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close this bookThe Biogas/Biofertilizer Business Handbook (Peace Corps, 1982, 186 p.)
View the document(introduction...)
View the documentInformation
View the documentMain Points of the Handbook
View the documentPreface
View the documentChapter one: An introduction
View the documentChapter two: Biogas systems are small factories
View the documentChapter three: The raw materials of biogas digestion
View the documentChapter four: The daily operation of a biogas factory
View the documentChapter five: The once a year cleaning of the digester
View the documentChapter six: Tanks and pipes: Storing and moving biogas
View the documentChapter seven: The factory's products: Biogas
View the documentChapter eight: The factory's products: Biofertilizer
View the documentChapter nine: The ABCs of safety
View the documentChapter ten: Conclusion: Profiting from an appropriate technology
close this folderAppendix
View the documentNew ideas
View the documentComposting
View the documentBioinsecticides
View the documentFerrocement
View the documentFacts & Figures
View the documentSources & Resources
View the documentFeasibility Studies
View the documentProblem solving
View the documentVocabulary


Partial composting (seven to ten days) can be used to prepare ground-up plants for biogas digesters. Complete composting (three to six weeks) can be used to make a solid organic fertilizer. With composting there is no methane gas, and compost does not have as much nitrogen as biogas sludge made from the same organic waste would. With compost the fertilizer is a solid, while 90 percent of the fertilizer from biogas digesters is a liquid. What follows is a method of composting adapted from the Filipino handbook, The Samka Guide to Homesite Farming.

When organic waste is composted, it becomes a biofertilizer that was made by organic waste rotting in air. When organic waste is digested in a biogas digester, it becomes a biofertilizer that was made by rotting where there was no air. Both types of fertilizer are very useful. The method that is chosen depends on local resources and needs.

The old method of making compost fertilizer is to pile plant wastes on the ground until the pile is 30 centimeters (1.0 foot) high, then to add animal manure. Ashes and lime are sprinkled on top of the manure. Another layer of plant waste is added, then more manure and lime, and in this way alternating layers are built up until the pile is about 1.5 meters (5.0 feet) high, wide, and long.

The compost pile is kept moist, but not wet or flooded with water. Most farmers cover their compost with a grass or banana leaf roof in the rainy season so it will not get too wet. In about three months the compost pile shrinks to about one tenth of its original size and has become a rich organic fertilizer which can be spaded into garden planting beds or plowed into field.

Now there is a better way to change organic wastes into rich organic fertilizer. Instead of waiting three months, compost fertilizer can be made in three weeks. And it will be a better fertilizer! The secrets of this quick process are as follows:

The first secret is to use plenty of organic waste that is high in nitrogen to add to the compost mixture, such as urine, fish and meat scraps, algae, and manure (see Facts and Figures section of Appendix). Most plant and crop wastes are high in fiber which means they are also high in carbon and will need lots of nitrogen to help them rot. Scientists call this the carbon/nitrogen ratio. It has been found that the best compost mixture is about 30 parts carbon to every one part nitrogen (C/N 30).

When a good mixture of organic matter has been collected, chop the plants into small pieces, 3.0 to 5.0 cm (1.0 to 2.0 inches) long, to speed up the rotting (decay) process. The rotting process is faster when there is more plant surface area exposed to moisture and air. The more pieces a plant is cut into, the greater the exposed surface area will be. If the plants can be ground, shredded, or pulped, the quality of the compost will be even higher. On a small farm there is no way to be sure of getting the right mix of carbon and nitrogen except by trying different combinations and observing the results. It is better to have too much nitrogen than too little.



-From the Samaka Guide to Homesite Farming


-Build your compost pile with alternating layers of fresh materials, dry matter, and manure.

All plant waste should be composted for four to seven days before being put in a biogas digester. If the plants are to be completely rotted, they must be composted for three to five weeks. The shorter times are for hot weather, and the longer times are for cold weather.

If there is not enough plant or animal waste with high nitrogen content, add chemical fertilizers containing nitrogen to the compost. This has three advantages.

· It will help the compost materials change quickly to organic fertilizer.

· It will make the chemical fertilizer into a better fertilizer which will be easier for plants to take up and digest.

· It will help prevent the nitrogen in the chemical fertilizer from evaporating or being washed away by the rain.

The second secret is to turn the compost pile every few days.

· If the right compost mixture is used, the pile will get very hot in the center.

· If it stays too hot too long, it will begin to smell bad and the pile will become dry.

· The solution is to push a bamboo stick into the center of the pile and pull it out after three minutes.

· If the stick is very hot, turn the pile. e If the stick smells bad, turn the pile.

