4 Fish Pond Construction

Construction of a large pond can be very expensive if labour is hired, machines are used, and expensive equipment is rented. For example, in the Philippines, a one-hectare pond having two concrete gates and walls 3m high x 3m wide recently cost US$1,522.56. Another pond, about 100m x 25m, with only a Rivaldi valve cost about US$680.

An interesting fact about fish pond construction is that whether the pond is large or small, expensive or inexpensive, ponds are all very much the same. A larger, more expensive pond will not necessarily be a better pond.

Here is an example of a good beginning for a new and small fish farmer:

A "backyard" fish pond was planned and sited very carefully by a farmer. The pond was dug by the farmer and constructed with bamboo pipes for water inlets and outlets. The construction itself cost no money. The farmer's only expense was a supply of fingerlings purchased from a nearby market. This fish pond, managed by the farmer and his family, produced enough fish for the family and some extra income from fish sold or bartered for goods needed by the farmer. The family ate well and suffered no major illnesses during the year.

Next year, the farmer plans to add another pond and to produce more fish for market. He will add a Rivaldi valve or a wooden monk to this new pond, because either of these will make ongoing management somewhat easier, now that there will be two ponds to manage (The bamboo pipe sometimes got clogged. This was no problemn to correct when there was only one pond. But it would take up needed time in a two-pond operation). Whichever the farmer chooses, the valve or the monk, he will make it himself with materials found locally, using money from the sale of his fish.

This farmer began his operation well. He started small and worked into a larger operation. However, even for the larger fish farm, he planned an expansion which was within his means. This kind of careful planning increases the farmer's chances of success -- and yours. And the scope of the project is something he can undertake on his own. He gained the knowledge and experience that he needs to expand his operation.

The following section presents a range of ideas for constructing fish ponds. The farmer can pick a combination of construction possibilities which best fit his own needs and resources.

IMPORTANT:

Stress that the "right" way in any situation is the way which:

* the owner can afford

* the owner can manage easily

* fits the owner's needs most completely

Construction should begin only after careful planning such as that outlined in the preceding sections on "Planning."

A fish pond has three main parts: the walls, the water inlet, and the drainage system. Walls are also called dams, dikes, levees, or bunds. This manual uses "walls." Whatever they are called, walls hold the water in the pond. They can be built using soil taken from inside the pond, or they can be built with soil taken from another place. They must be strong enough to withstand the pressure of all the water inside the pond: water constantly pushes against the walls. They must also be water-tight (impermeable), so the pond does not leak.

The water inlet, located above the pond water level, is used to let water into the pond and is closed off after the pond is filled.

The drainage system is used to empty the water from the pond when the farmer is ready to harvest the fish.

There are many ways of making inlet and drainage systems: the most important criterion is that they work. But the walls are especially important: they are all that keep the fish inside the pond. The walls must be built carefully.

Pond construction follows the same principles whether the pond is a single backyard pond or part of a large fish hatchery. These are the steps in pond construction:

* Survey the land

* Mark out the area of the pond

* Measure and mark out the walls

* Excavate the pond bottom, if necessary

* Build the drainage system

* Build the water inlet

* Build the walls

* Seal the pond bottom and walls

Each of these steps will be discussed in detail in the following pages.

Survey the Land

The first step in the construction of a fish pond is marking the area of the proposed pond. If the site chosen is a natural slope, the first thing to be done is to find out where the main wall will be built. The main wall should be marked off at the lower end of the pond, where the pond will be the deepest and the slope the greatest. This is where the pond's drainage system will be put. If the pond is to be on a flat area, the pond bottom itself must be made with a slope so the pond will drain. This is done by digging one end deeper than the other end. Remember: the main wall is always at the deeper end.

DETERMINING THE SLOPE

Even flat ground usually has some kind of slope, although it may be very little and hard to see. So, before constructing the pond, the land is surveyed to find out which way the land slopes and what that slope is.

