Aridisoils
A hot desert soil
Soil Type= Zonal
 Soil develops in response to the desert climate.
 Less than 250 mm of water a year.
 Diurnal temperature range. High daytime 45 degrees
Celsius. Night-time 0 degrees Celsius.
 Humidity is low.
 Found between 15 degree and 40 degrees North and South
of the equator.
 Found in North Africa (The Sahel), California and Australia.
 Aridisols are rich in calcium carbonate.
 Useless for agriculture unless Irrigation is available.
 Occupy 12% of the earths land area.
Soil Profile
Soil Profile
 Develop surface pebble layers called desert
pavements.
 Contain horizons in which clay, calcium
carbonate,silca, salts or gypsum.
 A horizon is light in colour as little vegetation
to add organic matter(humus) to the soil
profile.
 Arid soils contain whitish layers called calcic
horizons. They are a accumulations of calcium
carbonate, concrete and agricultural lime. They
form due to the process of calcification.
 They form an impermeable cement like layer of
hardpan known as calcite.
Factors affecting aridisols / Soil forming
factors
1.Climate
2.Relief
3.Living Things/vegetation
4.Parent material
5.Time
Climate
 Aridisols form in arid and semi arid regions.
 Annual rainfall less than 250 mm.
 Generally rain falls in short violent showers.
 Sun is high overhead and cloudless producing
temperature ranges during the day between 20 to 40
degrees Celsius. Dropping to well below freezing at night.
 This climate causes mechanical weathering especially
exfoliation ( onion weathering) creating angular scree
particles for soil formation.
 The lack of rainfall prevents chemical weathering from
occurring.
 Strong winds and sand storms can often remove fine
sand particles leaving coarser and heavier soil particles
behind.
Relief
 Relief has a major influence on aridisols.
 Valleys provide pathways for flash floods which
remove soils in the valley floors.
 Water flowing off mountainsides deposits mud, sand
and gravel( sediment) at the base of the slopes in
huge fan shaped deposits called alluvial fans.
 These fans sediment are an important parent
material(rock) for aridisols and influence the texture
and mineral content in soils nearby.
Living things/Vegetation
 Vegetation is scarce or absent
 Plants that thrive are ground hugging shrubs and short
woody trees. ( Yucca, Cactus and Mesquite shrubs. They
have adapted with the ability to store water.)
 There is rapid growth is vegetation after the torrential
downpours of rain.
 Plants and animals are closely linked with many animals and
insects using the pants as shelter and food.
 However the scarce vegetation cover is limited and restricts
soil building properties of micro organisms which could
convert organic matter into humus.
 Therefore Aridisols are mineral rich but lack humus.
Parent material and Time
 Influences the colour of the soil.
 They occur on parent material such as
crystalline rocks.
 Aridisols occur on gypsiferous material formed
from marine sedimentary rocks, on
unconsolidated sediments, or limestone.
 Desert soils that have developed on older
parent material are redder in colour than soils
developed on younger materials which are often
pale grey.
Characteristics of Aridisols
1. Colour
2. PH
3. Humus Content
4. Structure
5. Texture
6. Water Content
Colour
 Some Aridisols have the same pale
brownish colour.
 Others may be layered with browns, reds,
pinks and whites.
 The variation in colour is due to living
organisms, salinisation, weathering and
parent material.
PH
 Aridisols have a high calcium carbonate
and sodium concentrations making them
alkaline( very acidic).
Humus content
 Contain little organic matter, therefore little
humus.
 This is caused by a lack of plants which in
turn affects the soil building properties of
micro organisms that would normally convert
organic matter into humus
Structure
 Have a blocky structure.
 May also be a platy structure where the
clay content of the soil is higher.
Texture
 Have a coarse sandy or gravelly texture
because there is less chemical weathering.
 Coarse sandy soils have large pores so
they allow water to pass through (little
retention).
 These coarse sand and dust is found on
lower mountain slopes.
 Finer dust and sand particles are blown to
lower basin areas accumulating and
creating a deep well drained soil cover.
Water content
 Low water content, these soils are very
dry most of the year. This leads to very
little leaching.
 Low water content also is related to the
low humus content because of sparse
organic matter.
 Soils without a well developed litter layer
find it hard to retain water that falls on
them.
Process affecting aridisols
1.Calcification.
2.Salinisation
Calcification
This process involves the build up of calcium
carbonate near the surface of the soil. This can
happen when there is little rainfall in an area and
plants pull up water (through transpiration). There is
then an unequal amount of Calcium Carbonate found
naturally in the soil.
Salinisation
In hot areas rapid evaporation causes salts in the ground
to be pulled up through the soil towards the surface. Salt
on the surface builds up in a white crusty layer. Too
much salt near the surface causes plants (and animals) to
die. Too much salt is extremely difficult and expensive
for farmers to deal with. Salinisation is the build up of
salt near the surface.