6. FERTILIZING FISH PONDS

The use of fertilizers to increase the production of natural food for fish

6.0 Introduction

1. Fertilizers are natural or synthetic substances that are used in ponds to increase the production of the natural food organisms to be eaten by the fish. These organisms include phytoplankton*, zooplankton* and insects (see Chapter 10, ). They are all part of a complex food web* converging toward fish production. By increasing the availability of major nutrients, fertilizers promote the development of planktonic algae, which provide food for many fish. Fertilization also leads to the development of animals which feed on algae, including some fish such as the Chinese silver carp and the Nile tilapia.

2. When a fertilizer is added to a fish pond, the chemicals it contains dissolve in the water, where:

3. This second portion may also assist the development of bacteria, responsible for the decomposition of organic matter. The decomposition of organic matter may in turn release more nutrients back into the mud or water. The chemicals attached to soil particles may also later be released back into the water slowly, over a long period of time. They may also migrate deeper into mud and soil, where they will no longer affect the water body, unless the pond bottom is dried or ploughed (see Section 2.5).

4. Most of these phenomena are linked with and controlled by water quality and in particular temperature, pH, alkalinity and dissolved oxygen level.

The action of chemicals contained in fertilizers when dissolved in water

Different types of fertilizer

5. Pond fertilizers form two distinct groups:

6. Both types of fertilizer have advantages and disadvantages, as listed in Table 13. Select the most appropriate type of fertilization for your own needs. Selection may depend not only on local availability but also on the size of the farm.

(a) Small-scale fish farms tend to rely on organic fertilizers as they are cheap and available locally.

(b) Large-scale fish farms most often adopt inorganic fertilizers, as they are more easily stored and distributed.

7. The best results can often be obtained with the combined use of both types of fertilizer.

TABLE 13
Comparison of organic and inorganic fertilizers
Item
Organic fertilizers
Inorganic fertilizers
Storage
Difficult, only short time
Easy, possibly for long time
Distribution
Difficult, esp. on larger scale
Easy
Mineral content
Variable, low
Consistent, high to very high
Organic matter
Present
Absent
Effect on soil structure
Improvement
No
Direct food for fish
Yes
No
Decomposition process
Yes, with oxygen consumption
No
Price
Low to medium
High to very high
Cost per nutrient unit
Higher
Lower
Availability
Possibly in neighbourhood or even on own farm
Commercial suppliers only; sometimes imported
Direct pond fertilization
Possible by raising animals on or near the pond
Not feasible

Making the best use of fertilizers

8. When using fertilizers to increase fish production in your ponds, you should aim to establish and maintain a dense growth of planktonic algae (phytoplankton) and zooplankton, which should colour the water a rich shade of green. Such dense planktonic growth is often called a plankton bloom.

9. To establish and maintain a good plankton bloom at minimum cost, watch for the following points.

(a) Pond water and bottom soil should be neutral or slightly alkaline. Lime them if necessary (see Chapter 5).

(b) If present, bottom mud should be good quality, not too thick and mostly made of fine detritus; too much cellulose* slows down its decomposition. Control the emersed vegetation and the mud thickness, if possible by draining and drying (see Section 2.5).

(c) Reduce the competition for nutrients and sunlight by controlling the floating and submersed vegetation (see Section 4.9).

(d) Reduce the water exchange rate as much as possible to avoid draining away water rich in nutrients and plankton.

(e) Fertilize each pond according to its particular characteristics; for example, use more fertilizer:

(f) Add more fertilizer as needed according to the plankton density (see Section 10.1, Management, 21), using small amounts regularly, if possible.

Chemical qualities of a poor water supply
Phosphates
Less than 0.1 mg/l
Nitrates
Less than 2 mg/l
Potassium
Less than 0.1 mg/l
Calcium and magnesium
Less than 15 mg/l

10. If supplies of fertilizer are limited, give priority to the ponds where the availability of natural food is most important, for example nursery ponds and broodstock ponds.

11. Do not fertilize a pond if:

Deciding about the need for fertilizers

12. The Secchi disc transparency (see Section 2.3) can be used as a simple method for judging plankton turbidity and the need for additional fertilization of a fish pond. Depending on the value observed, control and manage the pond as shown in the following chart.

Fertilization needs
Secchi disc transparency
Management/control
Less than 25 cm
No fertilization:
Closely observe fish for signs of dissolved oxygen depletion (see Section 2.5)
Increase water inflow, if necessary
25-40 cm
No fertilization:
Regularly observe fish behaviour
40-60 cm
Routine fertilization necessary
More than 60 cm
Routine fertilization necessary, possibly with an increased dose

13. If you do not have a Secchi disc, you can use your arm instead. Stick your arm vertically underwater. As long as your hand is not visible when your elbow is at the water surface, there is no need for fertilization.

Note: avoid over-fertilization. It is both wasteful and dangerous for your fish.

14. The next sections tell you more about organic and inorganic fertilizers, and how best to use them.

Checking the need for fertilizers
by using a Secchi disc

 
By immersing your arm in the water

6.1 Inorganic fertilizers in fish farming

Different kinds of inorganic fertilizer

1. An agricultural inorganic fertilizer may contain several types of nutrients:

2. However, the fertilizers are named only according to the primary nutrients they contain.

(a) Fertilizers which contain only one or sometimes two primary nutrients retain their chemical name such as superphosphate (P) or ammonium phosphate (N + P) (see Table 14).

(b) Those with two or three primary nutrients, known as mixed fertilizers, are referred to by their NPK grade, that is, their percentages, as weight, of the three primary nutrients;

Example

100 kg of a mixed fertilizer called 8-8-2 contain 8 kg nitrogen, 8 kg P2O5 equivalent and 2 kg K2O equivalent, the remaining weight being mostly made up of inactive filler. Similarly, a 10-20-0 fertilizer contains 10 percent N, 20 percent P2O5 equivalent and 0 percent K2O equivalent.

Note: to determine how much pure phosphorus P or pure potassium K a mixed fertilizer contains, multiply the equivalent values as follows:

Example

The 100 kg of 8-8-2 mixed fertilizer (see previous example) contain:

3. Inorganic fertilizers most commonly used in fish farming are listed in Table 14. Exact concentrations of nutrients may vary from one supplier to another, according to the origin of the fertilizer. In addition, many types of mixed fertilizer are commercially available according to the local needs of plant crops. Contact your local office for agricultural development to obtain information about locally available inorganic fertilizers.

