Soil !!! some call it dirt. But it is..... Soil !!!
The essence of our being.
The medium in which crops are grown to feed humans, as well as animals. One
of our natural resources (soil, water, air, plant, animal).
Soil by volume, on the average consists of 45% mineral, 25% water, 25% air
and 5% organic matter (both living and dead organisms).
There are thousands of different soils throughout the world.
Freezing and Thawing
Heating and Cooling
Wetting and Drying
Grinding or Rubbing
Under the influence of physical
factors like deformation by heat and
cold, assault by wind, rain, hail and
ice, and the enormous levering
forces of water expanding into ice,
solid rock is shattered into smaller
pieces (see picture).
But however small these fragments,
they still have the same properties
as the parent rock.
Being formed under high pressure
and temperatures, the crystals of
the minerals in the rock are
somewhat unstable at surface
pressure and temperature.
Particularly when attacked by acids
that etch away the soluble
components in the minerals, the
crystals fall apart, albeit very slowly.
It is called spontaneous weathering,
but it is accelerated considerably
under the influence of vegetation
and its acids (chemical weathering).
SOIL FORMING PROCESSES
There are five key processes by which soil is formed, they are:
Leaching - leaching is the removal of soluble components of the soil column. As water
washes down through the soil it can carry away bases such as calcium, held as
exchangeable ions in clay-humus complexes, as well as acidification through the
substitution of hydrogen ions.
Eluviation - here soil particles held in suspension, such as clay, are removed (eg.
Illuviation - here soil particles held in suspension, such as clay, are accumulated (eg.
Podsolisation - podsolisation occurs when strongly acid soil solutions cause the
breakdown of clay minerals. As a result silica, aluminium and iron form complexes with
organic substances in the soil. These minerals are removed from the surface zone of
the soil and can accumulate in distinct dark sub-surface layers - very evident on
inspection. Upland heaths and moors often contain podsols.
Gleying - gleying occurs in waterlogged, anaerobic conditions when iron compounds
are reduced and either removed from the soil, or segregated out as mottles or
concretions in the soil. Marshy wetlands often contain gleyed soils.
It is important to realise also that soil types are closely related to the shape of the
landscape - or its 'topography'. Soil scientists use this to help them create soil maps.
An experienced eye can determine changes in underlying soil types when walking
through a landscape and observing changes in topography (and often vegetation too).
Factors in soil formation:
The material in which soils form is called "parent material." In the lower part of
the soils, these materials may be relatively unchanged from when they were
deposited by moving water, ice, or wind.
Few soils weather directly from the underlying rocks. These "residual" soils have
the same general chemistry as the original rocks.
More commonly, soils form in materials that have moved in from elsewhere.
Materials may have moved many miles or only a few feet.
Glacial till is material ground up and moved by a glacier. Sediments along rivers
have different textures, depending on whether the stream moves quickly or
slowly. Fast-moving water leaves gravel, rocks, and sand. Slow-moving water
and lakes leave fine textured material (clay and silt) when sediments in the water
Windblown "loess" is common in the Midwest. It buries "glacial till" in many
Soils vary, depending on the climate.
Temperature and moisture amounts cause different patterns of
weathering and leaching.
Wind redistributes sand and other particles especially in arid regions.
The amount, intensity, timing, and kind of precipitation influence soil
Seasonal and daily changes in temperature affect moisture
effectiveness, biological activity, rates of chemical reactions, and kinds
Slope and aspect affect the moisture and temperature of soil.
Steep slopes facing the sun are warmer, just like the south-facing side
of a house. Steep soils may be eroded and lose their topsoil as they
Thus, they may be thinner than the more nearly level soils that receive
deposits from areas upslope.
Deeper, darker colored soils may be expected on the bottom land.
Plants, animals, micro-organisms, and humans affect soil formation.
Animals and micro-organisms (such as moles, earthworms, bacteria, fungi and
nematodes) mix soils and form burrows and pores.
Plant roots open channels in the soils. Different types of roots have different
effects on soils. Grass roots are "fibrous" near the soil surface and easily
decompose, adding organic matter. Taproots open pathways through dense
Micro-organisms affect chemical exchanges between roots and soil. Humans
can mix the soil so extensively that the soil material is again considered parent
The native vegetation depends on climate, topography, and biological factors
plus many soil factors such as soil density, depth, chemistry, temperature, and
Leaves from plants fall to the surface and decompose on the soil.
Organisms decompose these leaves and mix them with the upper part of the
Trees and shrubs have large roots that may grow to considerable depths.
Time for all these factors to interact with the soil is also a factor.
Over time, soils exhibit features that reflect the other forming factors. Soil
formation processes are continuous.
Recently deposited material, such as the deposition from a flood, exhibits no
features from soil development activities.
The previous soil surface and underlying horizons become buried. The time
clock resets for these soils.
Terraces above the active floodplain, while genetically similar to the floodplain,
are older land surfaces and exhibit more development features.
Soil taxonomy at the highest hierarchical level identifies 12 soil orders.
The names for the orders and taxonomic soil properties relate to Greek,
Latin, or other root words that reveal something about the soil.
Sixty-four suborders are recognized at the next level of classification.
There are about 300 great groups and more than 2,400 subgroups.
Soils within a subgroup that have similar physical and chemical
properties that affect their responses to management and manipulation
The soil series is the lowest category in the soil classification system.
During the weathering
minerals in solution (cations
and anions), the basis of
oxides of iron and alumina
various forms of silica (siliconoxide compounds).
stable wastes as very fine silt
(mostly fine quartz) and
coarser quartz (sand).
These have no nutritious
value for plants.
Depending on temperature and rainfall,
new minerals are formed. The oxides
of iron and alumina combine with silica
to form clay. In temperate regions a
three-layer clay is formed, which is
weak, swells under moisture, and
clogs. It is able to absorb large
amounts of water but is rather heavy
on plant roots, blocking the oxygen the
soil organisms need. Because clay has
a charged surface area, it is able to
bind and retain minerals and nutrients
(Cation Exchange Capacity). The
valuable nutrition for plants won't leach
away easily in three-layer clays.
Two-layer clays are formed in hot,
humid tropical regions, producing
arable but easily dried soils. These
clays are not able to hold much water,
or nutrients, but are still very much
better than sand.