A good understanding of soil and its characteristics is one of the most
important of the many factors which must be considered for successful
freshwater fish culture. The purpose of this manual is to provide the
basic knowledge of the soil that is needed for the construction of ponds,
a water supply, canals, reservoirs, barrages, small dams, and for the
efficient management of fish ponds.
To do this, you will learn:
1.1 What is soil?
Soil is a complex mixture of living organisms, organic
matter, minerals, water and air. Take a handful of soil and look
at it closely. You can see that it is a mixture of many different
kinds of small particles.
Soil is made up of:
Organic particles of decayed plant and animal materials
which come from living plant and animal bodies;
Mineral particles such as sand, clay, stones or
gravel which were once parts of larger rocks.
Depending on their texture, structure and
consistency, various kinds of soil hold more or less water and air.
You will study the make-up of soil in more detail in Section 1.6.
1.2 Why is it necessary to investigate your soil?
Soil is your basic material
If you are going to make a success of freshwater fish-farming, you must
know your soil well. The bottom of your pond is soil. When you dig your
pond, you will use the soil you take out to build your dikes. If you are
going to build a reservoir to store water, you will build a dam using
soil. You will need to dig ditches or canals in the soil for a water supply
to your ponds.
How well does your soil hold water?
It is important to know how well your soil holds water. This is called
soil permeability. Soil which is permeable does not hold
water. Soil which is impermeable holds water. Before you build a fish-pond,
you will need to test your soil to see if its permeability is suitable
for building a pond. If the loss of water through seepage is too great,
you may need to seal the bottom of your pond, you may need to seal the
dikes, or you may find that you lose too much water from your supply canals.
Choose a site for the construction of your pond with good soil
where water losses through seepage will be minimal
(see Section 2.1, Water losses by seepage,
FAO Training Series No. 4);
When you are building a pond, good soil ensures strong and impermeable
dikes so that the water remains in the ponds. Wet
and swampy grounds are usually good for pond construction;
Avoid building-sites for ponds with holes or cracks,
anthills or rock outcroppings or with
the roots of large bushes or trees. Here, water losses
might be excessive and it could be difficult to seal the pond
If you plan to build a small reservoir, select
a good site for the dam with suitable soil located nearby for
How to conduct a soil survey
Before you begin to build a pond, you have to do a soil survey to see
if the site is satisfactory for pond building. You will learn how to do
this in Chapter 2, Planning and making a soil survey.
1.3 The origin and evolution of soil
What is the origin of soil and how do different rocks develop?
All soils fall into two main categories: the mineral soils and
the organic soils.
Mineral soils come from parent
rock called parent material. They develop over time as
the parent material is broken down by various physical, chemical and
biological processes due to climate, drainage, leaching, erosion,
vegetation, living organisms and human activity. This is called weathering.
For example, high soil temperatures break down rocks into smaller
fragments, owing to heating and cooling. The parent material is gradually
reduced to particles, larger areas come in contact with water, and
the chemical composition of the minerals present changes. Soluble
chemicals are washed down, or leached, to deeper parts of the soil
while less soluble products are left in the upper parts of the soil.
Further weathering takes place and, in time, mineral soils develop
to their actual state.
Organic soils come from
organic matter. They develop by the gradual accumulation
and decay of plant and animal material over time. It is a general
rule that a soil is called an organic soil if:
Either more than half of the upper 80 cm of soil is organic;
Or organic material of any thickness rests directly on parent
are a common type of organic soil. They form on poorly drained places
such as river valleys and lake shores which are often under water
and where decomposition of vegetal organic matter is very slow,
or even stops. The layers of organic matter, being formed by the
vegetation, then begin to pile layer by layer on top of the mineral
soil and can be several metres thick. Peat soils, made up of about
80 percent partly decomposed organic matter, have very high water
content and permeability.
There are two types of mineral soils
Some mineral soils develop from a parent material which is broken down
on location into small particles through weathering. These are called
Other mineral soils develop from small particles coming from mineral
soils developed in another location, transported for some distance, and
deposited. These are called sedimentary soils.
Soils developed from a local parent material: residual
Residual soils are usually found in the hills and extend down to the
foothills along the edges of valleys. They are rarely found in large level
areas but are usually found in areas ranging from gently sloping to quite
steep. Solid rock or partly decomposed rock material below the subsoil
indicates that residual soil was formed here.
