SOIL COMPOSITION
Inorganic and organic
components of soil
Abdulhaq Mehran
Roll No 1938
M.Sc. second year
Environmental science
What is soil?
Soil is the thin layer of material covering the earth's surface
and is formed from the weathering of rocks.
Soil is one of the most important elements of an ecosystem,
and it contains both biotic and abiotic factors. The
composition of abiotic factors is particularly important as it
can impact the biotic factors, such as what kinds of plants
can grow in an ecosystem.
Soil components
o Organic Matter
o Minerals
o Water
o Air
1. Soil organic component
Soil organic matter is any
material produced originally
by living organisms (plant or
animal) that is returned to
the soil and goes through
the decomposition process.
What’s the difference
between organic material
and organic matter?
SOM composition
Forms and classification of soil organic matter have been
described by Tate (1987) and Theng (1987).
Soil organic matter (SOM) is the organic component of soil,
consisting of three primary parts including:
small (fresh) plant residues and small living soil organisms
Active organic matter (decomposing)
stable organic matter (humus)
Soil organic matter serves as a reservoir of nutrients for
crops, provides soil aggregation, increases nutrient
exchange, retains moisture, reduces compaction, reduces
surface crusting, and increases water infiltration into soil.
Cont..
Fresh Plant Residues – Refers to plant residue, animal, or
other organic substances that have recently been added to
the soil and have only begun to show signs of decay. Does
not include surface residue cover.
Small Living Organisms or Soil Microorganisms –
Bacteria, fungi, nematodes, protozoa, arthropods, etc.
Active Organic Matter – Microorganisms and other
organic compounds are used as food by microorganisms.
Active soil organic matter decomposes faster than other
components of soil organic matter in response to
management changes.
Cont.
Humus or Stable Organic
Matter – product of microbial
decomposition, dark colored,
dynamic, substances composed
of residual organic matters not
readily decomposed by soil
microorganisms.
The process of humus
formation is called humification
El- carbon, hydrogen, Sulphur
and N
Degradation stages of
humus
The humus may be found in the following three stages of
degradation:
1. LITTER- the top floor is covered with dead organic parts
showing low degree of decomposition. These poorly
decayed dead parts of plants form litter.
2. DUFF- below the litter may be found a layer of partially
decomposed organic matter which is known as duff layer.
3. LEAF MOULD / HUMUS- when the duff is decomposed
completely into organic substances the decomposition
products generally called leaf moulds.
Role of humus in soil
It make the soil fertile.
It provide nutrients to the plants and microorganisms.
Because it is porous , it has got high capacity for retaining
water.
Humus make the soil porous, thus increase the aeration and
percolation which make the soil more suitable for plant
growth.
It also acts as weak cement thus binds the sand particles.
Presence of humus in the soil increase the rate of absorption
in plants.
The factors which influence the rate of humification are:
Nature of plants, animal or soil organism.
Rate of decomposition.
Temperature (increase in temperature up to a certain limit
increases the rate of humification).
Aeration and moisture, these increase the rate of
humification.
Soil microorganisms
Organism present in the soils are called soil organism.
Important group of soil organism are :
Flora: algae, bacteria (90% of total flora). Fungi , actinomycetes.
Bacteria: neutral soil
Fungi: Acidic soil
Actinomycetes: saline soil
Fauna: protozoa, Nematodes, mites, Earthworm etc.
Role of soil organisms
1. Decompose the dead organic matter and increase
plant nutrients in available forms.
2. Production of toxins eg fusaric acid.
3. Nitrogen fixation in the soil.
4. Mixing of soil.
5. Improvement in soil aeration.
6. Improvement in the aggregation of soil particles.
Factors Affecting Soil
Organic Matter
Inherent factors affecting soil organic matter such as
• climate
• soil texture
• Topography. ...
• Salinity and acidity. ...
• Vegetation and biomass production.
Soil Organic Matter Function
Nutrient Supply: Upon decomposition, nutrients are
released in a plant-available form. While maintaining current
levels.
Each percent of SOM in the top 6 inches of a medium
textured soil releases about 10-20 pounds of nitrogen, 1 to 2
pounds of phosphorus, and 0.4 to 0.8 pounds of sulfur per
acre per year.
