SOILS AND SOIL DEVELOPMENT
are indispensable resources, without which life could not exist.
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.
is made up of both insoluble mineral material and soluble chemicals in
most common minerals found in soils are combinations of the most common
elements of the earths crust, namely silicon, aluminum, oxygen, and iron.
and chemicals that make up soil come from many sources.
derived from the weathering of the underlying rocks or sediment.
Some are added
or moved by chemicals in solution.
come from the air or are derived from organic activity, some of which helps
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.
water also moves finer grained particles down through the soil.
removal of soil components from the topsoil is called eluviation.
particles then are deposited at a lower depth through illuviation.
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.
water affects the chemical composition of a soil moving the soluble chemicals
(nutrients) deeper into the soil profile. This depletion is called leaching.
eluviation, and illuviation are the causes of the characteristic stratification
soil water is held to the surface of the individual soil particles by surface
tension and this is called capillary water.
water can move in any direction in the soil profile from wetter to dry areas.
during dry periods
water also moves chemicals in solution. Lime deposits can form and produce a
cement-like layer called caliche.
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.
air supplies the oxygen and carbon dioxide necessary for plants.
also contain organic matter.
decayed remains of plant and animal material, partially transformed by
bacterial action, is called humus.
Humus is important:
in the extraction of nutrients
of food microscopic organisms
characteristics of soil include:
The color of
soil is indicative of the type of minerals in the soil and the amount of humus.
texture varies according to the size of the particles- clay silt sand -
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.
and texture influence porosity and permeability.
Porosity is a measure of how much space in a soil that can contain
Permeability is a measure of the rate at which fluids pass
through the soil.
acidity of alkalinity of a soil is measured on a scale of 0 to 14, called the pH
is a measure of the concentration of highly reactive hydrogen ions present in
the soil moisture.
acidity is good because the nutrients can be dissolved and are available to the
Too acidic and
all the nutrients get leached out
AFFECTING SOIL FORMATION
several factors that affect the development of soils including:
is usually the dominant factor in soil formation.
is derived form the weathered fragments of rock called parent material.
and animals affect soil formation.
roots and earthworms make the soil looser, and more porous.
cycles are important to plant growth.
younger the soil, the more the soil will be influenced by the parent
The older the soil than climate will have more
to do with the soils characteristics.
takes hundreds to thousands of years for a soil to reach maturity.
parent materials have not had time to develop into leached out soils.
Blown Material (Loess)
of the best soil for agriculture is on these surfaces.
time, soils develop vertical, distinct layering called horizons.
layers or horizons are distinguished by their different physical and
the broadest scale of generalization, there are three primary soil-forming
regimes that relate to climatic differences including:
Laterization is a soil-forming
process that occurs in humid, tropical and subtropical climates.
to no humus
horizon leached out except Fe and Al.
is reddish, coarse and porous.
B horizon has a heavy concentration of illuviated
Podzolization occurs in its purest form in the high middle
latitudes moist, cool summers long, severe winters.
Associated with coniferous forests.
(Iron and Aluminum Leached out)
Calcification occurs in dry climates where evaportranspiration exceeds precipitation.
(Very little humus)