· If the stick is dry, turn the pile.

The third secret is to keep the compost pile moist, but not wet, something like a wet rag out of which the water has been squeezed.

· The hot temperature in the compost pile causes much of the water to evaporate as steam.

· So every time the pile is turned, sprinkle water or sludge from a biogas digester on the compost to keep it moist. e If the pile is turned often and kept moist, it will smell sweet.

· If it smells bad, it is because the pile was not turned often enough. The bad smell is the smell of valuable nitrogen fertilizer escaping into the air.

To turn a compost pile, first gather the material from the outside of the old pile and place it in the center of the new pile. Being dry, it will need more water. Then take the material from the inside of the old pile and place it on the top and sides of the new pile. In this way, every time the pile is turned, the mixture alternates its position from inside to outside and from outside to inside.

A successful compost pile needs three things: a good mixture of organic materials broken up into very small pieces with enough high nitrogen content waste, the right amount of water or biogas sludge, and plenty of air.

If farm animals are kept in pens, the floors of the pens should be made of concrete so that channels and holes can be made to guide the manure and urine to holding areas where the wastes can be easily added to compost piles. The holding areas can be filled with shredded straw and leaves to absorb the urine and begin the rotting process.

For thousands of years people have used animal manure, ash, and plant wastes to restore to the soil the elements that plants need to grow strong and healthy. Plants take certain substances from the soil in order to grow. These substances are food for the plants. They are just as necessary to plants as rice, corn, meat, fish, and vegetables are to people.

When a family's rice sack is empty, the rice must be replaced or else they will starve. The same is true for plants; the farmer must return to the soil the substances that are taken from it when a crop is harvested or the next crop will starve. About 100 years ago scientists began to study what was in the soil that gave food to the plants through their roots. Today three elements are believed essential to all plant food. These three elements are: nitrogen (N), phosphorus (P), and potassium (K). There are secondary elements in addition to these three which are needed in small amounts, but which must be present in the soil to make healthy plants. They are: calcium, magnesium, sulfur, boron, iron, manganese, copper, zinc, molybdenum, and cobalt.

In fertile soil all of these elements are present in the correct amounts so crops grow well and harvests are big. Poor soil does not have enough of some of these plant foods so the plants are not healthy and harvests are small. Plant food can be made in factories and put into the soil to supply what is lacking or to replace what has been taken from the soil by harvested crops. Today, farmers all over the world are using manufactured chemical fertilizers to increase the yields of their crops. It has been found that fertilizers can increase the value of crops by as much as ten times the cost of the fertilizer. It is often said that "fertilizers pay; they do not cost."

The kind of fertilizer that should be used depends on two things: what kind of crop is to be planted and what the soil lacks that is needed in order to make that crop grow well.

When there is not enough nitrogen in the soil:

1) The plants have light green or yellowish leaves.

2) The plants do not grow big and they grow slowly.

3) The plants have stalks which are thin and stiff.

4) The plants have leaves which form a small angle with the stem.

When there is not enough phosphorus in the soil:

1) The plants are thin and short.

2) Beans have dark green or bluish color, like the leaves of plants during a long dry season.

3) The stems and leaves of corn and some vegetables become purple in color.

4) Grains are thin and light in weight.

When there is not enough potassium in the soil:

1) The leaves die along the edges. In bean leaves, gray spotted half circles form on the edge of the leaves.

2) The stems are weak and brittle.

3) Growth is slow.

4) Grains are thin and light in weight.

Leafy vegetables such as lettuce, cabbage, and mustard need more nitrogen the phosphorus or potassium. A complete fertilizer containing the three major elements should be used before planting. The fertilizer should be mixed very well with the soil. After two weeks if the leaves of the plants are not dark green in color, add fertilizer containing only nitrogen to the soil.

Plants which have fruits and seeds such as corn, mango, tomato, peanuts, string beans, potatoes, cucumbers, papayas, bananas, pineapples, peppers, and squash need as much phosphorus and potassium as they need nitrogen.

The appearance of the plants will tell when there is a serious lack of a fertilizer element. When in doubt as to what fertilizer element is lacking, the best thing to do is to use a complete fertilizer.

It is not only fertilizer which plants need. Plant food must be "seasoned." It must be prepared correctly, and it must be mixed with water. When people put salt and sugar on food, it makes the juices of the mouth and stomach come out in large quantities. Without these juices, the food we eat will not nourish us. If soil is too sour (too acid), plant food cannot be completely digested by the plants. In such cases, add seasoning by mixing fine powdered lime into the soil at least two weeks before planting. This will help the plants in absorbing and digesting the fertilizer. Finally, water is needed to dissolve the fertilizer and lime, so that the roots of the plants can take up the nutritional elements.