There are a number of ways which can be used to determine slope. The way outlined here probably would not be used by many farmers if they were building a pond on their own, but this is an accurate method of determining slope and should be encouraged if at all possible.

To survey the land for slope, some stakes (long, straight pieces of wood), some string (fishline, etc.), and a carpenter's level are needed.

Most farmers will not be familiar with the level, a device that has an air bubble trapped inside which rests between two drawn lines. When the level is placed on the ground, it shows whether the area is flat or sloped: if it is straight or flat (level), the bubble stays in the middle between the lines; if the land slopes, the bubble will move to the right or left of the lines, depending upon the direction of the slope.

Farmers who cannot find a carpenter's level can make a level by getting a small lightweight container. They should then place the container on a known horizontal surface, add water, and draw a line around the inside of the container at the water level. Then, if this container is placed on a slope, the water will shift away from the line to show the slope.

When all the equipment is gathered, measure the slope.

* Look at the land and decide which part is higher.

* Drive a stake or a piece of wood or bamboo into the ground at the highest point.

* Walk away downhill from the stake about 100cm. Drive another stake into the ground at this point.

* Tie string or fishline or vine (whatever is being used) between the two stakes. Attach the level to the string. Then move the string up and down on the stakes until the bubble is between the lines on the level, or the water level is even with the line marked on the container. This will mean the string is level between the stakes, even though the stakes are in the ground at different heights.

* Measure the height of each string by measuring from ground level to the place where the string is tied.

This drawing shows that one string is tied at 20cm; the other is tied at 25cm. Therefore, one end of the area is 5cm lower than the other. The distance covered by the string is 100cm, so the slope is 5% (over 100cm of ground, the elevation changed 5cm). Since a slope of 2-5% is good for a fish pond, this site has a satisfactory slope for a pond.

Other Ways of Determining Slope. As mentioned earlier, the above method of measuring slope is a good one, but it my be difficult for some people to do. It is possible to calculate slope roughly. A farmer, who realizes that what he is looking for is a way to place his pond so that the water can enter from the water source and drain away well, can figure the slope of his land by doing such things as rolling a ball or other round object and watching carefully to note where and how quickly the ball rolls. A good slope would mean a slow-rolling ball. A variation of this involves throwing a quantity of water, or a mixture of water and dye, on the ground and watching the path it takes and its speed as it moves along the ground.

It is important to consider slope carefully. A well-placed pond with good drainage is easier to care for and has more chance to be successful. It may be necessary for the pond owner to measure his land only once to find a good location. Or it may be necessary to repeat the measuring a number of times. This is probably a good thing to encourage since locations which look alike to the eye often have enough difference in slope to make a big difference to a fish pond. Also, determining slope is a larger project if more than one pond is being built. Then the Ponds must be laid out in relation to each other.

There may be several areas which have the correct slope, but only one which is good in terms of getting the water into the pond from the water source and out of the pond easily. For example, the farmer might like to drain his pond so that the water irrigates his fields. Therefore, he will want to keep this in mind when he decides upon the exact placement of his pond. Likewise, if he is building a pond on a hillside in back of his house, the slope may be perfect, but he will need to avoid drainage into his buildings.

Once the slope is found, the location of the main wall can be determined. Of course, if the pond is built on flat ground, it will have four walls. If the pond is a barrage pond, it may only have one wall. The number of walls depends upon the land. The shape of the land may mean that one wall or two walls or four walls will be needed.

Mark out the Pond Site

Measure, the Walls

Now that the slope is known, the place of the main wall is known. The main wall is at the end of the pond which will be deepest, and is the wall where the drainage system will go.

Mark out the main wall, and any other walls that will be built, with stakes. The walls, when finished, will be wide: it does not matter so much where the stakes are placed within the width of the planned walls, for they are to be used as height markers.