     TABLE 14
Concentration of primary nutrients in common inorganic fertilizers (in percentage, by weight)
Fertilizer
Phosphorus
Nitrogen
Potassium
Calcium present
Water solubility
eq. P2O5
P1
N
eq. K2O
K2
PHOSPHATE
             
Basic slag
16-20
7.0-8.8
-
-
-
40% CaO
Poor if Ca high
Superphosphate
14-20
6.2-8.8
-
-
Yes
High (85%)
 
Triple superphosphate
44-54
19.4-23.8
-
-
-
Yes
High (85%)
NITROGEN
             
Ammonium nitrate
-
-
33-35
-
-
-
High
Ammonium sulphate
-
-
20-22
-
-
-
High
Ammonium phosphate
20-48
8.8-21.1
11-16
-
-
-
High
Di-ammonium phosphate
48-52
21.1-22.9
18-21
-
-
-
High
Calcium nitrate
-
-
15-16
-
-
Yes
High
Sodium nitrate
-
-
15-16
-
-
-
High
Urea
-
-
42-47
-
-
-
High
POTASSIUM
             
Kainite (MgS04+KCI)
-
-
-
20
16.6
-
High
Potassium nitrate
-
-
13-14
44-46
36.5-38.2
-
High
Potassium sulphate
-
-
-
45-54
37.4-44.8
-
High
Muriate of potash
-
-
-
50-62
41.5-51.5
-
High
 

1Phosphorus: to be multiplied by 2.29 to obtain P205 equivalence
2Potassium: to be multiplied by 1.2 to obtain K20 equivalence

Selecting inorganic fertilizers

4. Generally phosphorus is the limiting primary nutrient most often missing in natural water supplies for good growth of planktonic algae. Therefore, phosphate fertilizers are usually the most effective inorganic fertilizers for fish ponds in most regions of the world.

5. Nitrogen fertilizers are sometimes useful, especially in temperate climates and in watersheds where agriculture is little developed. They are mostly used to avoid an unbalanced P:N ratio in the water, which is a risk when using phosphate fertilizers alone. If the P:N ratio is too high, blue-green algae may bloom instead of the more desirable green algae. In the tropics, where fixation of nitrogen by bacteria and algae is much more active, nitrogen fertilizers are less needed. In older ponds with a good layer of mud, they are usually unnecessary.

Note: some nitrogen fertilizers, such as ammonium compounds and urea, are acid forming. When applied to a pond, additional liming may be required to maintain water pH and total alkalinity within adequate limits (see Section 5.1).

6. Potassium fertilizers are not generally necessary except in specific locations where there is a deficiency in potassium. This situation may happen in ponds built in swampy areas or in peaty soils. Additional potassium may also be useful in ponds with a hard bottom and little aquatic vegetation.

7. Before spending too much money on inorganic fertilizers, you should check on the following.

(a) The chemical quality of the water supply: have at least one good chemical analysis carried out, checking on total phosphorus, phosphates, nitrogen, nitrates and potassium concentrations as well as total alkalinity and pH. If possible, check again during different seasons.

(b) The nature of the pond bottom soil (sandy/light or clayey/heavier) and its chemical characteristics should be examined, such as pH and concentrations of calcium and primary nutrients.

(c) The water solubility of the fertilizer is important (see Table 14), especially for phosphate fertilizers. Look for:

8. Further assistance is given in Table 15. Also remember that some of the substances you are using for pest control (see Sections 4.6 and 4.7) have a fertilizing effect. Calcium cyanamide, for example, contains 18 to 22 percent nitrogen.

  TABLE 15
Criteria for the use of inorganic fertilizers
 
Phosphate fertilizers
Nitrogen fertilizers
Potassium fertilizers
Water quality: desirable concentrations for good algal production:
Phosphates
> 0.2 mg/l
Total P > 0.4 mg/l
Nitrates
> 2 mg/l
Total N > 1.5-3 mg/l
Potassium
> 1 mg/l
-
Best P:N ratio
Enough total P present for total N to be used: P:N = 1:4 to 1:8
-
Inorganic fertilizer advisable for:
  • poor water/soil
  • acid soil or light soil and/or water poor in Ca: prefer basic slag
  • soil/water richer in Ca and/or heavy soil: prefer superphosphate
  • poor water/soil
  • new ponds
  • ponds with no bottom mud
  • nursery ponds
  • more intensive cultural system, at higher fish density
  • poor water/soil
  • water total alkalinity less than 25 mg/l CaCO3
  • ponds in swampy areas
  • peaty soil
  • hard pond bottom and little aquatic vegetation
Typical amount, per hectare, for one production cycle
30-60 kg eq. P205
40-100 kg N (check P:N ratio)
35 kg K2O to 60-80 kg in peaty soil

Mixing inorganic fertilizers with other substances

9. To save on time and labour, inorganic fertilizers are commonly mixed with each other or with substances such as organic fertilizers or liming materials.

10. However, you should avoid certain mixtures in which the components react with each other, and fertilizer quality deteriorates.

11. For maximum safety, check the chart below and apply the following rules:

(a) Never mix the following together and wait for at least two weeks between separate distributions:

(b) You may mix together, but only just before use:



Storing inorganic fertilizers

12. It is best to avoid having to store inorganic fertilizers for too long. Buy in small quantities, only as much as you need, and store them for the shortest possible time, particularly nitrogen and potassium fertilizers.

13. Protect fertilizers from humidity and rain by storing them on a wooden platform under a roof, by wrapping them in plastic sheets or by doing both of these things.

14. Do not buy any fertilizer which shows signs of being wet.

Protecting stored fertilizers
 
Wrap in plastic

Fertilizing your fish ponds

15. Unless the production cycle is very short, inorganic fertilizers are usually applied to fish ponds at regular intervals:

16. Application of inorganic fertilizers during the production cycle should be based on observations of both water quality and fish behaviour (see Section 6.0). The following chart will assist you in deciding whether or not to fertilize your pond.

Fertilize your pond inorganically if at least one of the following is present
Yes
No
Water temperature at noon
Above 16C
Below 16C
Secchi disc transparency
More than 40 cm
Less than 40 cm
Water pH at sunset
Less than 9
More than 9
Dissolved oxygen before sunrise
More than 3 mg/l
Less than 3 mg/l

Choosing the quantity of inorganic fertilizer

17. As you learned earlier, the kind and amount of inorganic fertilizer to use can vary greatly from one pond to another. It is therefore not possible to recommend any specific mixture or dosage which would give best results in all locations. Table 15, however, summarizes points to consider for further guidance.