Soils developed from a transported parent material: sedimentary
The soil particles making up sedimentary soils may have been
transported by wind or by water.
If the particles have been transported by wind,
the soil develops from a loess, which is usually
the best agricultural topsoil blown from other areas. It is common
in rolling or hilly topography. Loess is often quite fertile and
contains a fair amount of organic matter to great depths;
If the particles have been transported by water,
the soil develops from an alluvium and the resulting
sedimentary soil is an alluvial soil. Soil may be
transported by moving water such as rain, a river or a tidal area.
Sedimentation may occur in standing water such as a lake, swamp
or sea. The water may be fresh or saline (inland, coastal or delta
estuary). The transport may have taken place a long time ago,
or it may still be taking place today.
Alluvial soils are of the
greatest interest for fish culture. They are found in areas called
sedimentation plains where the topography is usually
slightly sloping or almost flat. This means that a minimum of earthwork
is necessary for the construction of fish-ponds. The composition
of alluvial soil often contains enough clay for water retention
and dike construction. A water source is usually close by, but not
always. Alluvial soils may be found in:
Old alluvium has been in
place long enough to show distinct layers caused by soil formation
processes. It is usually located high above present flood levels.
The topography is likely to be level or gently sloping.
New alluvium is found in flood plains. It has been
moved into place by recent floods and is still subject to flooding.
Soil layers are difficult to see. The topography is generally
level, but rolling low ridges and hollows are also found. These
soils are usually highly fertile.
Note: a sea or lake may have been present thousands of
years ago in a location which today is covered with forest or savannah.
This soil will be an alluvial soil, even though water is no longer present.
River flood-plains subjected to seasonal floods;
River deltas where river deposits are found together with a permanent
River estuaries where sedimentation is influenced by tidal movements
at the transition from fresh waters to marine waters;
Coastal plains where tides cause sea-water deposits.
1.4 The soil and the subsoil
of mineral soils
After soil has developed for some time, you can usually see two
major horizontal layers or strata which lie above the
parent material. The surface stratum is the soil which is made up
of the surface soil and the soil proper. The bottom stratum is the
The surface stratum: the soil
This is the layer where most of the biological action takes place,
such as animal diggings, animal droppings, growing plant roots,
decomposition of organic matter, and man's agricultural activities.
Circulation of air, water and chemicals is greatest here, and the
soil is softer. As water washes down minerals and organic materials
from the surface to the deeper parts, one can recognize within this
surface stratum two narrower layers which are:
The surface soil or the top layer which is usually
shallow and sometimes cultivated by man. The surface soil has
organic material and most of the feeder roots of the plants live
on it. It is darker in colour and, in certain cases, it may even
be black. At the bottom of this surface soil, you may find a shallow
layer of gravel;
The soil proper, the second layer, which is lighter
in colour and contains the roots of the larger plants such as
bushes and trees.
The bottom stratum: the subsoil
This is the deeper layer where only the largest tree roots will
penetrate. Circulation of air, water, and chemicals is reduced here,
and the soil is hard. The general appearance of this subsoil will
vary according to its origin:
If it is a residual soil, (see
above) the number of stones rapidly increases toward the bottom
of the subsoil until it reaches the parent rock;
If it is a, sedimentary soil (as shown here), the
soil layers may be narrower. Each of them may have its own composition
related to the way it was deposited. Usually, these soils are
quite thick and the parent rock is then found several metres below
1.5 Soil horizons in mineral soils
There are many different kinds of soil
You have already learned in Section 1.3 that there are many kinds of
soil and many variations of these kinds of soil. Soils may be shallow
or very deep, they may be leached* or saline, they may be
mature or immature. The characteristics of soil vary according to:
Local conditions, such as topography, climate, vegetation
and human activity;
The nature of the material from which the soil has developed;
The length of time the soil has been developing.
Soil horizons are layers which are characteristic of each kind of soil
As there are many kinds of soil and many variations of these soils, there
are also variations of the horizontal layers typical to all soils. Soil
layers tend to vary from place to place as to their number, individual
thickness, colour, physical and chemical characteristics.