The farmer has to plan the depth of his pond and the height of his wall. If the pond is going to be 2m deep at the deepest end, for example, the walls should always be at least 30cm higher than the water level for a small pond, and at least 50cm higher for a large pond. Also, the walls will settle after they are finished, so it is best to make the wall 10% higher than the desired final height of the wall. A 2m deep pond, therefore, would have walls with a total height at the deepest point of 2.5 or 2.6m [height of wall before it settles = depth of pond + 30cm (for small pond) or 50cm (for large pond) + 10% of depth and 30 or 50cm].

Tie strings to the stakes along the main wall line, at a height of 2.5 or 2.6m for a pond whose deepest end will be 2m. Use a levelling device to connect strings to the stakes marking the other walls, if the pond has other walls, at the same level as the string marking the height of the main wall. The strings are the building markers. When the walls reach the strings, they are the right height.

Dig the Pond Bottom

As stated before, the pond bottom must slope downward from the shallow end to the deep end to help drainage. The pond bottom usually has a slope of from 2 to 5%. (A slope of 2% would mean that for every 100cm change in length there is a 2cm change in height.)

The pond bottom must be clear of rocks, roots, trees, and stumps so that later, when a net is used to harvest the fish, the net will not get caught and tear. If the pond bottom is already smooth and slopes well, it can be left alone. Or, if the pond bottom only has grass on it, the grass does not need to be removed before the pond is filled. In fact, once water is added to the pond, the grass will die and rot and add nutrients to the water.

If the pond bottom does not already slope downward, excavate (dig out) the bottom area of the pond until a good slope for drainage is made.

Adjust the height of the strings tied to the wall markers if digging the bottom has changed the height.

Keep the soil which was dug out of the pond: when the pond walls are finished, the soil can be placed on top and planted with grass. This fertile topsoil will root grass easily; this grass will help keep the walls from eroding (washing away).

The pond bottom can be excavated by hand or by using machines, like bulldozers, if they are available. Remember: if the land for the pond is chosen well with regard to the natural topography, only a small part of the pond bottom will need to be dug out. The most important thing is to have the pond bottom slope so that the pond can be drained.

Build a Drainage System

A drainage system is anything that is used to empty the pond. It consists of the outlet system for letting water out of the pond and the drainage ditches which carry the water from the pond away.

As stated before, the best and easiest way to have a good drainage system is to build the pond in a place which provides a good slope -- on a hill, for example. This is the first step. Then, there are many different drainage systems which can be put into the pond. Some of these drainage methods are expensive; others are very inexpensive.

The drainage system must be built before the pond walls because some drainage devices go through the walls. (In some countries the drainage is done by knocking a hole in the wall of the pond. When the pond is dry and empty, the hole is patched up.)

One of the easiest ways to drain the pond is to place a bamboo or plastic pipe through the base of the wall into the middle of the pond. The end of the pipe which is inside the pond has a screen over it to keep fish from entering the pipe. The other end of the pipe, the end that is outside the pond, is plugged with wood or clay. To drain the pond at harvest time, the plug is pulled out.

Two other methods of draining the pond which work but are not used as often, are the siphon and the pump. A siphon is merely a flexible plastic or rubber tube. One end of the tube is in the pond near the bottom; the other end is placed on the ground outside the pond. A vacuum is produced in the pipe by sucking at the end outside the pond until water begins to flow out. The end of the pipe inside the pond must be kept in the water or the siphon will not work.

The pump is usually not a good idea for a farmer because the engines that are used to run the pumps are costly and often not available, or gasoline to run them is costly, or they must be given frequent attention so they will not break down.

All ponds must be drained for harvesting fish. Also, it is a good idea to let a pond dry out completely once every year or so to get rid of any unwanted fish and/or disease-causing organisms.

The following are some tested, effective drainage systems a farmer can consider for his pond.

RIVALDI VALVE

 This valve was named after a farmer in Paraguay who first used the system. It is an easy and good method to use in a small fish pond. A farmer who is building only one small pond for family use would find this valve a good choice for his needs.