18. It is best to determine the amount of fertilizer needed in each of your ponds by trial and error as follows:

(a) When the pond is full of water, make an initial application of phosphate fertilizer equivalent to 125 to 175 g P2O5/100 m2 or 12.5 to 17.5 kg/ha.

(b) If additional nitrogen is required, apply sufficient nitrogen fertilizer to give a ratio of 4 to 8 N for each phosphorus unit, taking into account if possible the nitrogen already present in the pond to avoid wasting fertilizer. The richer the water and the higher the fish density, the higher this nitrogen proportion should be.

Example

You want to plan a fertilization programme for your pond. You obtain superphosphate containing 20 percent P205 equivalent (Table 14) and decide to apply it at the rate of 150 g P2O5/100 m2

(a) How much superphosphate do you need per 100 m2?

(b) How much nitrogen do you need per 100 m2 for a ratio P:N = 1:6?

(c) How much nitrogen fertilizer should you apply per 100 m2?

If there is little nitrogen already in the pond or if you are not sure of the levels, add nitrogen according to the phosphorus and nitrogen amounts calculated above. You will need 400 g N. If, for example, you use ammonium nitrate with 35 percent N (Table 14), you need to apply (100 g x 400 g) 35 g = approximately 1 140 g/100 m2 pond.

If you know what the nitrogen level is (for example, by measuring it at the time), you may save on fertilizer. If, for example, total nitrogen N = 3 mg/l = 3 g/m3 and water is 1 rn deep:

(c) Seven to ten days after applying the first fertilizer dose, measure the Secchi disc transparency before applying a new dose, and decrease or increase the previous amounts of fertilizer accordingly.

(d) Repeat this process at regular intervals of seven to 15 days so as to maintain the SD transparency at between 40 and 60 cm throughout the production cycle.

(e) Keep checking on water quality and fish behaviour to further modify your fertilization programme if necessary (see criteria in chart above).

Note: if potassium fertilizer is required, use 0.35 to 0.80 kg K2O/100 m2 according to local conditions (see Table 15).

Distributing inorganic fertilizers

19. To apply inorganic fertilizers, remember the following recommendations.

(a) Apply regularly at very short intervals of preferably seven to 15 days, especially if the pond bottom is sandy and little mud has built up over it.

(b) For each treatment, use small doses of fertilizer.

(c) Before the treatment and for a few days after, reduce the water inflow as much as possible.

(d) For best results, never throw solid fertilizers directly into the pond water. This is especially important for phosphate fertilizers, because the bottom mud or soil can quickly turn the soluble phosphorus into insoluble compounds, which are then of limited use for the pond water.

20. There are two main methods for fertilizing ponds.

21. You may use the dry inorganic fertilizers directly and let them dissolve slowly. Water currents help to disperse the dissolved chemicals through the entire pond area. In small ponds, use at least one fertilizing point per 1000 m2 of water area. In larger ponds, use two to three points per hectare.

22. A number of methods for distributing dry inorganic fertilizers are illustrated:

(a) From a wooden post, suspend a small bag made of cotton or burlap, about 30 cm underwater. In this bag enclose the seven- to 15-day dosage of fertilizer for the water area concerned. At the end of this period, empty the insoluble filler from the bag and add a new dosage of fertilizer. You could also use a perforated can or a basket.

(b) Use a floating perforated container such as a woven basket or a plastic can with holes, attached to the inside  of a car tyre inner-tube. Use this container as you were told in item (a) above.

Woven basket suspended inside an inner-tube

(c) Submerge a wooden platform 30 cm underwater and set it at least 30 cm above the pond bottom. The doses of fertilizer are placed on the platform, either directly, if there is no risk of it being swept away or, when water currents are too strong, in open fertilizer bags.

23. For better results you may use dissolved inorganic fertilizers in two ways.

(a) For periodic pond fertilization, dissolve the required dose of fertilizer well in a large container such as a clean 200-l metal drum filled with pond water. Using buckets, distribute this solution evenly over the whole pond surface from the banks or, if necessary, from a boat.

(b) For continuous pond fertilization, build one or more emersed wooden platforms in the pond. On these platforms, permanently install a large container such as a 200-l metal drum, equipped with a small outflow valve about 10 cm above its base. You will need about five such containers per hectare. Fill each container with 100 to 150 l of water and dissolve the appropriate dose of fertilizer in it. Open the valve just enough to let the solution flow out steadily over a period of several days.

Metal drum with valve to release solution

6.2 Organic fertilizers: animal manures

1. In many instances, especially for small farmers, organic fertilizers are the most effective way of increasing natural food supply in ponds to improve fish production.

Different kinds of organic fertilizer

2. Several kinds of organic material, mostly waste materials, can be used as organic fertilizers. Most common are the following:

3. In the next paragraphs, you will learn about animal manures. You will learn more about the other kinds of organic fertilizer in Sections 6.3 and 6.4.

Animal manures as organic fertilizers

4. As pond fertilizers, animal manures have such great advantages that they should be preferred whenever' possible.

5. As direct food, they can partly replace supplementary feeds (see Section 10.3). For example, on manuring days additional feeding may be cancelled. Some fish, such as the Nile tilapia, may even be produced in large quantities without any additional feeding.

6. They are a source of additional carbon dioxide (CO2), which is very important for the efficient utilization of the nutrients present in the water. This is especially so when used together with inorganic fertilizers.

7. They increase the abundance of bacteria in the water, which not only accelerate the decomposition of organic matter (see Section 2.0), but also serve as food for the zooplankton, which in turn also increases in abundance.

Beneficial effects of organic fertilizers

8. They have beneficial effects not only on the bottom soil structure but also on the bottom fauna, such as the chironomid larvae.

9. However, animal manures also have some disadvantages, mostly related to their low content in primary nutrients, their negative effect on dissolved oxygen content and the reluctance of some fish farmers to use animal wastes directly in fish ponds.

The composition of animal manures

10. The chemical composition of organic manure varies greatly according to the animal from which it originates - namely the species, age, sex, the nature of its diet - and according to the way the manure is handled, namely its relative freshness, conditions of storage and rate of dilution with water. In some cases, total wastes made of dung and urine are available, while in others only solid wastes can be collected.

Note: it is best to use cattle and pig manures fresh. Chicken and duck manures can be stored.

11. Throughout the world, most animal manure is obtained from a limited number of species such as buffalo, cattle (bullock, dairy cows or fattening beef), horses or donkeys, sheep, goats, pigs, rabbits and poultry (chicken, ducks, geese). Examples of solid manure composition in primary nutrients NPK, on a dry weight basis, are given in Table 16.

