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Soil
What is Soil?
Components of Soil
Soil has five components
1. Mineral particles
2. Organic matter
3. Water
4. Air
5. Living organisms
1. Mineral Particles
There are five types of mineral particle in soil and
these are defined by their size.
Gravel greater than 2mm in size
Coarse Sand between 2mm and 0.2mm
Fine Sand between 0.2mm and 0.02mm
Silt between 0.02mm and 0.002mm
Clay less than 0. 0.002mm
1. Mineral Particles, soil texture
The proportion of each type of mineral particle
makes up the soil texture.
A soil texture diagram can be used to calculate
the type of soil on the basis of the proportion of
sand silt and clay
1. Mineral Particles, texture
silt
clay
sand
A soil is composed of 30% clay 20% silt and 50% sand =
Sandy,clay, loam
1. Mineral Particles, clay
The most important mineral
particle in soil is clay.
Clay particles are very
small, less than 0.002 mm,
and, usually are flattened
They have an enormous
surface area. 1gm of clay
can have a surface area
between 5-250 metre2
1. Mineral Particles, clay
The clay particles are charged, with many negative and some
positive charges on their surface, and this gives them many
important properties
1. Mineral Particles, clay
Clay particles are colloidal, that is, if placed in water,
they form weak hydrogen bonds with water
molecules which causes them to stay in suspension.
1. Mineral Particles, clay
Cations bonded to clay e.g. positively
charged ions of magnesium and calcium
The clay particles form part of the cation exchange capacity of soil.
The clay particles bind to positively charged ions (cations) of
nutrients such as magnesium and calcium. If a soil lacks clay it
would inevitably be less fertile since it will lack the capacity to hold
many nutrients in the soil.
1. Mineral Particles, clay and nutrients
Clay particles have few positive charges so that negatively charged
nutrient ions such as nitrates
are much more easily leached
from soils.
2. Organic Matter
The majority of soils contain large amounts of dead organic
matter, DOM. This comes mostly from the remains of plants,
leaves, roots etc, These plant remains are broken down by
decomposers and saprobionts into humus; the process is called
humification. Humus is made of dead organic matter but it does
not contain recognizable plant remains. The DOM and the humus
are important in soil for several reasons.
Plants
Dead
organic
matter
Decomposers/
saprobionts
humification
humus
carbon dioxide
Decomposers/
saprobionts
mineralisation
minerals
carbon dioxide
2. Organic Matter, water
Water forms hydrogen bonds with the dead organic
matter and the humus in soil. If a soil lacks dead
organic matter it is less likely to be able to retain water.
In a sand dunes as succession proceeds the organic
content of the soil increases and as it does so, the ability
of the sandy soil to hold water increases.
2. Organic Matter, structure
Soil structure is the way in which the mineral skeleton and
the organic matter in the soil are aggregated together. There
are a number of different types of soil structure or peds.
2. Organic Matter, structure, peds.
Ped
Diagram
Description
Crumb
Typical of rich topsoils under grass or deciduous
woodland. Soil organisms thoroughly mix DOM with
the mineral soil. Well aerated and with good water and
nutrient retention, it is an ideal soil for plant growth.
Blocky
Angular blocky structure is typical of deeper, clay-rich
subsoils and develops as the soil cracks during dry
seasons. In wet seasons the clay swells and the cracks
close up again, hence this structure is only slowly
permeable leading to poor drainage conditions.
Platy
Platy structure is not normally found in natural soils
but develops as a consequence of compaction caused
by inappropriate use of the soil (normally in wet
conditions).
2. Organic Matter, nutrients
Like the clay particles the dead organic matter forms part of the
cation exchange capacity of soil. The D.O.M binds to positively
charged ions, cations, of nutrients such as magnesium and calcium
and retains them in the soil.
2. Organic Matter, Summary
Soil
Structure
The organic matter interacts with the mineral particles and the
living organisms to produce the soil structure, ped.
Nutrients
The organic matter and the clay form the cation exchange
capacity of soil. Without clay and organic matter the ability of
the soil to hold nutrients will be reduced.
Water
capacity
Organic matter forms hydrogen bonds with water molecules and
increases the ability of the soil to retain water.
3. Water, gravitational water
A good soil needs to contain water. The water in soil can be divided
up into a number of different categories.
Precipitation
Precipitation
Precipitation
Soil surface
Gravitational water
moves through the soil
by gravity until it
reaches the water table.
As it does so,
it causes the
Translocation of
materials through
the soil.
Water table
3. Water, hygroscopic water
Hygroscopic water: this water is tightly joined (by hydrogen
bonds) to the particles in soil particularly clay and DOM
Because the hygroscopic
water is so strongly
attached to the clay and
DOM it is difficult for
plant roots to use this water
3. Water, capillary water
Capillary water: water molecules can form hydrogen bonds with each
other. This bonding creates surface tension which will hold the water
in the soil
Hydrogen bonds
Because the capillary water
is not strongly attached to
the soil it is easy for plant
roots to absorb.
hygroscopic water
3. Water, drainage
Feature
Good Drainage
Poor Drainage
Pore
Space
*
Sandy soils have large pore
spaces and are well drained
Clay soils have small pore
spaces an are poorly drained.