The Rivaldi valve is a flexible plastic pipe. Place the pipe on the ground before the wall is built. Build the wall. Then turn up and tie the pipe to a stake. Tie the pipe end at a level which is somewhat above the usual level of the water in the pond. Keep the pipe up and tied to the stake until it is time to drain the pond. Then, untie the pipe and let it lie on the floor of the pond until the water is out of the pond. At other times, the pipe works as an overflow to let out water after a heavy rain: when the water level in the pond reaches the top of the pipe, water will flow down the pipe and out of the pond.

The Rivaldi valve should have a screen over the end inside the pond to keep fish from going out of the pond while the pond is being emptied or drained.

ELBOW JOINT

A variation of the Rivaldi valve, this consists of two metal or plastic pipes connected by an elbow joint. The joint lets the upper pipe be turned down to drain the pond. The joint is screwed onto the ends of the two pipes, one of which extends under the wall and the other above the surface of the water. This drainage method is also called a "turn-down" pipe because it is actually turned on its side to drain the pond.

BOTTOM-WATER OVERFLOW

This drain takes water directly from the bottom of the pond where oxygen levels are the lowest. The Rivaldi valve and elbow joint do this also, but each of these requires that the pipe be lowered so the pond can be drained. The bottom-water overflow regulates the depth of water without any need for moving the pipes. When. new water is added to the pond, the less-oxygenated water at the bottom drains out automatically.

This type of drain is relatively complicated and usually difficult to build. For a small fish farm operation, it would probably not be worth the effort.

DOUBLE SLEEVE OVERFLOW

This drainage system is built like the turn-down pipe, except a large pipe is placed over the section of pipe which extends above the pond's surface. This outer pipe should be longer and wider than the inner pipe, which is placed so that it is about equal in height to the depth of water desired in the pond.

When fresh water is required in the pond quickly because the water is too warm for the fish or because the oxygen levels are low, all the farmer has to do is to add water to the pond. The double-sleeve overflow automatically drains the stale water from the bottom of the pond.

SLUICE

A sluice can function in a number of ways in a pond. It can be a screened gate in a water channel going into the pond, or a drainage gate leading water out of the pond.

In a pond, a drainage sluice gate is anchored into the main wall by extending the sides of the sluice into the wall so the sluice structure stands upright. The sluice is constructed at the center of the main wall before the dike is built.

The sluice can be made of wood, cement, or brick. It can have one or two wooden gates which are removed to empty or fill the pond. A sluice also can have a screen gate to keep unwanted fish from entering at an inlet and pond fish from leaving at the outlet.

IMPORTANT:

The wooden gates of the sluice must fit into the slots well, but easily. The wood will swell to make a tighter seal as it is soaked by the water in the pond. The slots (grooves) can be filled with several strong, long, narrow boards which have been bevelled or notched so that they fit together tightly. Or the slots can be filled with single pieces of wood. When single pieces of wood (or a number of boards which have been fastened tightly together) are used in a sluice, the pond is drained and the water flow regulated by lifting the entire wooden structure out of the groove to a height which allows some or a lot of water to flow out of the pond. When separate boards are used in the grooves, the boards are taken out one at a time. If a small flow out of the pond is desired, only one board may be taken out. To drain the pond, all the boards are removed. In a sluice having two wooden gates, the space between the gates can be packed tightly with earth. This will help seal the water into the pond.

MONK

The monk is very much like the sluice, but it is not built into the pond wall the way the sluice is. Sometimes the back of the monk does touch the wall, but it is not built into the wall. Also, a monk is never used at the inlet as a sluice can be.

A monk-type drainage system controls the level of water and prevents fish from escaping when the pond is being filled. It also allows for good drainage of the pond. The completed structure consists of a horizontal drainage pipe and the vertical structure, or monk. The drainage pipe must be placed before the walls are built; the monk may be built outside the pond, and placed inside later.