12. Chicken droppings are the richest in nutrients. Pig dung is usually richer than sheep or goat dung. Manures from cattle and horses are poorer in nutrients, especially when the animals feed on grass only. Their fibre content is relatively high. Buffalo dung is the poorest manure of all.

   TABLE 16
Examples of the NPK composition of animal manures1 (percent of oven-dry weight)
Animal/poultry
Country
Nitrogen N
Phosphorous P
Potassium K
Buffalo dung
China
1.50
0.55
0.40
India
0.75
0.20
2.00
Horse dung
India
1.88
0.52
1.00
USA
2.00
1.20
0.80
Cattle dung
India
1.65
0.44
0.83
UK
2.98
0.41
1.78
Sheep dung
India
1.55
0.70
0.72
USA
1.89
1.35
0.54
Goat dung
(Asia)
2.04
0.73
0.47
Pig dung
China
2.66
1.37
1.47
USA
3.03
1.66
1.60
Rabbit droppings
-
1.72
1.30
1.08
Goose droppings
Hungary
0.6
0.22
0.83
Duck droppings
Mean values
2.15
1.13
1.15
Hungary
1.00
0.62
0.50
Chicken droppings
China
5.14
1.98
2.03
India
2.87
1.28
1.95
USA
4.59
2.33
1.96

1 R.V. Misra and P.R. Hesse, Comparative analyses of organic manures, FAO/UNDP Regional Project RAS/75/004, Project Field Document 24, 97p.

How much manure do animals produce

13. The total amount of manure produced daily by various animals depends mainly on their live weight, as shown in Table 17. Pigs, for example, produce a daily average of about one-tenth of their live weight in total wet wastes, consisting of solid wastes and urine. A little less than half of this is made up of solid manure.

14. Depending on the conditions under which the manure can be collected, it may be mixed with other kinds of organic matter such as:

15. In these cases, the quantity of organic fertilizer collected daily may be higher. Its quality also changes according to the materials added to it.

Average yearly weight of total animal manure produced
by various farm animals (per 100 kg of live weight*)


* Based on manure free from bedding and collected
from confined animals Note: see also Table 17

    TABLE 17
Estimated daily production of farm animal wastes
Animal /poultry
Live weight
LW(kg)
Total wet wastes1 per day
Solid wastes per day
Dry Matter
Total fresh wastes1
(Solids only)
Total dry matter1
(Solids only)
Total wet wastes1
Solid wastes
kg/100 kg   LW/day
% LW
kg
% LW
kg
%
g/d
%
g/d
Buffalo
250
6.4
16
4.5
11
17
2720
19
2100
   
500
5.7
28
4.0
20
 
4760
 
3800
   
Cattle
210
6.2
13
4.7
9
 
1950
 
1800
6.0
0.95
350
6.0
21
4.3
15
15
3150
20
3000
(4.5)
(0.85)
450
6.0
27
4.2
19
 
4050
 
3800
   
550
6.2
34
4.4
24
 
5100
 
4800
   
Horse
380
5.0
24
3.5
13
20
4800
23
3000
5.0 (3.5)
1.0 (0.7)
Sheep/goat
30
7.0
2.1
3.5
1.0
28
590
35
368
7.0 (3.5)
1.9 (1.2)
Pig
13-36
10.0
2.5
5.3
1.3
 
350
 
390
   
36-54
8.0
3.0
4.3
1.6
14
420
20
480
8.0
1.1
54-72
6.0
3.5
3.2
1.9
 
490
 
570
(4.3)
(0.9)
72-90
4.5
3.5
2.7
1.9
 
490
 
570
   
Duck
2-3
11
0.275
-
-
43
118
-
-
11.0
4.7
Chicken
1-1.5
7
0.080
-
45
36
-
-7.0
3.2
   

1 Solid wastes and urine

Selecting the best animal manures for your fish pond

16. If several types of manure are available, choose the best for fertilizing your pond according to the following criteria.

(a) The manure should be easily soluble and dispersable in water. Liquid manure or solid poultry wastes are preferred, because cow or horse dung usually contains a lot of insoluble cellulose especially if mixed with stable litter.

(b) It should be in small particles rather than in lumps.

(c) Use it as fresh as possible. Large losses of nitrogen and carbon occur during storage, especially if the manure is left in the open air and in the rain.

(d) Make sure it has a high nutrient content, as discussed above (see also Table 16).

(e) Manure should be easy to collect. Housed or corralled animals produce more concentrated manure than free- roaming ones. Animal housing can be designed to improve the collection and distribution of manure to the ponds (see Chapter 7).

Note: in new ponds with sandy soil, cattle manure with higher fibre content may be preferable to help form the bottom mud.

When to fertilize your ponds with animal manure

17. Fish ponds are usually fertilized with animal manure at least ten to 15 days before stocking with fish. In drained ponds, the manure is applied to the pond floor just before refilling with water (see paragraphs 30 to 33). In undrained ponds, the manure is applied to the water (see paragraphs 36 to 38).

18. After the first application, the pond should be fertilized at regular Intervals throughout the fish production cycle. For best results, fertilize your ponds with manure frequently, at short intervals preferably not less than once a week. Daily applications are best.

Recent research has shown that best fertilization results are obtained in ponds where a constant flow of organic carbon is maintained through the entire food web (see Section 10.1). Dense populations of planktonic algae, bacteria and zooplankton are thus established and kept relatively stable, preventing sudden high peaks of abundance of various groups. The water quality, such as dissolved oxygen content, also remains much more stable and favourable for fish production.

19. As you have already learned (see Section 6.1), you should monitor your pond carefully during fertilization to avoid fish losses. This is especially important if you are using animal manure. Continue to fertilize a pond only if:

20. If you do not have enough animal manure to fertilize all your fish ponds, give priority to those where its effects are especially beneficial:

Fertilize your pond organically
Yes
No
Water temperature at noon
Above 20 C
Below 20 C
Secchi disc transparency
More than 40 cm
Less than 40 cm
Water pH at sunset
Below 9
Above 9
Dissolved oxygen before sunrise1
Above 3-4 mg/l
Below 3-4 mg/l

1 Depending on the fish species present in the pond (see Section 2.5)

Using animal manure safely

21. Because of the increased demand of dissolved oxygen caused by the addition of organic matter to the pond water, you should limit the amount of animal manure to be applied at any one time. This safe maximum amount is usually expressed in kilograms (kg) of dry matter (DM) per hectare (ha) per day (d) abbreviated as kg DM/ha/d.

