M. Fokhrul Islam
More than 70 per cent of the population in Bangladesh is engaged in agriculture. Therefore, any technology that can influence agriculture or gets influenced by the agricultural practices becomes a subject of concern not only to the biogas user but also to the nation as a whole. Bio-gas technology is becoming popular in rural Bangladesh in view of escalating costs of fuels as well as soaring prices of chemical fertilizers which our farmers find difficult to buy for their sustainable crop production.
The first biogas plant was constructed in 1972 at the premises of Bangladesh Agricultural University (BAU), Mymensingh. Thereafter through several projects under different organizations constructed about 24 thousand biogas plants in different parts of the country. Infrastructure Development Company Limited (IDCOL), a state-owned non banking financial institution, is implementing National Domestic Biogas and Manure Programme (NDBMP) with the participation of different partner organizations (POs) and with support from the Netherlands Development Organization (SNV) and Kreditanstalt für Wiederaufbau (KfW) since July 2006. Under this programme a total of 37,629 biogas plants will be constructed during 2006-2012.
Two major outputs from a biogas plant are the gas and the bio-slurry. Bio-slurry is the decompose product produced from organic materials in a reduction process in presence of anaerobic microbes in the biogas digester. It is an excellent soil conditioner as it adds humus and supports the micro-biological activity in the soil increasing the soil porosity and water holding capacity. It has greater fertilizer value for crops and feed material for earthworms, livestock and fishponds. The loss of nitrogen is minimum in case of bio-slurry as compared to Farm Yard Manure (FYM) and compost due to anaerobic condition in the digester.
Declining in soil fertility is a common scenario in Bangladesh though magnitudes vary in different Agro-Ecological Zones (AEZ). Decline in soil fertility occurs through a combination of lowering of soil organic matter and loss of nutrients. In Bangladesh, depletion of soil fertility is mainly due to exploitation of land without proper replenishment of plant nutrients in soils.
A medium fertile soil should have an organic matter content of 1.8-3.4%. But in Bangladesh, most soils have less than 1.7%, and some soils have even less than 1% organic matter. The average organic matter content of top soils has decline by 20-46% over past 20 years due to intensive cultivation. To arrest further declination of soil fertility proper use of bio-slurry alone or in combinations with chemical fertilizers may be good options. Bio-slurry considered as quality organic fertilizer is environmental friendly, has no toxic or harmful effects and can help to a great extent rejuvenate the soils by supplying organic matter, considerable amounts of macro and micro nutrients. This organic fertilizer has also liming effects. The poultry litter fermented organic fertilizer is more effective in acid soils to reduce acidity and thereby protects crops from harmful effects of aluminum.
The farmer needs to use chemical fertilizer to increase the crop production. However, if only chemical fertilizers are continuously applied to the soil without adding organic manure, productivity of land will decline. On the other hand, if only organic manure is added to the soil, desired increase in crop yield cannot be achieved. Neither bio-slurry nor chemical fertilizer alone is enough to meet demand of soil-crop systems. Fertility trials carried out in Bangladesh and elsewhere have revealed that optimum results can be achieved through the combined application of both chemical and organic fertilizers. NDBMP supported on-station and on-farm trials conducted by Bangladesh Agricultural Research Institute (BARI) in major AEZs with major crops found that yield can be increased by 12-25%. In countries where biogas technology is well developed, for instance in China, there are evidences that productivity of agricultural land can be increased to a remarkable extent with the use of bio-slurry in combination with chemical fertilizer.
Organic fertilizer
Bio-slurry is a 100% organic fertilizer most suitable for organic farming. This fertilizer can effectively be used for all high value fields and horticultural crops including vegetables, fruit, flowering as well as ornamental plants and roof top garden. This organic fertilizer can also profitably be used for forest nurseries, public parks and roadside plantations. Organically produced crops and fruits are healthy and nutritious, and have better shelf life as well as higher market value. Demand for organically produce crops are increasing day by day in Bangladesh and elsewhere in the world.
An important point to be noted is that the anaerobic fermentation in the biogas digester does not result in any absolute increase in total nutrient content of bio-slurry compared to that of the dung fed to the plant. But the organic fraction of nitrogen is converted into a form that is more readily usable by plants. The fertilizer value attributed to the bio-slurry is due to its enrichment in humic substances (8.6 to 21.6% humic acid) and macronutrients which are supplied to soil.
Fresh bio-slurry should be allowed to settle for few days in the slurry pit and mixed with water at the ratio of 1:1-2 before applying to the fruit or vegetable crops grown close to the house with the help of bucket or pale. The ammonia content of the fresh slurry can harm the tender fruits and vegetables if applied immediately.
By employing simple collection and preservation techniques, the quantity and quality of bio-slurry in terms of weight and plant nutrient content can be considerably improved and preserved for later use in field or garden crops. It is advisable to construct at least two slurry pits so that each of them can be emptied alternatively. The pit volume should be equal to the volume of the biogas digester. To protect the bio-slurry in pits from sun and rain, shade should be provided by constructing a roof made of local materials such as bamboo and rice/wheat straw over the pits or from corrugated iron sheet or any other more permanent materials. Make the structure sufficiently strong to withstand winds. Creeper vegetables can be grown on the roof of shade.
To derive maximum benefit from organic manure application, the compost should be well decomposed and be of good quality. Use of un-decomposed organic manure should be avoided, as it will do more harm than good. Un-decomposed materials when applied to the soil attract insects and take a longer time (i.e. more time than the life cycle of the crop) before the plant nutrients present in them are converted in the form that can be assimilated by the plants. It is essential to know whether the compost has attained the stage of maturity before applying it in the field. In practice, mature compost can be identified from its physical appearance. Matured compost has a dark brown colour whereas undigested slurry is light brown or greenish. When pressed between two fingers, matured compost is friable in consistency and can be easily distributed in the field.
3 million plants
It has been estimated that there is potential of establishing about 3 million biogas plants in Bangladesh. If the 65 percent of the potential is tapped (2 million), there is a possibility of producing 12 million metric tons of bio-slurry (dry weight basis) per year from the average family-sized (2.4 m3) biogas plants. If calculated in terms of nutrients, 165,000 ton of N; 92,400 ton P, 40,800 ton of K and 39,600 MT of S would be available per year as fertilizer. If a farmer applies 6 tons of dry bio-slurry in crop field, s/he can reduce 60 kg urea, 48 kg TSP and 60 kg MoP fertilizer for selected crop. In Bangladesh, if properly managed, bio-slurry could play a major role in supplementing the use of expensive chemical fertilizers.
The writer is Bio-Manure Management Adviser, SNV Bangladesh.
Source: www.weeklyholiday.net, 11th June 2010
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