Humus
content
The presence of humus in clay
soils improves drainage
The presence of humus in sandy
soils reduces drainage
Structure
Soils with a crumb ped tend
to be well drained as are
structurless sandy soils
Clay Soils with a blocky
or platy structure tend to be
poorly drained
Bedrock
Permeability
Rocks such as sandstone,
limestone and chalk are
highly permeable
Rocks such as granite are
impermeable
* Please notice the difference between porosity and permeability
3. Water, soil thermal capacity
Feature
Effect
Absorption of
Solar radiation
The amount of light absorbed will be related to latitude,
aspect, albedo and vegetation cover.
Water
content
Water has a high specific heat, that is it takes a lot of
energy to raise its temperature, the higher the water
content of the soil the more sunlight it will take to heat it
Exposure
In areas exposed to high winds the evaporation of water
from the soil will lower the soils temperature
Respiration by
microorganism
The decomposition of dead organic matter in soil by
respiration releases heat as a waste product.
4. Soil Air, Aeration
Aeration:
A function of pore space determine by particle size
4. Soil Air, Aerobic conditions
A good soil will contain air. The oxygen in the soil air can have
a variety of different functions:
It is used by the roots of plants in aerobic respiration. This provides
them with energy for the active uptake of nutrients and growth.
It used by decomposers and saprobionts in aerobic respiration
speeding the rate of decomposition of dead organic matter and thus
the recycling of nutrients.
In the nitrogen cycle the presence of oxygen speeds up decomposition
and nitrification by bacteria and fungi.
DOM decomposition Ammonia nitrification Nitrites nitrification Nitrates
4. Soil Air
Anaerobic conditions
Waterlogged soils such as those found in the peat soils of
Sphagnum Bog and Fen contain very little oxygen. The
consequence of this is that the breakdown of dead organic
matter is slow since few decomposers, such as earthworms,
can survive in the anaerobic conditions. Decomposition by
saprobionts such as fungi and bacteria is also slow in the
absence of oxygen, since anaerobic respiration is less efficient
In Anaerobic conditions denitrifying bacteria convert of nitrates
to Atmospheric nitrogen.
5. Soil Organisms
Soil is to a large extent formed and created by the living
organisms the live in it and on it.
Organisms
number per teaspoonful weight (kg) per hectare
Bacteria
90,000,000
1,000-2,000
Fungi
200,000
2,000-5,000
Algae
30,000
~500
Protozoa
5,000
100-500
Nematodes
30
~200
Earthworms
<1
0-2,000
Other soil animals
<1
0-500
~ 91 million
~ 6,000
Total
Which group of organisms is missing from this table?
Plants
5. Living organisms
Plants
Pioneer species begin the process of soil formation, binding the soil
particles together and reducing erosion. The plants add dead organic
matter which improves structure, nutrient content and water holding
capacity. As a plant succession proceeds so the soil quality improves
allowing other species to come in.
Fungi
Fungal mycelia are found throughout soil. Fungal hyphae release
enzymes which break down dead organic matter allowing the nutrients
to pass to plants. Many plants form close associations with fungi,
called mycorrhiza. The plants give the fungi carbohydrates in return
for nutrients
Animals
There are many types of animals found in soil. Earthworms are
important decomposers they eat dead leaves and break them down in
digestion, thus they begin the process of humification, helping to
recycle nutrients to plants. The passage of their bodies through the soil
creates channels which develops aeration, drainage and the formation
of a good crumb ped.
5. Living organisms
Bacteria
Bacteria are found in large numbers in soil. One metre2 of soil can
contain 1011 bacteria and typically at least 5000 species. Bacteria are
important saprobionts, breaking down dead organic matter.
Several types are involved in the nitrogen cycle. Free living and
mutualistic nitrogen fixing bacteria such as Rhizobium convert
atmospheric to ammonia compounds. Nitrifying bacteria convert
Ammonia to nitrites and then nitrites to nitrates. Nitrates are the
main source of nitrogen for plants. Denitrifying bacteria convert nitrates
to nitrogen gas in the absence of oxygen.
Sandy Soils and Clay Soils
Property
Sandy soil
Clay Soil
Texture
Coarse
Fine
Structure
Structureless
Forms large clods when wet
Becomes hard and cracked when dry
Porosity
Large pore spaces, good
aeration, rapid drainage
Small pore spaces, poor
aeration, slow drainage
Water
holding
capacity
Poor water retention, little
capillary water, no waterlogging
Good water retention, large amounts
of capillary water, easily
waterlogging
Temperature
Warm due to low moisture content
Cold due to high moisture content
Nutrient
retention
Low, rapid leaching
High, not leached, clay particles
attract nutrient cations and some
anions
To Find the Water Content of Soil
1.
Weigh soil to find mass (mass 1)
2.
Place in an Oven at 110 C0 till the
soil reaches constant mass,
(Final mass = mass 2)
3.
Mass 1 - Mass 2 = Water content of soil
To Find the Organic Content of Soil
4.
Take dried soil and weigh to find mass (mass 3)
5.
Place in a crucible and heat with a bunsen burner
burning off the organic matter. Burn till the soil
reaches constant mass, (burnt mass = mass 4)
6.
Dried mass 3 – burnt mass 4 = organic content of soil
To Find the pH of Soil
1.
Add equal quantities of barium sulphate.
(barium sulphate flocculates the colloidal particles
i.e it causes the suspended particles to stick together
and fall out of solution allowing you to see a colour
change
2.
Add universal indicator solution
3.
Observe the colour change
http://www.royagcol.ac.uk/soils/book/p20.htm