22. The safe maximum amount of animal manure in cooler and temperate climates is 60 kg DM/ha/d or 0.6 kg DM/100 m2 /d; in warmer and tropical climates, the safe maximum is 120 kg DM/ha/d or 1.2 kg DM/100 m2 /d.

23. To determine how much dry matter (DM) a specific manure contains, you can measure it yourself. Take exactly 100 g of the manure you want to use and place it in a dry container such as a tin. Weigh the manure and the container (W1grams). Place the open container in an oven and heat it to about 180 C for four hours. Weigh the manure and container again (W2 grams). The difference (W1 - W2 = W3 grams) is the moisture content (in percent) of the manure. Obtain the dry matter weight DM (in g) = 100 g - W3. This is also the percentage of dry matter present in your manure.

Example

You weigh 100 g of cow manure collected from a farm and place it in a tin. The total weight W1 = 185 g. After drying the sample in an oven, you find W2 = 145 g. The moisture content of the manure is W3 = 185 g - 145 g = 40 g. The dry matter content of your manure is DM = 100 g - 40 g = 60 g or 60 percent.

24. Alternatively you can use Table 17, next to last column, which gives you the average dry matter content in percent of the solid wastes, for fresh manures. These values are only approximate and may be very different from the real value of the actual manure you want to use. For example, fresh cow manure usually contains 20 percent DM but dried cow manure may contain as much as 80 percent DM. It is thus always best to measure the DM content if you can.

25. Then, you can easily determine:

Note: if you are using liquid manure, you should be even more careful because of its relatively high content in ammonia, a very toxic gas for fish. Do not apply more than 1000 litres/ha/d = 10 l/100 m2 /d.

Examples

(a) 100 kg of fresh solid pig manure contains about 20 kg dry matter. If you have 375 kg, it contains 20 kg x 3.75 = 75 kg dry matter. In a cool climate, you can safely apply 60 kg DM/ha/d. Therefore in this case, you could daily apply the available pig manure to 75 kg DM 60 kg DM/ha/d = 1.25 ha of ponds.

(b) Maximum amount of fresh solid manure to be safely applied per day in 100 m2 of tropical fish pond is shown in the chart (see also Table 17):

Solid   manure
Maximum amount (kg fresh/100 m2 /d)
Buffalo
6.3
Cattle
6.0
Horse
5.2
Sheep/goat
3.4
Pig
6.0
Duck
2.8
Chicken
4.8

Beware: if you do not use manure every day but only once a week, it does not mean that you can safely place on one day seven times as much manure into your pond. The safe maximum amount remains the same. If you wish to use more manure, you should reduce the interval between two consecutive applications. Place the manure two or three times a week.

Controlling the amount of animal manure to apply

26. The amount of animal manure to be applied to a particular pond varies greatly, depending on factors such as climate, water and soil quality, characteristics of the manure and kind of cultural system (type of fish, rearing density, length of rearing period). As for inorganic fertilizers, it is impossible to recommend any treatment valid under all circumstances.

27. For best results according to your particular case, you may apply one of the following procedures.

(a) As an approximate guide, in small tropical rural ponds generally from 100 m2 to 300 m2 in size, apply once or preferably twice a week, one of the following:

(b) For more controlled application in a warm climate, at relatively low rearing densities of fish:

(c) In a warm climate, at higher rearing densities of fish:

Example 1

In the tropics, you wish to manure a 400 m2 pond stocked with a low density of fish, using fresh chicken manure containing 25 percent dry matter. You may proceed as follows.

(a) Ten days before stocking, apply fresh chicken droppings at the equivalent rate of 15 kg/DM/100 m2 or 15 x (100 25) = 60 kg droppings per 100 m2. You will need 4 x 60 kg = 240 kg droppings to manure your pond.

(b) Seven days after stocking, you measure SD transparency = 60 cm. Apply fresh chicken manure to the pond at the equivalent rate of 0.5 kg DM/100 m2 or 0.5 x (100 25) = 2 kg droppings per 100 m2 . Apply 4 x 2 kg = 8 kg droppings to the whole pond.

(c) Seven days later, you measure an SD transparency = 55 cm. Manure the pond at the increased equivalent rate of 0.8 kg DM/ 100 m2.

(d) Every week, check on water quality and adjust manuring.

Example 2

In the tropics, you wish to manure an old 250 m2 pond stocked with 10 kg of Nile tilapia fingerlings using fresh solid pig manure containing 20 percent dry matter. You may proceed as follows.

(a) Ten days before stocking, apply the pig manure at the equivalent rate of 10 kg DM/100 m2or 10 x (100 20) = 50 kg pig manure/ 100 m2 . To manure your pond, you will need 2.5 x 50 kg = 125 kg fresh pig manure.

(b) Three days after stocking, you measure SD transparency = 50 cm. Manure the pond at the equivalent rate of dry matter/100 m2 equal to 1:10 of the fish weight/100 m2 . As there are 10 kg fish/ 250 m2 = 4 kg fish/100 m2, you need (4 kg 10) = 0.4 kg DM/100 m2. You will need 0.4 x (100 20) = 2 kg pig manure per 100 m2 or (2 kg x 2.5) = 5 kg pig manure for your pond.

(c) If you have enough manure, apply 5 kg pig manure each day thereafter for two weeks, carefully checking the water quality and fish behaviour in the pond.

(d) After two weeks, if the water quality permits, slightly increase the daily amount of pig manure, for example up to 6 to 6.5 kg/day. If necessary, adjust this amount during the next days.

(e) Every two weeks, slightly increase the daily amount of the manure until you reach the safe maximum of 6 kg/100 m2 /day or (6 kg x 2.5) = 15 kg/day for your pond.

Mixing animal manures

28. If you do not have enough of one kind of animal manure, you may have to mix two kinds of manure to obtain the total amount of dry matter required for your ponds.

29. Calculate how much of the second manure (M2 in kg) you require as:

M2 = [DM tot - (M1 x DM1)] DM2

where

M1 = available quantity of first manure, in kg;
DM1 = dry matter content of first manure, in percent;
DM2 = dry matter content of second manure, in percent;
DM tot = total dry matter needed to manure the ponds, in kg.

Example

The ponds of your fish farm cover an area of 2 ha or 2 x 10000 m2 = 20000 m2 . To manure the pond bottoms properly, you require 10 kg DM/100 m2 and a total dry matter of 10 kg x (20000 m2 100 m2) = 2000 kg. This is equivalent to 2000 x (100 25) = 8000 kg of chicken droppings, at 25 percent dry matter content. However, you can obtain only 3000 kg of chicken droppings, and the rest of the manure has to be solid pig waste, 20 percent dry matter content.