Always remember that chemical fertilizers work best when used only as a supplement to organic fertilizers. The more organic material that is mixed with chemical fertilizers, the better it will be for the plants and for the continued fertility of the soil. A large percentage of expensive chemical fertilizers can be washed out of fields by rain and evaporated into the air. If chemical fertilizers are mixed with organic fertilizers, the soil will be able to hold more of the nutrients from being washed away or evaporated.

Organic fertilizers, which add humus (decayed plant matter) to the soil, are necessary to profitable farming and to successful home gardening. Chemical fertilizers can be a valuable supplement if there is not enough organic fertilizer. When chemical fertilizers are used: mix them with compost first and make it into an enriched organic fertilizer--this will benefit the crops, the soil, and it will reduce costs to the farmer.

What follows is another way to profit from plant wastes. It will not directly help biogas systems, but it should be of help to farmers who have plant wastes. This method is especially good for crop wastes, such as straw from grain crops.

Crop waste can be:

1) Composted for a week until the fibers are broken down enough to be used in a biogas digester.

2) Composted for a month and used as compost fertilizer.

3) Composted by the following method which is adapted from an article by Art Bell in the magazine, "Peace Corps/Philippines" of December, 1979.

In order to clear fields, crop wastes are often burned. Every time plants are burned, nitrogen and other fertilizer compounds are wasted that the crops need.

Nitrogen is energy--plant energy; to burn it is to lose that energy. It is like burning fertilizer instead of spreading it on the fields. Using all of a field's crop wastes as fertilizer for future crops, returns approximately 25 percent of the nitrogen, 40 percent of the phosphorus, and 75 percent of the potassium, along with the humus which holds the soil together, and stops erosion.

In order to understand how to make use of plant energy, the situation of the rice farmer can be used as an example. The rice is harvested, threshed, the grain is sacked, and now the fields are covered with six-inch strands of rice stubble, and there are large piles of rice straw. The rice straw is often burned, a process that burns up thousands of pesos of plant nutrients.

If this stubble and straw is plowed under, what would happen? How could this waste be made to rot, to decompose fast enough and completely enough so that another crop could be plants in two to three weeks? In nature there is what is called a carbon/nitrogen ratio which must be kept in balance for normal plant growth to occur. This is just a scientific way of saying that energy for breaking down plant and animal matter, which is usually carbon, comes from nitrogen. This decay process is carried out by bacteria working on the carbon materials, and the size of the bacteria populations increase in proportion to the amount of nitrogen that is present in their surroundings.

Plowing under a large amount of plant waste, such as rice stubble and straw, calls upon the soil to supply a large amount of nitrogen for the bacterial decay of all this plant material. This process is the first priority. Plants growing in the soil will be starved for nitrogen as long as there are high levels of carbon plant matter left to be decayed by the bacteria.

To offset this nitrogen starvation and speed up the decomposition process, a nitrogen fertilizer such as sulphate of ammonia or urea is added. What this means is that the farmer can subtract this from the basic fertilizer application for the next crop. In practice, many farmers still use their normal fertilizers, and the added nitrogen plus the increased organic matter produces crops with much higher yields.

Normally, broadcasting (scattering) 30 kilograms of nitrogen per hectare (three bags sulphate of ammonia or one and a half bags of urea per hectare), followed by plowing under the crop wastes; and if the soil is not very moist, a light irrigation is all that is needed to break down the plants.

It should be pointed out that this nitrogen cannot be lost by leaching from the soil because it is captured and held within the bacterial cells that are decaying the plants. When the last of the plant matter is broken down into basic organic plant nutrients, the bacteria die and the nitrogen inside their cell bodies becomes available for new plants as nutritional nitrogen and other plant nutrients.

The time required for this decomposition process is only two to three weeks. It should be noted that this process is the same for all crops, whether they are cereal grains such as rice and wheat, or crops such as corn, cotton, or vegetables. For very woody-stemmed crops like cotton, pull the plants up and chop or grind them into six inch or smaller pieces. Spray the chopped-up plants with a water solution of sulphate of ammonia or urea, which will supply the 30 kilograms of nitrogen per hectare. Then immediately plow the plants under and if the soil is not very moist, follow with a light irrigation. Again, all this crop waste will be decomposed and part of the soil within the same two to three weeks as with the lighter, less woody crop wastes.

This is a simple process which takes a little work and a little time. Who knows, maybe some enterprising person can put together a well-paying business, making fertilizer out of all the plant wastes that are being dumped or burned.