The drainage pipe runs from the back of the monk under the pond wall. It should be between 20 and 40cm in diameter; if piping of this diameter is not available, two pipes may be used. For good drainage, place the pipes 30 to 40cm lower than the pond bottom. Make sure the drainage pipe is on solid ground so that the pipes do not bend. Bent pipes are difficult to clean out when clogged.

The monk itself is a structure which is closed on three sides and open in the front. The open side should face the inside of the pond and should be at least 30cm wide; the entire monk should be at least 40cm above the surface of the water.

The two parallel sides of the monk, and the bottom, have grooves cut in them: a monk may have two or three grooves. One groove, or part of a groove is always for the screen. The other groove(s) is for the boards.

Monks can be made of wood, concrete or brick. A wooden monk should use strong wood -- 4 to 5cm thick.

A concrete monk should be reinforced with metal. Before the concrete is poured, a wooden form shaped like the monk is made and oiled. A frame, slightly smaller than the wooden form, is made of chicken wire, or some other strong wire, and set down inside of the wooden form. The concrete is then poured into the form. A good concrete mixture for monks is 1 part cement, 2 parts clean sand, and 4 parts crushed stone, by volume.

If the monk is made of concrete, the grooves can be shaped by bending iron rods into a "U" shape. Remember, the grooves must be sunk into the sides and bottom of the monk.

The grooves are filled by using a series of boards -- wide enough to fit the grooves well and between 20 and 30cm high. Each board has a hook on it so it can be lifted from the groove easily; the boards may also be bevelled or notched so that they fit together well.

If the monk has three grooves, the first groove can be a large screen. The screen is what keeps the fish from escaping as the pond drains. However, if the monk has only two grooves, a smaller screen can be placed above or below the boards in the first groove. Placing the screen at the bottom allows water to drain out from the bottom of the pond.

HERRGUTH MONK

This is a monk with three grooves. A large screen is in the first groove. The large screen is better than a smaller one because it does not get clogged up as easily as a small screen.

The second groove holds a series of boards. The lowest board can be a small screen. Water flows through the large screen in the first groove, and through the small screen in the bottom of the first series of boards, up and over the third series of boards into the drainage system.

There are other ways this kind of monk can be built. For example, the second groove could be filled by a large wooden gate (one piece of wood or several fastened together) which could be raised and held up to allow a flow of water from the bottom of the pond. It is this flow of water from the bottom of the pond which is important.

The Herrguth monk would probably not be used in a pond which is filled by rainwater. In these ponds -- sky ponds -- a regular monk is used, and the space between the two wooden gates is packed with mud to make a watertight seal which lasts for the fish-growing season and is removed when the pond is drained for harvest.

SOME NOTES ABOUT MONKS

Be careful with screens. Bamboo slats can be used instead of screening if the fish are large. But for fry, the holes should be less than 2mm in diameter. Often the screens are made by poking small holes in sheet metal. The screen mesh can get larger as the fish grow.

A valve is sometimes placed on the drainage pipe behind the upright part of the monk. This is used to control the draining speed and is easier to do than to move the boards in the grooves.

A large catching ditch can be made in front of the monk to help with taking fish out of the pond when the pond is being drained for harvest.

DRAINAGE DITCHES

Drainage ditches are channels which should be dug on the bottom of the pond to help the water flow out. Lining the ditches with stones helps the water flow. A small family pond does not require this system of drains. The only real requirement for drainage is a gentle slope.

This is the time to build other ditches which may be needed. For example, if the farmer wants to use the water from his fish pond to irrigate his land, he will want to construct the ditches or channels which will carry the water from the pond to the field or to storage tanks for use later. Therefore, the farmer must consider carefully where the water which is draining from a pond is going to go. If the pond is being fully drained, and the pond is built on flat ground, he should build drainage ditches around the outside of his pond to drain the water away from the walls. These ditches should be 30-40cm deep.