Calculate how much pig waste (M2) you will need to mix with the chicken droppings before treating your ponds, as follows:

and therefore M2 = [2 000 kg - (3 000 kg x 0.25)] 0.20 = 1 250 kg 0.20 = 6250 kg.

Applying animal manures to a drained pond bottom

30. The dry bottom of a drained pond should be manured at least two weeks after it has been limed (see Section 5.4). Inorganic fertilizers may be applied at the same time, except if they contain too much calcium such as basic slag (see Section 6.2).

31. It is usually best not to spread the manure all over the pond bottom but instead to stack it in heaps or in rows at regular intervals.

Note: a number of methods for the placement or the distribution of animal manure in various situations are illustrated below. However, these illustrated examples are general in nature and must be adapted according to local conditions (quality and quantity of manure available, water quality, weather, etc.).

If the water flow can be controlled, stack the animal manure
at regular intervals over the pond bottom in heaps ...


... or in rows

If the water flow can not be controlled, and the water
exchange might be too great, stack the animal
manure at regular intervals along the pond banks ...

32. In new ponds with a sandy bottom, the manure should be spread all over the pond bottom area and worked into the surface soil. It is preferable to use either cattle and/or horse manure with stable litter or chicken droppings.

33. As soon as the manure has been applied, start filling the pond with water.

... however in new ponds with a sandy bottom, spread the animal manure
over the entire bottom area and work it into the surface soil

Applying animal manures to water-filled ponds that have not yet been stocked

34. If the fish have not yet been stocked in the pond, pure animal manure may be applied evenly over the entire water surface.

Applying pure animal manure from a boat

35. However, if it is manure mixed with stable-litter, it is best to stack it in heaps along the banks. Mix these heaps from time to time.

Stacking animal manure mixed with stable-litter in heaps along the banks

Applying animal manures to water-filled ponds that have been stocked

36. Once the pond has been stocked with fish, you may  apply liquid manure directly over its entire water surface.

Applying liquid animal manure from the banks

37. However, solid manure should only be applied indirectly. Use one of the following methods:

(a) A floating basket attached to the inside of a car tyre inner-tube. Prepare the amount of manure to be applied to the pond. Fill the basket with some of this manure, place it in the inner-tube and attach two long ropes. Pull the floating basket from bank to bank across the entire water surface. The manure will soften and gradually dissolve in the water. Refill the basket when necessary until all the manure has been distributed.

Applying solid animal manure using an inner-tube and basket

(b) Manure heaps. Place the manure underwater in heaps at regular intervals along the pond banks.

Stacking animal manure in heaps along the banks

(c) A long crib built next to the dike on one or more sides of the pond. If you are using this kind of crib (see Sections 6.3 and 6.4), spread and mix the manure the full length of the crib.

Detail of a long crib

(d) In smaller ponds, it is best to build a crib in each of the two shallow corners of the pond. Fertilize the cribs as you were told in item (c) above.

Detail of a corner crib
 
Put a crib in each of the two shallow corners

38. When planning the fertilization of your ponds, remember the following:

(a) It is preferable to use animal manure as fresh as possible. Check if it is available when you need it.

(b) There should be at least a 15-day interval between liming and manuring.

(c) Apply manure preferably in the early hours of the day, about two to three hours after sunrise.

(d) Best results are obtained by combining manuring with inorganic fertilization. Additional phosphorus and nitrogen are usually beneficial to maintain a good plankton bloom (see Section 6.1).

(e) Always maintain and adjust your fertilization by checking water quality (see paragraph 19) and fish behaviour (see Section 2.5).

Note: remember that if you raise farm animals, it is an advantage to keep them close to your fish ponds. In some cases, you may even raise the animals in the pond, which is then automatically and continuously manured without extra cost or work. You will learn more about how to integrate animal husbandry with fish farming in Chapter 7.

6.3 Other organic fertilizers

1. Several organic fertilizers other than animal manures are commonly used for small-scale fish farming. These fertilizers are usually wastes that can be obtained cheaply from local sources. Part of the material is directly consumed by the fish, while the rest boosts the growth of the various food-web organisms used as food by the fish.

2. Among the most commonly used organic fertilizers other than animal manures, you can find:

Note: it is best to use blood for direct fish feeding (see Section 10.6).

Note: wastes such as rice hulls, sugar-cane stalks and sawdust are rich in cellulose, which decomposes very slowly in the pond. Do not use them too much, unless you are trying to establish a good pond bottom on sandy soils.

3.The average amounts of these organic fertilizers to be applied to small ponds are given in Table 18. Apply them regularly, avoiding overloading the pond with several weeks' supply. Check the water quality to control the quantities used. With the exception of slaughterhouse wastes and cassava tubers, these organic fertilizers are applied to the pond water in one or more heaps. You can also use either long or corner cribs as you were shown carlier. The fertilizing material is stacked and compacted inside to initiate underwater composting (see Section 6.4). As described above, it is best to turn over or at least mix the heap of decomposing material every week, before adding new organic matter.

TABLE 18
Organic fertilizers commonly used in small-scale fish farming
Organic fertilizer
Average amount applied at regular intervals
Animal manures
See Section 6.2
Cattle rumen content
(slaughterhouse wastes)
10 kg/100 m2/week
Cottonseeds for in-pond composting
(Agro-industrial wastes)
8 kg/100 m2/week
Biogas slurry
10-15 kg/100 m2/week
Cassava tubers
fermentation
50-100 kg/100 m2/week
10-25 kg/100 m2 /day
Vegetation for in-pond composting
(soft, aquatic or terrestrial)
20-25 kg/100 m2/week
Compost
(see Section 6.4)
20-25 kg/100 m2/week
50 kg/100 m2 pond bottom

4. Vegetation such as cut grasses, vegetable wastes and rotting fruits may be used for simple composting (see Section 6.4) in the pond itself.

(a) Build a frame of wood or bamboo about 2 m x 2 m depending on the size of the pond.

Wood or bamboo frame

(b) Attach it in the shallow end of the pond, about 1 m from the pond bank.

(c) Chop 200 to 250 kg of vegetal matter into small pieces, per 100 m2 of pond water.

(d) Heap all this vegetation into the frame so that it decomposes underwater. Compact the heap well by trampling it with your feet.

(e) After seven to ten days, turn the heap over and remove all undecomposed pieces that have remained hard.

(f) Add finely chopped vegetal matter on top of the heap and compact it well. You will need 20 to 25 kg/100 m2 pond.