The principal layers discussed in Section 1.4, The
soil and the subsoil of mineral soils, are subdivided into thinner layers
called master horizons. Each master horizon may be further
subdivided into subhorizons.
How do the soil horizons develop?
The physical properties of the soil stratum, from the surface of the
ground to a depth of about 1.5-2 metres or occasionally deeper, are influenced by seasonal changes of water content
and temperature and by various biological agents such as roots, worms,
insects, and bacteria.
Below the B master horizon, the character of the soil is determined by
the type of parent-rock from which it was formed, how it was deposited
and by later geological events.
The soil additions, movements,
How are these soil horizons labelled?
The labelling system used by soil scientists to designate soil
horizons is complex. Here, the more simplified system shown in Table
I designates master horizons
by the capital letters H, 0, A, E, B, C, and R.
Subhorizons will be designated
by adding a number to the letter of the master horizon. This is done
for each master horizon from top to bottom. Master horizon B, for
example, can be subdivided into subhorizons B1, B2, B3 .
TABLE 1 Simplified definitions and designations of soil master horizons
Organic horizon formed (or forming)
from accumulation of organic material deposited on the soil
surface. It consists of at least 20-30 percent organic matter,
the composition depending on the type of vegetation from which
the organic material results. It can form only in the absence
of air, when soils are continuously waterlogged.
Organic horizon as above except that
it is not saturated with water for more than a few days a year.
It contains at least 35 percent organic matter, consisting generally
of partly decomposed organic material.
Mineral horizon formed (or forming)
at the surface or adjacent to the surface. It either shows an
accumulation of humified organic matter (humus) intimately associated
with mineral particles or has a morphology acquired by soil
formation. It is somewhat darker than the underlying B-horizon,
the decomposed organic material being mixed with mineral material
by biological activity.
Eluvial horizon, situated beneath
an H-, O- or A-horizon. It contains less organic matter, shows
an accumulation of unweatherable minerals, and is lighter in
colour. It is formed by loss of iron, fine clay, etc., which
have been transported and accumulated in the underlying B-horizon.
Mineral horizon of unconsolidated material
from which the soil has developed. Bottom gravelly and stony
material which allows root development is also included here.
Layer of continuous indurated rock
sufficiently coherent when moist to make spade digging impracticable.
This rock may contain cracks but these are too few and too small
for significant root development. In fact, this is not a real
Identification of soil horizons: the soil profile
The easiest way to identify and describe separate soil horizons is to
look at a fresh soil profile. A soil profile is a vertical
section made through the soil that shows the thickness and sequence of
the individual horizons. To identify soil horizons, do as follows:
Look at the soil profile and identify the master horizons;
Make a drawing of the soil profile showing the master horizons. Label
each horizon as shown;
Study each master horizon separately and, if they are present, determine
the subhorizons. Label the subhorizons by numbering them
from the top to the bottom of each master horizon, as shown;
When you have finished the drawing of the soil profile, measure the
depth of the top and bottom of each horizon and write these depths
on your drawing. Depths should be measured in centimetres from the top
of the soil (immediately below any layer of leaves or other non decomposed
vegetation) to the top limit of each horizon and to its bottom limit.
Identify, draw and label each master horizon
Measure each horizon
Write depths on drawing
Note: if you see changes in the width of the horizon in
the profile, add a note giving the range of this change. In the illustration,
the top of the B2 subhorizon ranges from 51 to 62 centimetres and the
bottom from 90 to 94 centimetres. So, the B2 subhorizon width varies from
32 to 39 centimetres.
1.6 Soil composition
You have already learned in Section 1.1 that soil is a complex mixture
of living organisms, organic matter, minerals, water and air. In this
section, you will learn a little more about some of these soil constituents.
Organic matter in soil
Some organic matter is large enough to see, such as small leaves,
twigs, rotted pieces of wood and worms. Other organic matter is
so small that you cannot see it. This is called humus
and it is found in the soil in the colloidal state.*
Humus comes from dead plants and animals which decompose in the
soil. You cannot see it as you can see minerals, but you know it
is in the soil because of its colour. Humus
makes the soil darker than usual, or even black. Humus particles
have the property of strongly attracting soil minerals to their
surface through adsorption.*
Organic matter in soil
Minerals in soil
Minerals are present as particles of various sizes. In some cases,
these particles may join together to form clumps of larger size.
