonomy, USDA
     
  2. Andosols = France
     
  3. Black Dust Soils = Indonesia
     
  4. Soapy Hills = West-Indies
     
  5. others: volcanic ash soils
     

• click here to read more about ( volcanic landscapes)
  

Parent material:

• mainly volcanic ash, but also tuff, pumice, cinders and other volcanic ejecta (for pictures on stones go to:  http://www2.volstate.edu/svinson/geo100/minerals.html).
  
• The mineralogy differs in various regions but volcanic glasses, feldspars, Fe/Mg-minerals, Al-Humus-complexes and quartz are widely distributed.
  
• Mixture of  amorphous non-crystalline minerals in the clay fraction and clay minerals
  

Environment:

• Undulating to mountainous, humid, arctic to tropical regions with a wide range of vegetation types.
  

Fig.2 Andosols worldwide

( Source: FAO, 2001.)

• The total Andosol area worldwide is estimated at about 110 Mio. ha (~ 1 %). More than half of this is situated in the tropics.
  
• Major concentrations of Andosols are found around the Pacific Rim, Rift Valley Africa, Japan, New Zealand, pacific islands, Europe (Island, Italy).
  
• Many Andosols occur in densely populated areas.
  

• Must have an andic or vitric horizon within 25 cm of the soil surface.

  Fig.3 Vitric Andosols, Chile

  ( Source: ISRIC, NL.)

  
  

Definition vitric:

1. 10 % or more  volcanic glasses in the fine earth fraction (can be checked with x10 hand lens) (coarse texture)

2a. ( BD) = > 0.9 Mg/ m³ or

2b. Amorphous Al and Fe content > 0.4 % (acid oxalat extractable) or

2c. P-retention > 25 % and

3. Thickness of the horizon > 30 cm

Definition andic:

Fig.4 Andic horizon

( Source: http://sol.ensam.inra.fr)

1. > 5 % organic carbon (dark colored, fine textured)
   
2. < 10 % volcanic glasses, many  allophanes and/or Al-Humus-complexes
   
3. 10 % or more clay
   
4. BD at field capacity (no prior drying) of < 0.9 Mg/ m³
   
5. Amorphous Al and Fe > 2 % (acid oxalat extractable)
   
6. P-retention > 70 %
   
7. thickness > 30 cm
   

• Andosols are characterized by the presence of either an andic or a vitric horizon. An andic horizon is rich in allophanes or Al-humus complexes whereas a vitric horizon contains an abundance of volcanic glass (see World Resources Report 94, 2001).
  

a. Genesis under cool-humid and acid conditions

• Formation of acid Andosols occurs predominantly on acid substrate (rhyolite, older tuff, ignimbrite) under cool-humid and acid soil conditions (pH 3.5 to 5).
  
• Release of Si and Al³⁺ during rapid weathering of volcanic parent material.
  
• Al (and Fe) and organic acids and organic matter form stable and immobile Al-humus complexes. Si competes with humus/organic acids for Al. The free Si is either washed out from the soil or forms opaline Si (if not sufficient Al is present).
  
• The more humid: higher production of organic matter and stronger leaching of Si.
  
• [note: under acid conditions plant material and organic matter form a thick layer on the soil surface. During decomposition organic acids are released.]
  
• As Al protects organic matter from decomposition (is toxic for microorganisms) a thick dark organic horizon may form.

  Fig.5 Formation of a melanic horizon: intense dark color and high content of OM

  ( Source: FAO, 2001.)

  
  

b. Genesis under warm-humid, less acid soil conditions (pH 5.5 to 8) and lower organic matter production

• Fast and high release of Si, Al and Fe from weathering of volcanic parent material.
  
• Al forms together with Si and Fe amorphous, non-crystalline minerals as allophane, imogolite and ferrihydrite.
  

c. Characteristic features to consider

1. Youngest and least weathered part of the pedon is the surface layer
   
2. The formation of Andosols is further complicated if there is repeated deposition of fresh ash forming different horizons.

   Fig.6 Formation in various layers

   ( Source: FAO, 2001.)

   Fig.7 Formation in various layers: haplic Andosol, Japan

   ( Source: ISRIC, NL.)

   
   
3. Is often confounded with horizons that developed pedogenically.

   Fig.8 Ash layers from various volcanic eruptions. Here, a non weathered 200 year old white ash layer overlies a red-brown 8000 year old ash layer (Pacific Northwest of USA). Was first considered wrongly as Podzol (albic-spodic sequence).