(g) Seven to ten days later, turn the heap over, remove all hard pieces and add another 20 to 25 kg/100 m2 of finely cut vegetal matter.

(h) Repeat this procedure about every week.

6.4 Composting

Composting and fish production

1. Composting involves the intensive decomposition by micro-organisms of organic materials, generally under controlled conditions. This process makes it possible to utilize a wide range of cheap wastes, residues and natural vegetation for the production of a clean, dry material rich in organic matter and primary nutrients. This material is called compost.

2. As you have previously learned (see Section 6.3), compost may be used in fish ponds as an organic fertilizer. In drained ponds, compost is spread over the bottom area at the rate of 50 kg/100 m2 and mixed with the upper soil layer before refilling. Later, compost can be regularly applied to the pond to fertilize the water, with the great advantage that it does not increase the demand for dissolved oxygen as much as other organic fertilizers. For this reason, compost is particularly suitable for applying to nursery ponds.

3. Compost can also be used directly as a low-cost feed for a number of fish species, such as catfish and Nile tilapia, in simple fish farming systems (see Section 10.2).

4. Some of the compost can also be used to increase the production of your vegetable garden. You should use it whenever available.

What does composting involve?

5. Composting is carried out by different groups of micro-organisms such as bacteria, fungi* and protozoa*, which require mostly carbon (C) and nitrogen (N) for their development. It is to obtain these substances that they decompose the available organic matter. This decomposition process will be particularly active under the following conditions:

6. The better these conditions, the faster the compost will be ready.

The C/N ratio of organic materials

7. The C/N ratio is the ratio of the carbon content to the nitrogen content of the organic material concerned. It is usually expressed as a single number that tells how many times more carbon is present than nitrogen.

Example

If the C/N ratio of pig manure averages 14, this organic material contains 14 times as much carbon as nitrogen.

8. The C/N ratios of organic materials commonly used to make compost in rural areas are given in Table 19. From this it may be seen that:

  TABLE 19
Average C/N ratios of compostable materials
Material
C/N ratio
less than 20
20-40
40-70
80 or greater
Animal manures, solid

Cattle, horse, buffalo

-
20-25
-
-

Sheep, goat

-
20-29
-
-

Pig

14
-
-
-

Poultry

10
-
-
-
Animal husbandry

Animal bedding (incl. urine)

4
-
-
-

Farmyard manure, average

14
-
-
-
Slaughterhouse wastes

Mixed wastes

2
-
-
-

Blood

3
-
-
-

Bone meal

8
-
-
-
Fish wastes
4
-
-
-
Crop residues

Straw from barley, wheat, rice

-
-
-
80-100

Stalks, leaves: sugar cane

-
-
-
115

maize

-
-
55
-

cotton, millet

-
-
70
-

potato

-
25
-
-

groundnut

-
19
-
-

soybean

-
32
-
-
Cotton seed/s

Vegetable residues

13
-
-
-

Tobacco leaves

13
-
-
-

Fruit wastes

-
30
-
-

Groundnut shells

-
36
-
-

Rice husks

-
-
-
150

Cotton seeds, delintered

5
-
-
-

Oilseed cake, neern

4.5
-
-
-
Household wastes
-
25
-
-
Vegetation, aquatic
Seaweed
-
19
-
-
Water hyacinth
-
20
-
-
Vegetation, terrestrial
Grasses, mixed: young
16
-
-
-

old

-
40
-
-
Ferns
-
35
-
-
Lucerne, red clover
18
-
-
-
Tree leaves: general
-
-
55
-

Borassus sp.

-
32
-
-

Tamarindus sp.

-
27
-
-

Cassia sp.

12
-
-
-
Miscellaneous
Biogas slurry (from cattle dung)
-
20
-
-
Brewery wastes
15
-
-
-
Sawdust: rotted
-
-
-
200

fresh

-
-
-
+500

Making a good C/N mix

9. Some organic materials such as soybean stalks and leaves, groundnut shells, old grasses and weeds and some fruit wastes with a good C/N value of 30 to 40 can be used directly for composting. In most other cases, several different types of material with low and high C/N ratios need to be mixed in the right proportions to give suitable C/N ratios.

10. To mix in the right proportions, proceed as follows.

(a) Make a list of the materials you plan to use for making compost. Obtain their C/N ratio from Table 19.

(b) Group them into two categories according to their C/N ratio, a low C/N less than 40 or a high C/N greater than 40.

(c) If you use more than one material in one of these groups, calculate the average C/N of that group for the quantities of each material available.

(d) Calculate the proportions (on a dry weight basis) in which you should use the materials from each group to have C/N within the 30 to 40 range.

(e) Calculate how much material of each group should be used to mix together according to their respective dry matter content, depending on their respective moisture, as shown in the example below.

11. Composting materials with a very high C/N, such as sawdust or wood chips, not only can be activated by adding materials with a low C/N but also by adding:

at the rate of about 1 percent of the weight of the high C/N raw materials.

12. Do not add lime or ashes to the composting material as this may increase the losses in nitrogen as the pH increases.

Example

You plan to use pig dung (C/N 14), chicken droppings (C/N 10) and rice straw (C/N 110) to make compost.

You have two groups of materials:

In the first group, you plan to use 30 percent pig dung and 70 percent chicken droppings. The average C/N of the group equals
(14 x 0.30) + (10 x 0.70) = 11.2.

Calculate the proportions to be mixed from each group using a simple graph (see below) based on:

For best results, you should compost (on a dry weight basis) using a mix made of:

If the average dry matter content of the pig dung is 20 percent, that of the chicken droppings 25 percent, and that of the dried rice straw 80 percent, then you will need the following materials for each 100 kg of composting material:

Two general types of composting

13. Composting may be done under two kinds of conditions:

14. Each of these types has specific characteristics, summarized in Table 20. In some systems, both types of composting can occur, for example aerobic composting in the outer parts of the material and anaerobic composting in the inner area where little oxygen is present. For fish farming, composting is usually practised in two ways:

TABLE 20
Particular characteristics of composting methods
Characteristics
Aerobic composting
Anaerobic composting
Presence of oxygen
Necessary, for oxydation process
No, for reduction process
Losses of nitrogen
Yes, important (NH3)
Reduced, if good sealing
Losses of carbon
Yes, important (C02)
Reduced, if good sealing
Production of heat
Important (up to 65C or more)
Very small
Destruction of pathogens
Yes
No
Moisture content
To be controlled, best 40-60%
Not important
Composting method
  • in heap, above ground level
  • in heap. deeper under water
  • in pit, below ground level
  • in sealed heap, above ground level
  • in heap, at water surface
  • in sealed pit, below ground level


   Preparing compost in the pond (aerobic/anaerobic)

15. You have already learned how to compost aquatic and terrestrial vegetation in the pond itself, where it can be heaped into a bamboo frame. Long cribs or corner cribs in the pond are also commonly used for such purposes.