Water Inlet

All ponds, except for those filled directly by a spring or by rainwater, need water inlets. The water inlet must be constructed so that it supplies adequate quantities and quality of water, and so that it does not allow unwanted fish or other materials to enter the pond. This usually means there must be a channel of some kind to bring the water to the pond from the source and a filter of some kind to keep the water which goes into the pond clean and free from predators.

A water inlet can be as simple as a bamboo pipe of good diameter running from a water source through the wall into the pond. Remember: the inlet pipe should be placed above the water level so that incoming water drops into the pond. In some areas, such things as bamboo strips are tied to the end of the inlet pipe which is placed over the pond. The water flow into the pond is broken up by the strips and the water picks up and takes more oxygen from the air into the pond water.

If the pond is large or is a stream-fed barrage pond, a sluice makes a very good water inlet. The sluice can be one piece which controls flow when it is lifted to various heights, or the sluice can be a series of boards slipped in and out of the grooves.

It is better to filter most pond water as it goes into the pond. Filters are not needed if the water is clean and clear and the farmer knows the source is free from unwanted fish. But if the water is muddy, or has lots of leaves or debris in it, a filter helps keep the water quality good.

A filter can be placed at the beginning, middle or end of the channel which brings water to the pond. Usually filters work best near the water inlet. Filters can be made very simply. Remember they must keep unwanted fish out and pond fish in.

A wire screen makes a good filter. The picture above shows a sluice with a gate with fine screening to strain incoming water of pieces of debris and other unwanted fish and materials. Note the screen fits into the water channel exactly.

The horizontal screen at the left is very effective. Here the screen is placed so that the water passes through as it falls into the pond. This screen merely juts out from the wall at the inlet.

In the version below the horizontal screen has a vertical screen wall attached to it. This short wall prevents fish from going over the screen.

In any variations of these kinds of filters, the screens should be assembled into one piece for easy removal as a unit for cleaning.

There are other ways of filtering the water:

A nylon mesh bag makes a good filter, as long as it is partially submerged in the pond so that it does not tear as the bag catches fish or other material from the water source. Check it periodically.

A sand and gravel filter is particularly useful for cleaning out fish and eggs. It requires building a smaller pond or tank at the water inlet. If a filter is built in the earth it must be lined with a waterproof liner.

A saran fibre filter is basically like a wire screen that is placed horizontally underneath the water inlet. However, it is placed in a box standing in the water and uses saran fibre material instead of wire. (See drawings next page.)

These filters all have good and bad points. All must be cleaned often to remove debris that collects in them from the water source. The best filters are the sand and gravel filter, and the saran filter, but these are more costly than the others.

The farmer should examine his water source carefully before deciding on the kind of filter. If the water is very muddy, or has lots of leaves and grass in it (organic matter), he can use the wire screen. If the water source is free of organic material, the mesh bag will work because it is not likely to be torn. If the water contains unwanted fish and eggs, as well as a lot of organic matter, the saran filter or the sand and gravel filter is best.

To clean the filters, remove them and clean them with a brush and fresh water. Or flush the filter with water in the opposite direction of the normal water flow. This is called backwashing.

IMPORTANT:

Filters must be kept clean to be of any use. These filters should be cleaned each time water is let into the pond.

SILTATION TANK

One other structure which should be built at the water inlet, when necessary, is called a siltation tank. Silt is the mud that is suspended (floating) in water. Silt can become a problem when it clogs the gills of the pond fish so they cannot breathe. If the water source has a lot of mud in it, a siltation tank should be built at the inlet to the pond, or at the inlet to the first pond, if it is one of a series.

The siltation tank can simply be a smaller pond. The water flows into this pond and is kept there until the mud falls out of the water and settles on the bottom. Then the clear water is let into the fish pond. Siltation could also be done in a storage tank made out of old oil drums, etc. The important thing is that something be constructed or set up so that the silt has a chance to fall out of the water before that water goes into the pond.