   ( Source: http://soils.usda.gov/.../orders/ andisols.html )

   
   
4. Thin ash layers may just rejuvenate the surface soil material

   Fig.9 Vitric Andosol, Chile

   ( Source: ISRIC, NL.)

   but thicker layers bury the soil. A new profile will then develop in the fresh ash layer while soil formation in the buried A-horizon takes a different course in response to the suddenly decreased organic matter supply and the different composition of the soil moisture.

   Fig.10 Many Andosols contain buried soil

   ( Source: FAO, 2001.)

   
   
5. Strong variability (locally and temporarily) of parent material depending on wind direction and volcanic ejecta during eruption.

   Fig.11 Ignimbrite zonation, Mazama, Oregon (6,845 yr ago)

   ( Source: www.geology.sdsu.edu/.../ pinnacles_page.html)

   
   

• In Fig.11 the pale ( felsic) lower part is of rhyolitic origin and the darker upper part ( mafic) stems from andesitic volcanism. The zonation is inverse to the zonation of the material in the magma chamber before the volcanic eruption.
  

6a. Morphological characteristics

• Profile development mostly is AC oder ABC with a dark Ah (often 20 to 50 cm thick) on top of a brown B or C horizon. The A-horizon is thicker and darker as those of other soils in the region.

  Fig.12 Dark A-horizon

  ( Source: FAO, 2001.)

  Under grass often mollic Andosols develop

  Fig.13 Mollic Andosol, Chile

  ( Source: ISRIC, NL.)

  whereas under forest vegetation umbric Andosols may form.

  Fig.14 Umbric Andosol, Japan

  ( Source: ISRIC, NL.)

  
  
• In some Andosols the surface soil material is smeary (feels greasy) due to solid-liquid changes under pressure ( thixotropy). This makes Andosols susceptible to landslides, usually after prolonged heavy rains, as the high water holding capacity of andic horizons increases their weight.
  

6b. Physical characteristics

• The bulk density (BD) of Andosols is low (< 0.9 Mg m^-3). It is recommended to calculate contents on a volume basis, using BD.
  
• The quantity of available water is generally greater than in other mineral soils.
  
• Most Andosols have a good internal drainage, high porosity, high water infiltration, good aggregate stability.
  

Note: excessive air-drying of Andosol material will irreversibly deteriorate water holding properties, ion exchange capacities, soil volume, and ultimately the cohesion of soil particles through irreversible shrinkage of the colloidal surfaces. Therefore it is necessary to work on field moist samples.

6c. Chemical characteristics

1. Based on weathering intensity, composition of volcanic parent material

   Fig.15 Soil components of Basaltic, Andesitic and Rhyolitic soils

   and climatic conditions, the chemical properties of Andosols differ greatly.

   Fig.16 Chemical variability of Andosol soils

   ( Source: Van Wambeke, 2003.)

   
   
2. Andosols are variable-charge soils (non-crystalline amorphous structure of minerals), i.e. the cation exchange capacity ( cation exchange capacity) depends on the pH of the soil solution.

   Fig.17 Andosol No. 905 contains only allophane and imogolite: develop negative charges (CEC) and positive charges (AEC), depending on the pH of the solution. Andosol No. H-1-1 contains Al-humus complexes: has only negative surface charges (CEC) which increases with increasing pH.

   ( Source: Van Wambeke, 1992.)

   
   
3. Depending on the composition of Andosol soils, i.e. their quantity of amorphous allophanes and Al-humus complexes phosphorous (P)-retention is very high (up to 70 %; higher than for Ferralsols). Strongest P-fixation occurs through allophanes and imogolites (great reactive surface).

   Fig.18 Andic material fixes all of the added P (5, 25, 50, 75 ppm). The P-retention correlates with the density of active Al of the colloidal fraction and the high specific surface of the amorphous materials.

   ( Source: Buol et al. (2003).)

   
   

Andosols have a high potential for agricultural production. By and large, Andosols are fertile soils, particularly Andosols in intermediate or basic volcanic ash and not exposed to excessive leaching. The strong P fixation, however, is a problem.

Fig.19 Umbric Andosol, Ecuador

( Source: FAO, 2001.)

Fig.20 Dark surface colour is typical for many Andosols

( Source: FAO, 2001.)

Fig.21 Potato fields, Ecuador

( Source: FAO, 2001.)

Fig.22 Farmers burn the soil, Ethiopia

( Source: FAO, 2001.)