Dr. Md. Faruque Hossain :
Agricultural practices in the floodplain of Bangladesh are based on traditional knowledge and maintaining soil fertility is almost entirely dependent on locally available resources. The recycling of organic matter from farm and household waste is the major source of plant nutrients in the traditional system. Farmyard manure, compost and green manure are the traditional sources of plant nutrients. Traditionally, farmers in the Bangladesh cut and carry leaves and twigs from a variety of plants found on their farms, lands and either incorporate them into the soil as green manure or use them as green mulch or burning for cooking. These practices are most common in the country where they are mainly used in rice (Oryza sativa L.) and vegetable nurseries, and in rice fields. Fallen leaves and rice straw from various plants are also used as compost. These leaves and straw are collected in the spring or after harvest of rice, used as animal bedding and ultimately become as farmyard manure or compost.
Farmers have identified the leguminous and non-leguminous crops, shrubs and annual plants that have good manuring value. This local knowledge has developed over centuries of farmer experimentation and the importance of these composts and manures is widely recognized. There is several plant species commonly used as green manure. Usually those plants species have little or no value as fodder and become green manure by default. Many of them are multipurpose and locally available, and many of them are perennials. Farmers have a preference ranking for different species based on their value as manure, their availability and their multi-functionality. The degree to which local farmer preference ranking agrees with formal experimental results and the prospect for these green manures in the subsistence farming system of the Bangladeshi farming fields is discussed below.
There are however, some interesting features of farming in Bangladesh suggesting that farmers’ may have a more complex view of soil fertility. For instance, it is common practice for farmers to remove surface soil from paddies to construct embankments, homesteads or for brick making. This may suggest that, (a) farmers may not realize that a decrease in soil fertility occurs with soil depth. Or this may be a strategy to remove soil where fertility has declined to access ‘fresh’ soil below, which they perceive to be more fertile; (b) the monetary return from the use of soil in this way has greater value than that of fertility for crop production; (c) soils perceived to be of low fertility are selected to be used as construction materials; and (d) farmer’s recognized that soil fertility can be eventually replenished after removal of topsoil by the addition of vast quantitative of manure and inorganic fertilizer.
Scientists also report that rice yields may be declining under such intensive cultivation in several Asian countries including Bangladesh. However, the evidence on yield trends comes from experimental stations rather than farmers fields. In response to such concerns, some organizations are promoting organic farming. For example PROSHIKA, a non-government organization (NGO) recommends ‘ecological farming’ as an alternative to the use of chemical fertilizers. Ecological (organic farming) as promoted by PROSHIKA is based on use of ‘quick compost’ (a mixture of cow dung, rice bran and oil cake in the ratio 4:2:1) and recycling of plant residues to soils instead of reliance on chemical fertilizers and pesticides. Farmers are also encouraged to add more farmyard manure, household wastes, oil cake and green manures. Farmers working with PROSHIKA pointed out that fields, which they converted to organic practice, were relatively soft and more friable.
The knowledge farmers had about the traditional green leaf manuring plant species to be found in the Bangladesh was gathered together in various formal and informal ways. In order to understand and verify the farmers’ indigenous knowledge as far as these species were concerned, plant tissues (leaves) were analyzed for their nutritive values (nitrogen, phosphorus and potassium) and, in formal field experiments, the manuring performance of many of the varieties were tested on rice crops. Boro, broadcast aus, transplanting aman of different varieties are preferred by farmers. When their nutrient values were analyzed, Boro came in first place. The other varieties were also found to be quite rich in major plant nutrients. It was clear that farmer preference agreed well with the nutritive values of the plants studied. Plant varieties that were less preferred which although it had good nutritive values caused itching during puddling and rice transplantation. The nutritive values of other varieties were found to be comparatively poor. When green leaf manure from Boro and chemical fertilizer (100-22-42 kg N, P and K ha-1) were applied in on-farm rice field experiments and their effects compared, Boro consistently out yielded the chemical fertilizer over several seasons and locations. Similarly, rice yields from experimental plots treated with aus and amans were consistently better than yields from those plots where chemical fertilizers had been used. This proved that farmers, by experimenting over the generation, had been able to select species with superior qualities.
