1. GENERAL BACKGROUND

1.0 Purpose

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:

• How to investigate your own soil by conducting your own simple tests;
• Why some tests require skills and equipment too specialized for you, and should thus be performed by a soil laboratory;
• How to understand the technical language of soil scientists and civil engineers so that you can make use of soil analyses and soil laboratories.

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.

Remember ... 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 bottom properly;
If you plan to build a small reservoir, select a good site for the dam with suitable soil located nearby for its construction.

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 rock.

Peat soils 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 residual soils.

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 soils

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 soils

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 high water-table;
• 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 subsoil. 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 the surface.

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;

Example

Residual soils differ from sedimentary soil; parent materials as diverse as granite*, basalt*, gneiss* and micaschist* develop into different types of soils.

• The length of time the soil has been developing.

Example

Mature soils are those which are old and well developed; immature soils are those which are new and not fully developed.

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.

The upper part of the mineral soil, the A master horizon, is subject to the mechanical effects of weathering and to the loss of some of its constituents through leaching. In the lower part of the soil stratum, the B master horizon, some of the substances leached out from above are precipitated and accumulated .

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, changes, losses

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 .

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* water). 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

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 FINES

NOTE: key for mineral soil particles Clay Silt Sand Gravel   NOTE: 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 Institution.

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 diameter;
• 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).

Inorganic silt

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.

Organic silt

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.

Inorganic clay

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.

Organic clay

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

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.

1.8 Some examples of particular names for soils

Hardpan

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.

Loess

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.

Bentonite

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.

Black-cotton soil

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".

Lateritic soil

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.

Acid sulphate soil

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).