Water-Holding Capacity:
It has the ability to absorb and hold up to 90 percent of its
weight in water. clay holds great quantities of water, but
much of it is unavailable to plants.
Cont.
Soil Aggregation:
Organic matter improves soil aggregation, which improves soil
structure.
Erosion Prevention:
Because of increased water infiltration and stable soil
aggregates erosion is reduced with increased organic matter.
2. Soil inorganic component (soil Minerals)
Soil minerals play a vital role in soil fertility since mineral surfaces
serve as potential sites for nutrient storage. However, different
types of soil minerals hold and retain differing amounts of
nutrients.
The most common minerals found in soil are;
phosphorus, and potassium and also, nitrogen gas. Other, less
common minerals include calcium, magnesium, and sulfur.
1. Sand
2. Silt
3. Clay
Sand
Sand particles range in size from 0.05 millimeters to 2.0
millimeters and consist of pieces of weathered rock, such as
quartz, magnetite or other mineral or shell fragments.
Silt
Silt is larger than clay and smaller than sand and may feel
gritty between your fingers. Silt particles range from 0.002
millimeters to 0.05 millimeters. Like sand, silt results from
the weathering of rocks and consists of grains of quartz,
feldspar and other minerals. Silt-rich soil will retain moisture
for plant growth, but at times, it can limit drainage and
make the soil difficult to till. Silt itself does not contain plant
nutrients; however, existing nutrients in the area may stick
to the surface of the silt particles in the garden soil.
Clay
Clay is another fine-grained sediment that is smaller than
silt. Clay particles are smaller than 0.002 millimeters and
result from significant weathering of rocks. Minerals, such
as feldspars, degrade over time to form clay. Unlike sand
and silt, the aluminum-silicate minerals that degrade to clay
will provide important plant nutrients such as iron, calcium,
magnesium and potassium. Existing nutrients will cling to
the clay soil particles and help to feed plant growth in the
root area.
Soil texture
Soil texture refers to the proportion of sand, silt and clay
sized particles that make up the mineral fraction of the soil.
For example, light soil refers to a soil high in sand relative to
clay, while heavy soils are made up largely of clay.
Texture is important because it influences:
the amount of water the soil can hold.
the rate of water movement through the soil.
how workable and fertile the soil is.
How to determine soil texture
The bottle test (jar Method)
Step-1
Put 5 cm of soil in a bottle and fill
it with water;
Step-2
Stir the water and soil well, put the bottle down, and do not
touch it for an hour. At the end of an hour, the water will
have cleared and you will see that
the larger particles have settled;
Step-3
• At the bottom is a layer of sand;
• In the middle is a layer of silt;
• On the top is a layer of clay. If the water is still not clear, it is
because some of the finest clay is still mixed with the water;
• On the surface of the water there may be bits of organic
matter floating;
• Measure the depth of the sand, silt and clay and estimate the
approximate proportion of each.
based on its texture –
sand, silt and clay.
However, the
percentage of these can
vary, resulting in more
compound types of soil
such as loamy sand,
sandy clay, silty clay, etc
3. Water
Water is another component of soil. Water can make up
approximately 2% to 50% of the soil volume. Water is
important for transporting nutrients to growing plants and soil
organisms and for facilitating both biological and chemical
decomposition. Soil water availability is the capacity of a
particular soil to hold water that is available for plant use.
The capacity of a soil to hold water is largely dependent on
soil texture. The more small particles in soils, the more water
the soil can retain. Thus, clay soils having the greatest water-
holding capacity and sands the least. Additionally, organic
matter also influences the water-holding capacity of soils
because of organic matter’s high affinity for water. The higher
the percentage of organic material in soil, the higher the soil’s
water-holding capacity.
Water is held in the soil in the
following forms
I. Gravitation water
II. Capillary water
III. Hygroscopic water
IV. Water vapour
V. Combined water
4. Air(soil Atmosphere)
air is the next basic component of soil. Because air can
occupy the same spaces as water, it can make up
approximately 2% to 50% of the soil volume. Oxygen is
essential for root and microbe respiration, which helps
support plant growth. Carbon dioxide and nitrogen also are
important for belowground plant functions such as for
nitrogen-fixing bacteria.
The amount of carbon di oxide increase with the increase in
depth of the soil due to decomposition of accumulated
organic matter and abundance of plant roots.
References
Thank you
