World Reference Base for Soil ResourcesMineral Soils conditioned by Parent MaterialMineral Soils conditioned by TopographyMineral Soils conditioned by a wet (sub) Tropical Climate

Excursus: Bulk density


Table of contents

  1. Introduction
  2. Parent material and environment
  3. Regional Distribution
  4. Definition
  5. Genesis
    a. Formation in the dry zone
    b. Formation in the wet (humid) tropics
  6. Characteristics of Arenosols
    a. Morphological characteristics
    b. Physical characteristics
    c. Chemical characteristics
  7. Management and Use of Arenosols
1. Introduction

Fig.1 Landscape with quartz-rich sands
( Source: FAO, 2001.)

  • The name Arenosol comes from the Latin word arena = sand.
  • Arenosols belong to the Reference Soil Groups set #3 of the 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).
  • Arenosols consist of sandy soils developed in
    a. Residual sands, in situ after weathering of old, usually quartz-rich soil material or rock
    b. aeolian sands, i.e. recently deposited sands as occur in deserts and beach land.
  • They are internationally known as:
    1. Psamments, Psammaquents : Soil Taxonomy, USA
    2. Red and yellow sands: Brazil
    3. Podsolic soils: Australia
    4. Classe des sols peu évolués: France
2. Parent material and environment

Parent material

  • Unconsolidated, in places calcareous, translocated sand
  • Relatively small areas of Arenosols occur on residual sandstone


  • Arenosols occur from arid to humid, from extremely cold to extremely hot regions  = azonal soils
  • Landforms vary from recent dunes, beach ridges and sandy plains under scattered (mostly grassy) vegetation to very old plateaus under light forest

    Fig.2 Arenosol landscape, developed in situ
    ( Source: FAO, 2001.)

    Fig.3 Arenosol landscape developed recently
    ( Source: FAO, 2001.)

    Fig.4 Arenosol (Senegal)
    ( Source: FAO, 2001.)

    Fig.5 Arenosol-landscape South Africa
    ( Source: ISRIC, NL.)

3. Regional Distribution

Fig.6 Arenosols worldwide
( Source: FAO, 2001.)

  • Arenosols are among the most extensive soils in the world covering about 900 Mio. ha or ~ 7 % of the land surface.
  • Vast expanses of deep aeolian sands are found on the central African plateau between the Equator and 30o southern latitude (Kalahari sands).
  • Other areas occur in the Sahelian region of Africa, in the Sahara desert, the Middle East, China and central and western Australia.
4. Definition
  1. A texture, which is loamy sand or coarser to:
    a. a depth of at least 100 cm from the soil surface, or
    b. To a plinthic, petroplinthic or salic horizon between 50 and 100 cm from the soil surface
  2. < 35 % rock fragments or other coarse fragments
  3. No other diagnostic horizons other than an ochric,

    Fig.7 In the dry zone, an ochric A horizon is the only diagnostic horizon.
    ( Source: FAO, 2001.)

    yermic or albic horizon,

    Fig.8 Albic Arenosol, South Africa
    ( Source: ISRIC, NL.)

    or a plinthic, petroplinthic or salic horizon below 50 cm from the soil surface  see LECTURE NOTES ON THE MAJOR SOILS OF THE WORLD

    Fig.9 Arenosols in the Sahelian region
    ( Source: FAO, 2001.)

5. Genesis
  • The development of Arenosols differs between arid and humid climates. Arenosols in the dry zone show minimal profile development because
    - Soil forming processes are at a standstill during long periods of drought
    - And/or because the parent material is of young age
  • Arenosols in the wet tropics are either formed in
    - Young sand deposits of alluvial, lacustrine or aeolian origin
    - Or represent the ultimate in soil formation, i.e. prolonged weathering of quartz-rich rocks (granite, sandstone and quartzite) and constitute the thick albic E horizon of a Giant Podzol.

a. Formation in the dry zone

  • Sandy parent material is abundant in areas where sand accumulates after selective transportation of the weathering material by wind [note: transportation by water is discussed under Fluvisols, set #4: Mineral Soils conditioned by Topography).
  • Aeolian (wind-borne) sands are deposited by wind action, either in dunes or in extensive sheets (cover sand areas)

    Fig.10 Podzol under sand
    ( Source: FAO, 2001.)

  • During transport, selection of particles occurs; the wind speed and the size, shape and density of the minerals determine how far a particular grain will be transported.
  • This sorting of grains results in deposits that consist of pure sand with a uniform particle size. Many aeolian sand deposits show characteristic large-scale cross bedding, indicative of sand deposition on the slip face of dunes

    Fig.11 Schematic dune structure
    ( Source: FAO, 2001.)

    Fig.12 Cross-bedding induced by aeolian sand deposits
    ( Source: FAO, 2001.)

  • Soil formation in such dunes or cover sand areas is minimal until the dune is colonized by vegetation (= fixed dune)

    Fig.13 Fixed dunes are formed when transported sand settles in the lee of an obstacle such as brush or a piece of rock. It grows in size and more sand settles, i.e. the dune grows.
    ( Source: FAO, 2001.)

  • Then, humus can accumulate in the surface soil and a shallow, ochric surface horizon can develop

    Fig.14 Minimal profile development (long dry periods, low humus content and vegetation), e.g.. aridic Arenosols contain 0.2 % organic carbon.
    ( Source: FAO, 2001.)