The silt must be removed from the siltation tank or pond every so often. The silt which is removed should be used in gardens and fields: it is very fertile.

Build the Walls

The walls (dikes, dams, levees) have to withstand the pressure of all the water in the pond. They also have to be watertight to keep the water inside the pond.

The construction of the walls depends upon the kind of soil in which the pond is being built.

A soil which is a mixture of sand and clay is best. If pure clay is to be used, it must be mixed with other soil before it can be used. Pure clay will crack and leak. Do not use turf, humus, or peaty earth. All stones, pieces of wood, and other materials which might rot or otherwise weaken the wall must be removed before building begins. If the soil contains enough clay, the walls can be built by placing layers of soil 20cm deep over the drainage pipes and tamping each layer down until it is compact.

The finished height of the wall should be about 30cm above water for small ponds and 50cm above water for large ponds. The width of the wall at the top should be about equal to its height. For a large pond, the wall is never less than 1m wide at the top; most walls are built so that two people can walk side by side along the top.

Tamp the soil down with a simple tamping tool. Some people use a large rock or even their own weight by jumping up and down on the soil. The important thing is that the soil must be packed down very tightly.

One way to build pond walls in soil that does not have a lot of clay or is very sandy is to build a "key." The key is made of clay soil (it can be pure clay) and adds strength to the walls. To make a key, dig a trench (or shallow hole) about 1m deep and 1m wide in the center of the places where the walls will be. Then bring clay soil and pack it tightly into the trench. Also put a thick layer of clay soil on the pond bottom and pack that down tightly. The clay layer on the bottom and the key run together as shown. This connection of the bottom and the key helps prevent leaking. The drainage pipe should be placed in the clay lining.

If the farmer has a soil which is a mixture of clay and sand, and he is not sure it is strong enough, he may still wish to build a clay key. Or he can build a key using the same soil used in the wall. This key must be packed down very tightly.

The type of soil determines the ways in which the pond can be prepared so water does not leak out (see "Seal the Pond Bottom", below).

The soil also determines the slope of the walls. Soil with a lot of clay in it can have a greater slope on the outside wall than on the inside wall. A typical wall is built with an outside slope of 1:1 and an inside slope of 1:2. A slope of 1:2 means that for every change in length of 2m there is a change of 1m in height.

Once the walls are constructed, the farmer should plant grass on them. The grass roots help to hold the wall together and prevent erosion of the soil. However, NEVER plant trees on the wall. As the tree roots grow they will crack and destroy the wall.

Seal the Pond Bottom

The last step in pond construction is sealing the pond bottom so that it does not leak. If the soil has a lot of clay in it, no special sealing is needed. If the bottom is sand or gravel, it should be sealed to help it hold water. One way to seal the pond is to build a clay core into the wall and extend the clay over the bottom of the pond as a lining. This kind of sealing must be done when the walls are built. After the walls are built, there are other methods you can use for sealing the pond.

A pond can be sealed using hollow cement blocks, but this is expensive. Another method of sealing the bottom calls for using a sheet liner made of polyethylene plastic, or a rubber liner. The waterproof sheet is placed on the pond bottom and around the sides in one piece (the farmer may have to tightly seal several sections together), then covered with soil.

Another technique, recently developed in the USSR, is called a "gley" or "biological plastic." "Gley" can be made in the pond in this way:

* Clear the pond bottom of debris, rocks, and all other materials.

* Cover the pond bottom and sides completely with animal manure. Apply the manure in an even layer.

* Cover the animal manure layer with banana leaves, cut grasses, or any vegetable matter. Make sure that all of the manure is covered.

* Put a layer of soil on top of the vegetable layer.

* Tamp the layers down very well.

* Wait 2 to 3 weeks before filling the pond.