Mineral particles have different names according to their size,
such as boulder, stone, cobble, gravel, sand, silt
or clay. Some of these particles may be easily seen,
but some of the finer soil particles, such as silt and clay, may
be seen only with a microscope. The finest soil particles, called
colloid clay *, are invisible. Colloid clay particles
also have the property of strongly attracting soil minerals to their
surface through adsorption.*
Mineral particles are classified on the basis of their size. According
to the country and the purpose of the survey (engineering, agriculture
or soil conservation), different systems of classification are used,
although some efforts have been made to standardize. Some different
systems of classification are shown in Table 2.
Minerals in soil
Water in soil
In the soil, water may exist in two forms. These are free water
and bound water. Free water is found in the soil
pores,* and bound water is found attached to
the soil particles as a film (cohesion*water) or it
is adsorbed* at the surface of the soil particle (adhesion*
Soil permeability is
the movement of free water through the soil pores, cracks and holes.
This will be discussed further in Chapter 9. Bound water may be
of great importance, especially when the soil particles are very
fine, because of its direct influence on some of the engineering
properties of a soil such as its shrink-swell
potential (see Section 10.4). When the soil particles are very
fine, such as with clay particles, water and chemicals may be very
strongly adsorbed at their surface. Clay water content may thus
vary greatly. Clay particles may adsorb up to 600 percent of their
dry weight which may correspond to a swelling* ten
times their original volume when dry.
Water in soil
Air in soil
Soils also contain a proportion of air in the free pore spaces
between soil particles and clumps or aggregates. It is also found
in the cracks and holes, resulting from the activities of living
organisms such as worms, termites, ants, rodents and plant roots.
The volume of the soil atmosphere is mostly determined by the physical
nature of the soil, its water content, its relative degree of compaction*,
and the importance and nature of the activities of living organisms
Average pore space in percent of total volume
TABLE 2 Various systems of classification of mineral soil particles (sizes of particles in mm)
* USC list clay and silt in a single category called
NOTE: key for mineral
Clay Silt Sand Gravel
Symbols used for silt, sand, and gravel
F = fine
VF = very fine
M = medium
C = coarse
VC = very coarse
International System (Atterberg Classification)
US Department of Agriculture (USDA)
American Society for Testing Materials (ASTM) and US
Public Roads Administration (USPRA)
US Bureau of Soils (USBS)
Massachusetts Institute of Technology (MIT) and British
Standards Institute (BSI)
Unified Soil Classification (USC), US Corps of Engineers
(USCE), US Bureau of Reclamation (USBR), Indian Standards
1.7 Basic types of soil
You have already learned in Section 1.3 that soils are either mineral
or organic and vary according to their origin. Mineral soils originate
from parent material and develop either locally (residual soil) or following
transport (sedimentary soil). Organic soils generally originate through
the accumulation of plant materials (organic soil).
You have also learned that the basic constituents of soils are varied.
In particular, the sizes of the various materials may vary greatly from
one soil to another. According to the main constituent present in a soil,
one may define the following basic soil types.
Gravel and sand
Of the particles which make up the soil, sand and gravel can usually
be recognized most easily as non-coherent pieces of visible
rock. If you take some dry sand in your hand, it runs through your fingers
like water because sand is not a stable material. Sandy soils are easy
to work and do not stick to tools. Air and water circulate through them
very easily. You can tell the difference between gravel and sand by their
particle size, as defined in Table 2. For
the purpose of this manual, the following sizes are used:
Particles of sand are smaller than 0.2 cm (or 2 mm) in
Particles of gravel measure from 0.2 to 7.5 cm in diameter;
Particles larger than gravel are usually called stones
(7.5-25 cm) or boulders (larger than 25 cm in diameter).
Particles of silt are much smaller than particles of sand; they are not
visible to the eye and they are much closer together. Silt does
not let water through as easily as sand does and it is less permeable.
If you crush dry silt, it will form dust but not one as fine as clay dust.
Silty soils do not crack when dry and do not stick to tools when wet.
Silty soils are harder to work than sandy soils but less hard to work
than clayey soils.