  • Sand grains of Arenosols often have coatings:
    1. Brownish through clay and carbonates
    2. Reddish through hematite ( ferrugination) -> relic feature of more humid climate in the past

      Fig.15 On grassland planted too late to Panicum maximum, the first May storms beat down on the poorly covered soil and separated the humus and clay from the coarse sand. Sheet runoff carried away the light topsoil particles (coloured dark grey), leaving behind sheets of red sand as evidence of eroded soil. ORSTOM Station, Adiopodoumé Côte d'Ivoire (5% slope).
      ( Source: t1765e0j.htm)

b. Formation in the wet tropics: residual sands

  • Residual sands are the result of prolonged weathering of quartz-rich rocks such as granite, sandstone and quartzite. Extensive, horizontal sandstone plateaux occur in tropical shield areas. Well-known examples are:
    a. Precambrian Roraima sandstone formations on the Guiana Shield
    b. Voltaian sandstone formations in Western Africa
  • These sandstone formations have a deep weathering mantle of bleached, white sands (= color of quartz) that are rich in quartz, poor in clay and excessively drained

    Fig.16 Albic Arenosol, North Brazil
    ( Source: Zech und Hintermaier-Erhard, 2002.)

  • a thick albic E horizon is formed, in places over a dark illuviation horizon at several meters depth, e.g. in Australia: E-horizon up to 13 m depth

    Fig.17 In the humid zone: strongly developed tropical Podzol with thick albic E eluviation horizon (200 cm) became excluded from the group of Podzols -> are albic Arenosols (or Giant Podzols).
    ( Source: FAO, 2001.)

  • If the underlying spodic horizon starts within 200 cm, the soil is classified as Podzol but where the spodic horizon (Bhs, Bs) starts deeper (beyond the taxonomic control section) the soil is classified as albic Arenosol.
6a. Morphological characteristics
  • In the humid zone:
    • If the spodic horizon (Bs, Bhs) starts below 200 cm -> albic Arenosols or Giant Podzol;
    • If the spodic horizon (eluvial horizon starts within 200 cm -> Podzol

Fig.18 Arenosols in the dry zone, i.e. recently deposited aeolian sands have an ochric A over massive C-horizon: A(E)C.
( Source: FAO, 2001.)

6b. Hydrological and physical characteristics
  • Arenosols have weak capillary transport due to prevailing coarse sand texture.
  • Depending on the grain size distribution and organic matter content, the available water storage may be as low as 3-4 %, in places it can reach up to 15 – 17 %.
  • High infiltration (varies between 2.5 and 25 cm/hour), and may be 250 times faster than in clay soils (0.01 – 0.1 cm/hour).
  • High ( bulk density) (BD; between 1.5 and 1.7) due to many large pores.
6c. Chemical characteristics
  • Most Arenosols in humid tropical regions are strongly leached soils with a low nutrient content and a very tight nutrient cycling between vegetation and surface soils (O-horizon and shallow A-horizon).
  • Arenosols in the dry regions are normally rich in bases. The organic carbon content of most surface horizons are normally less than 0.5 %.
7. Land use and management
  • Arenosols in arid lands, where the annual rainfall sum is less than 300 mm, are predominantly used for extensive grazing.

    Fig.19 Arenosols in Senegal (dry season)
    ( Source: FAO, 2001.)

    Fig.20 Arenosols in Senegal (rainy season)
    ( Source: FAO, 2001.)

    However, overgrazing is an enormous problem

    Fig.21 Uncontrolled grazing without soil conservation measures destroyed much of the vegetation in the Sahelian zone of the Senegal. Without vegetation, topsoil is eroded and the land becomes infertile.
    ( Source: Agency for International Development)

  • Dry farming is possible where the annual rainfall sum exceeds 300 mm (300 to 600 mm).

    Fig.22 Onions on coastal Arenosols
    ( Source: FAO, 2001.)

    Fig.23 Irrigation
    ( Source: FAO, 2001.)

    Fig.24 Growth is surprisingly good thanks to a regular supply of rabbit manure.
    ( Source: FAO, 2001.)

    Fig.25 Rabbit manure not only acts as a fertiliser but improve also the water holding capacity of the soil.
    ( Source: FAO, 2001.)

    Fig.26 Groundnut can be cultivated in Arenosols
    ( Source: FAO, 2001.)

    Fig.27 Land use of Arenosols: intercropping
    ( Source: FAO, 2001.)

    Fig.28 Cash Crops
    ( Source: FAO, 2001.)

    Fig.29 Sugar cane on irrigatet Haplic Arenosols in Natal, South-Africa.
    ( Source: FAO, 2001.)

    Soil conservation is a prerequisite for sustainable management

    Fig.30 Erosion control: barriers against shifting sands
    ( Source: FAO, 2001.)

  • Albic Arenosols are best left under their natural vegetation. As nutrient elements are all concentrated in the biomass and in the top 20 cm of the soil, removal of the vegetation results in infertile  badlands without ecological or economic value. Permanent cultivation of annual crops would require management inputs that are usually not economically justifiable.

    Fig.31 Albic Arenosol, South Africa.
    ( Source: ISRIC, NL.)