16. You may use a simple material, such as grass or cottonseed; if one of the materials such as rice straw has a high C/N ratio, it would be better to alternate layers of this material with layers of a low C/N such as animal manure, cotton seeds and waste fruit or vegetables.

17. The heap should be well compacted underwater, for example by trampling each layer well. Build the heap somewhat higher than the water surface, as its height will slowly decrease. Every week, add some new layers of material to build it up again.

18. For best results:

Preparing compost on land

19. It you decide to prepare compost on land, it is easier to use the aerobic method. It is then most important to ensure that air is always present within the composting pile to maintain rapid and full decomposition of the organic materials. For this, remember the following.

(a) Start building a new pile of composting materials with a first layer of coarse vegetal material at least 25 cm high, for example using stipes of banana leaves, straw or sugar-cane stalks. This layer should help air circulation while also absorbing any nutrient-rich liquid draining from the layers above.

(b) Cut the composting materials in small pieces of 3 to 7 cm.

(c) Pack all materials loosely, leaving air spaces between them. Never compact this compost pile.

(d) Do not build your pile too high, to avoid it compacting under its own weight.

(e) Keep the pile moist but not wet. Too much water results in a lack of air circulation. Protect your pile from rain (too wet) and from sun (too dry).

(f) Turn the pile over from time to time to aerate it and to avoid too high a build-up of heat in its centre. Drive a stick into the middle of the pile. Wait a few minutes before taking the stick out. If the pile is too hot, dry or smelly, it is time to turn it over.

20. There are two ways to pile up the material:

21. Remember that you will need water for composting. To simplify the use of compost piles, it is better to place them near a source of water, for example a pond, and near one of the main sources of composting materials such as an animal shed.

Preparing compost from grass cuttings

22. To prepare a cheap compost from grass cuttings under tropical conditions, you can use a simple heap method as follows.

(a) Prepare your materials. You will need:

(b) Select a clean area and compact the surface soil. Mark the corners of a rectangular area 2 m x 4 m with four poles, 1.50 m above ground level.

(c) Within this area, and starting from its perimeter, build a 40-cm layer of grass cuttings, about 250 kg.

(d) On top of this, spread evenly about one-quarter of the soil. Incorporate it into the grass layer by shaking.

(e) Moisten the layer with 100 l of water, using a watering-can.

(f) Repeat this process three times to build a four-layer heap about 1.50 m high.

(g) Protect the heap under a small shed built, for example, with bamboo and grass. It is best to close the sides with movable straw screens.

(h) Turn the heap over about 25 days later. Rebuild the heap from 30-cm layers, watering each layer with about 100 l of water.

(i) Repeat this last process about 16 days later.

(j) Depending on temperature and materials, your compost should be ready seven to eight weeks after starting.

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Preparing compost from water hyacinth

23. Under tropical conditions, to prepare a cheap compost from water hyacinth (Eichornia crassipes), a floating aquatic plant, use the heap method as follows.

(a) Prepare your materials. You will need:

(b) Mix well together the dried and freshwater hyacinth plants.

(c) Mark the corners of a square area 3.5 m x 3.5 m with poles, 1.50 m above ground level.

(d) Within this area, build a first layer about 25 cm high with coarse material.

(e) On top of this first layer build a 40- to 50-cm layer with the plant mix.

(f) Insert the bamboo pipes vertically into this last layer, down to the first layer and about 0.8 to 1 m apart.

(g) Continue to build up the heap around the bamboo pipes with the rest of the plant mix.

(h) Protect the heap from rain and sun.

(i) After about 14 days, turn the heap over, watering it if it is too dry.

(j) About one month later, your compost should be ready.

Note: instead of bamboo you can also use stalks from plants such as corn tied in bundles of five or six stalks.

View of plan for placement of 12 bamboo pipes into pile
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Preparing compost from plant materials and animal manure

24. A simple method of preparing compost in tropical rural areas from various materials such as tree leaves, grass, household wastes, rice husks, straw and animal manure, is the following:

(a) Mark the corners of a square area 1.5 m x 1.5 m with poles about 1.5 m above ground level.

(b) Within this area, build a first layer of coarse material about 25 cm high. Cut the rest of the material into small pieces.

(c) Add a second layer of 5 to 10 cm made of low C/N material, preferably animal manure. Moisten as necessary.

(d) Build up your heap until it is about 1.5 m high by adding more layers. Alternate 20-cm layers made of high C/N material with 10-cm layers made of low C/N material. If you do not have enough animal manure, sprinkle some nitrogen fertilizer on top of each high C/N layer (see Section 6.1). Moisten each layer so that it is damp but not soggy.

(e) If it does not rain, you may need to sprinkle water on top of the heap every three days.

(f) Turn over your pile after ten to 14 days. Check its heat production and moisture regularly.

(g) Your compost should be ready after another ten to 15 days.

Note: you can protect the composting heap from sun and excessive rain by covering it with plenty of straw and by rounding off the top of the heap.

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Preparing compost using the pit method

25. With this method, several pits are dug side by side. They can be of any size, although it will be easier if they are not too deep. A practical size is 1.5 m x 3 m by 1.5 m deep.

26. To prepare compost so that it becomes continuously available, proceed as follows.

(a) At the bottom of the first pit, build a first layer of coarse material, 30 cm deep. Chopped banana stems, banana tree leaves and straw are good.

(b) Add a second layer about 10 cm thick of a mix of low C/N material, including animal manure if available, and moisten as necessary.

(c) Set up, or lay across, bundles of grass, stalks or sticks, or bamboo pipes (see above), to provide air spaces in the composting pile.

(d) Add more layers, alternating 20-cm layers of high C/N material with 10-cm layers of low C/N material, moistening each layer as necessary. Add some more air spaces across.

(e) When the pit is full and the pile rises above the ground, cover it with plenty of straw.

(f) Sprinkle water over the pile as necessary every three days.

(g) After about three weeks under tropical conditions, turn the pile over by transferring it to an empty pit next to it. Moisten as necessary and cover the new pile with straw.

(h) Start composting in the first empty pit while checking the process in the second area.

(i) Depending on the climate and the kind of materials used:

(j) Transfer the pile from the first pit into the second pit.

(k) Start composting in the first pit again.

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