CHAPTER 12
SOILS AND SOIL DEVELOPMENT
INTRODUCTION
Soils
are indispensable resources, without which life could not exist.
Soil
is a dynamic resource capable of supporting a vegetative cover.
Soil act as a buffer zone where the lithosphere,
atmosphere, hydrosphere and biosphere can interact.
PRINCIPLE SOIL
COMPONENTS
Soil
is made up of both insoluble mineral material and soluble chemicals in
solution.
The
most common minerals found in soils are combinations of the most common
elements of the earths crust, namely silicon, aluminum, oxygen, and iron.
INORGANIC
MATERIALS
These elements
and chemicals that make up soil come from many sources.
Some are
derived from the weathering of the underlying rocks or sediment.
Some are added
or moved by chemicals in solution.
Other parts
come from the air or are derived from organic activity, some of which helps
disintegrate rocks.
SOIL WATER
Precipitation
brings water to the soil. When rain hits the surface, some of it runs off and
some gets absorbed into the ground.
The water that
percolates down is pulled by gravity and is called gravitational water.
GRAVITATIONAL
WATER
Gravitational
water also moves finer grained particles down through the soil.
This
removal of soil components from the topsoil is called eluviation.
The
particles then are deposited at a lower depth through illuviation.
ELUVIATION AND
ILLUVIATION
By these
processes, the upper soil becomes coarser (eluviation) over time and less able
to hold water.
In some cases,
the subsoil gets very dense (illuviation) and a clay hardpan is formed.
LEACHING
Gravitational
water affects the chemical composition of a soil moving the soluble chemicals
(nutrients) deeper into the soil profile. This depletion is called leaching.
Leaching,
eluviation, and illuviation are the causes of the characteristic stratification
of soils.
CAPILLARY
WATER
Some
soil water is held to the surface of the individual soil particles by surface
tension and this is called capillary water.
Capillary
water can move in any direction in the soil profile from wetter to dry areas.
Storage
during dry periods
Capillary
water also moves chemicals in solution. Lime deposits can form and produce a
cement-like layer called caliche.
SOIL AIR
A
large part of soil (up to 50%) is made up of the voids between soil particles.
When not filled with water, these spaces are filled with air.
Soil
air supplies the oxygen and carbon dioxide necessary for plants.
Worms
ORGANIC MATTER
Soils
also contain organic matter.
The
decayed remains of plant and animal material, partially transformed by
bacterial action, is called humus.
Humus is important:
Catalyst
in the extraction of nutrients
Improves
soil structure
Holds
water
Source
of food microscopic organisms
CHARACTERISTICS
OF SOIL
The
characteristics of soil include:
Color
Amount
of Humus
Texture
Structure
Acidity/Alkalinity
COLOR
The color of
soil is indicative of the type of minerals in the soil and the amount of humus.
Red
and Yellow
Light
Gray
Black
TEXTURE
Soil
texture varies according to the size of the particles- clay silt sand -
gravel.
Texture
is important in the ability of soil to hold water and air.
Loam soils are those with textures that are a mixture of
silt, clay and sand which are ideal for agriculture.
STRUCTURE
Structure
and texture influence porosity and permeability.
Porosity is a measure of how much space in a soil that can contain
water.
Permeability is a measure of the rate at which fluids pass
through the soil.
ACIDITY AND
ALKALINITY
The
acidity of alkalinity of a soil is measured on a scale of 0 to 14, called the pH
scale.
This
is a measure of the concentration of highly reactive hydrogen ions present in
the soil moisture.
A little
acidity is good because the nutrients can be dissolved and are available to the
plant.
Too acidic and
all the nutrients get leached out
FACTORS
AFFECTING SOIL FORMATION
There are
several factors that affect the development of soils including:
Climate
Parent Material
Organic Activity
Topography
Time
CLIMATE
Climate
is usually the dominant factor in soil formation.
Precipitation
Temperature
Humus
PARENT
MATERIAL
Soil
is derived form the weathered fragments of rock called parent material.
Residual
Parent Material
Transported
Parent Material
ORGANIC
ACTIVITY
Plants
and animals affect soil formation.
Tree
roots and earthworms make the soil looser, and more porous.
Vegetative cover.
Plant Life
Available Humus
Humus Soil
Fertility
NUTRIENT
CYCLES
Nutrient
cycles are important to plant growth.
Mid-Latitude
Climates
Tropical
Climates
Dry
Climates
TIME
The
younger the soil, the more the soil will be influenced by the parent
material.
The older the soil than climate will have more
to do with the soils characteristics.
Generally, it
takes hundreds to thousands of years for a soil to reach maturity.
TIME
Transported
parent materials have not had time to develop into leached out soils.
River
Sediments (Nile)
Glaciated
Outwash
Wind
Blown Material (Loess)
Volcanic
Activity
Some
of the best soil for agriculture is on these surfaces.
SOIL PROFILE
DEVELOPMENT
Over
time, soils develop vertical, distinct layering called horizons.
These
layers or horizons are distinguished by their different physical and
chemical properties.
SOIL FORMING
REGIMES
At
the broadest scale of generalization, there are three primary soil-forming
regimes that relate to climatic differences including:
Laterization
Podzolization
Calcification.
LATERIZATION
Laterization is a soil-forming
process that occurs in humid, tropical and subtropical climates.
Little
to no humus
A
horizon leached out except Fe and Al.
Topsoil
is reddish, coarse and porous.
The
B horizon has a heavy concentration of illuviated
materials
PODZOLIZATION
Podzolization occurs in its purest form in the high middle
latitudes moist, cool summers long, severe winters.
Associated with coniferous forests.
Abundant
Humus
Acidic
(Iron and Aluminum Leached out)
Pine
Barrens (NJ)
CALCIFICATION
Calcification occurs in dry climates where evaportranspiration exceeds precipitation.
Alkali
Dusts (Caliche)
Grasslands
(Abundant Humus)
Deserts
(Very little humus)
Not
Leached Out