Note: inorganic silt has a smooth appearance, just like
clay, and it is often mistaken for clay. However, silt may be readily
distinguished in the field from clay with the shaking test. It is important to make
this distinction because some silty soils may become very unstable when
wet as, for example, when used for dike construction and placed under
water. On the contrary, clay is a stable construction material.
Particles of inorganic silt are mixed with finely divided particles of
organic matter, some of them still visible, such as shells and plant material.
The colour of the soil varies from light to very dark grey. Generally,
organic silt has an odour of decaying organic matter.
Clay is the finest part of the soil and some clay particles
are not even visible under the microscope. It has strong binding
properties for water and chemicals. Most clay can be easily recognized
because when it loses water it cracks and forms very hard lumps. Clay
adsorbs water very slowly, but it will hold a lot of water once it has
adsorbed it. It may then swell and more than double its volume. Clay becomes
very sticky when wet and, if you hold it in your hand, it will stick to
your fingers. When clay soils are wet, they are often too sticky to work
and when they are dry they are too hard to work.
Note: you can tell the difference between inorganic clay
and inorganic silt by using the shaking test. No powder comes from
the surface of dried clay when it is rubbed between the fingers. Also,
the colour of inorganic clay is usually yellow, red or white.
This is a clay containing finely divided organic matter. Its colour is
generally dark grey or black. Generally, organic clay has a strong odour
of decaying organic matter.
Peat is a truly organic soil made of visible fragments
of decayed plant material. Its colour varies from light brown to black.
It has the odour of organic matter.
Note: the major part of most
kinds of soil is usually made up of a combination of two different
soil types. They are called composite soils and named
according to their major and minor constituents.
A soil offering an exceptionally strong resistance to penetration by
boring tools. It is usually a very dense mineral soil of clay, sand and
gravel that has been cemented together to form a rock-like layer. It will
not soften when wet and a pick has to be used to dig in it.
A wind-transported sediment, usually light brown in colour. The size
range of most of the particles is very narrow (0.01 -0.05 mm). The particles
stick together strongly because of a calcareous or clayey binding material.
Root penetration is extensive.
A clay with a high content of montmorillonite*, a very fine
clay. Bentonite usually develops through the chemical change of volcanic
ash. When water is added, dry bentonite swells more than other dried clays.
However, on drying, it also shrinks more. Bentonite may be used to seal pond bottoms which
are not impermeable enough.
A heavy clay soil generally containing 40 to 50 percent clay, mostly
montmorillonite*, little organic matter and a high proportion
of calcium carbonate. The colour varies from light to dark grey, black
or blue-black. When wet, it becomes highly sticky, very soft and swollen,
with reduced bearing capacity. When drying, it shrinks considerably, 20
to 30 percent. Large cracks appear at the surface and may extend as deep
as 3 m. The depth of such soil generally varies from 1 m to 3.6 m or more.
It is commonly found in warm and relatively dry climates. In India, such
soils are termed "Regur".
An old name for a very uniform tropical soil, typical of the humid tropics.
Intensive and continuous weathering over a very long period has resulted
in leaching* of chemicals (such as silica), in accumulation
of iron and aluminium salts, and in formation of clays. Biological activity
is high, particularly under forest, and there is an extensive root system.
Generally, soil colour is reddish or yellowish. When there is a ground-water
influence within the 0- to 125-cm zone, a firm
iron-rich clay material (plinthite*) is usually formed as red mottles*.
When exposed to the air, it dries out and becomes irreversibly hard (laterite
or ironstone), forming a hardpan (see above) or hard concretions.
An acid sulphate soil is characterized by its great acidity (pH less than 4 see Section
4.1) and by the presence of generally abundant yellow mottles. These mottles point to the
presence of an iron sulphate compound (jarosite), formed through exposure
to the air (oxidation)* and bacterial action, from a mineral
containing iron and sulphur-pyrite. Such soils are found either in saline
areas such as coastal mangroves or in freshwater areas such as river plains.
Freshwater acid sulphate soils occur extensively in Southeast Asia, for
example, in the Plain of Reeds, Mekong delta and in the Bangkok Plain,
Thailand. The use of these soils for fish culture
should be carefully planned (see Section 4.2).