Another example of farmer ingenuity in green manuring can be found in relay planting. Here rice bran and niger are planted as green manure under the planted maize crop (Zea mays L.). These green manure crops were dug into the soil after the maize harvest and before the rice were transplanted in a rice-wheat-maize rotation. Farmers in Bangladesh developed this procedure when they realized that an intensive cropping system placed heavy demands on plant nutrients and that growing a green manure such as Sesbania would mean relinquishing a food crop, something no subsistence farmer could afford. Relay planting on two sites was studied in a three-year long field experiment. The plots manured with relay planted Boro rice as a green manure produced rice yields equivalent to a plot treated with chemical fertilizer applied at 100-22-42 kg N, P and K ha-1 based on Bangladesh Agricultural Research Council (BARC) with no negative effect on the grain yield of the maize crops.
There is another example, based on BARC recommended and traditional farmer nitrogen application rates, the yield response to current farmer added nitrogen in the field was very low, suggesting that at current yield levels nitrogen application rates used by farmers are generally in excess of crop requirements. Given this situation it is easy to understand why the recommendation of PROSHIKA to stop applying urea fertilizer and maintain organic inputs appears to lead to a benefit for farmers. From all these examples it can be concluded that Bangladeshi farmers make efficient use of their natural resources in providing for the necessities of their subsistence way of life.
Low and declining soil fertility is a major production constraint in the Bangladesh and it is becoming increasingly critical to secure sustainable soil productivity. Intensification of crop production, deforestation and soil erosion is the main factors involved in declining soil fertility. Chemical fertilizers have been seen as a way of sustaining soil productivity by which soils body are becoming sick. However, the extent to which farmers can depend on this input is constrained by the inaccessibility of the village areas, lack of technical know-how, the low purchasing power of the majority of farmers and the difficulty of making the right type of chemical and organic fertilizer available in the right amount at the right time. Farmers themselves are becoming increasingly concerned about the deteriorating physical properties of the soil as the volume of chemical fertilizer increases and the amount of organic manure declines.
Most of the indigenous green manure species can be used for many purposes. Daincha (Sesbania), for example, is used as a live fence, a hedge plant, and as an excellent green manure, which is also frequently used to control crabs in rice fields. Moreover, these species help conserve soil and are said to cause minimal shade problems to food crops. Whilst growing such plant species in agricultural lands as fences and along riverbanks causes no harm, there are limitations to the scale on which green manure can be used. If green manures were to be employed on a large scale, a major gap would develop between demand and availability.
Therefore, one should not expect the total nutrient supply to come from green manure alone. Moreover green manure involves considerable human labor because the green biomass has to be cut and carried. Some plant species such as ankhitare and siplikan are beginning to disappear very quickly because of over exploitation. Attention must be given to the conservation and proper utilization of these important plant species. Traditional knowledge is often neglected in agricultural research and development and the price paid for this omission is that new technologies are often adopted with little enthusiasm and resolve.
Despite the many efforts being made to introduce exotic green manures such as Sesbania sp., their use is far from widespread in Bangladesh because of altitude constraints, the fact that they compete with arable crops during the growing season and because seeds are often unavailable and difficult to germinate. Using indigenous green manure is a traditional practice. Therefore, it is less likely to encounter the same adoption problems and communication barriers that confront new technologies. Identification, conservation, and utilization of locally available green manure will contribute significantly to meeting the total nutrient demand of local crops without creating environmental problems or placing cash demands on subsistence farmers. These examples reinforce my belief that agricultural research, especially in the case of subsistence farming, should be based on local knowledge.
(Dr. Md. Faruque Hossain, Professor and Environmental Scientist, Department of Operations Management, American International University-Bangladesh. Email: [email protected]; [email protected])