ILEIA Newsletter • 14 nº 2-3 • September 1998
부르키나파소의 전통적인 덮기(mulching) 방법 -Fidèle G. Hien
도입부르키나파소 전체, 특히 중앙 고원지대에서 1990년대 이전 시행된 지도 프로그램은 농경-목축 체계의 강화하기 위한 투자는 효과적으로 실제 수요를 충족시키지 못하는 것으로 입증되었다. 그러나 지도사업으로 제안된 방법과 결합된 물과 흙의 관리를 위한 전통 기술과 방법은 때때로 연구자와 지도자들의 관심을 끄는 놀라운 결과를 올렸다. 부르키나파소의 Centre Nord에서 전통적인 덮기는 이러한 방법의 하나다. 심각한 가뭄이 들었던 1974~1985년 이후 전통적인 덮기의 사용이 놀랄 만큼 증가했다. 밭에서 물을 보존하기 위한 전통적인 기술로 처음 사용된 덮기는 침식 방지와 토양 관리 기술과 통합되면서 점진적으로 향상되었다. 오늘날 그 방법은 그 지역에서 물과 토질을 관리하는 효과적인 방법으로 인식된다.
Centre Nord의 농업덮기가 개발된 Centre Nord는 Mossi 고원의 일부이다. 덮기는 현재 그 지방의 대부분, 특히 Sanmatenga, Namentenga, Bam, Passoré에서 실행되고 있다. 1985년에서 1996년 사이 인구는 35%까지 증가했다. 그때 평균 인구밀도는 예를 들어 1평방킬로미터에 52명이었다. Centre Nord는 고도 300m의 평지이다. 강우량은 500~800mm이다. 토양의 대부분은 철분을 함유하고, 겉이 딱딱하고 얕다(47%). 깊고 무거운 흙(16%)은 가장 좋은 농업잠재력을 지닌다. 평균적으로 단 토지의 43.5%만 경작할 수 있다. 1990년 농사에 알맞은 땅의 24%는 매우 저하되었다(BUNASOLS-MAE 1990). 자연식생은 지나친 방목과 땔감 채취를 위한 착취로 심각하게 저하되었다.
Mossi 고원은 토지의 개발 수준이 수용력 이상으로 진행되고 생산 체계는 악화의 소용돌이에 빠진 지역으로 간주되었다(Kessler and Boni 1991; Zoungrana
and Zoungrana 1992; Hoek vd et al 1993; Hien 1995). 휴한기는 매우 짧아지고 영양분이 격감했다. 유기물(1년 1헥타르에 -1.37톤)과 영양분이 비극적인 상태였다: 질소(1년 1헥타르에 -15~-20kg)와 인(1년 1헥타르에 - 2kg).
Centre Nord에선 두 생산 체계가 발견되었다: Mossi족의 농경-목축 체계와 Peulh족의 목축 체계. 열화로 인하여 두 체계는 점점 계곡의 비옥한 땅을 두고 경쟁하게 되었다. Barning과 Dambré(1994)에 따르면, 농민의 사회경제적 상황은 꽤 약하다: 농민의 43%는 최저생계를 유지하며, 그들은 가난하고 농업에 투자할 길이 없다. 농민의 단 19%만이 부유하고 작물이나 가축 생산에 투자할 여유가 있다고 간주될 수 있을 뿐이다. 평균적으로 곡물 생산 상태는 수입 상태(1년에 -3000 F.CFA)와 마찬가지로 마이너스(1년 1인당 -49kg)이다. 가축 생산에 대한 투자는 가장 좋은 결과를 주고 덜 위험하다.
덮기: 기원과 발전According to the farmers of Tagalla, Sanmatenga province, mulching is an old technique used to improve soil conditions. Today, it has spread over large parts of the central plateau. Its reappearance was particularly striking after 1974 and more so after 1984, times of severe drought in the Sahel.
Mulching consists of spreading the plots to be cultivated with 6000 kilogram per hectare of straw from Loudetia togoensis, an annual herb typical of the superficial soils of the Sudano-Sahel region (Lafay andt Ranson 1995) at the end of the dry season. After it has flowered, the straw of Loudetia togoensis can be collected freely from fallow land. It is not a very appetising plant and provides poor forage. This straw, together with household refuse, chaff and dung are the inputs traditionally used by the farmers of the Centre Nord. Dung is the most preferred source of organic matter. Chemical inputs are generally too expensive to be used for cereals, the subsistence crop. Mulching seems to be the simplest solution and the one most within the reach of the farmers of the region who want to improve their soils (Slingerland and Masdewel 1996). Sowing takes place one or two days after the first major rains and there is no tillage before planting. There are three successive weedings in order to incorporate and ensure that the straw is well decomposed before the end of the growing season.
The decision-making criteria used by farmers as to whether mulch should be applied take three factors into consideration: soil type, the type of crop, and the field type. Lafay and Ranson (1995) point out that mulching takes place primarily on so-called "hot" soils. During the 1994 season in the village of Tagalla, mulch was applied in a 100 percent of cases to rehabilitate the denuded and crusted soils known as Zippellé in the traditional classification of land (Schutjes 1991). on shallow and gravel soils, the Zegedega soils, mulch was applied to 44 percent of farmland. Clay-loam or sandy-clay soils found on slopes, and known as Bolé, were mulched in 37 percent of cases. The so-called "cold" soils - sandy soils or Bissiga and heavy soils of the lower areas known as Baongo - are generally less involved and formed respectively 28 percent and 8 percent of the mulched fields in 1994. on these soils the straw is generally burned two days after sowing in order to prevent weeds. For the farmer, mulching aims above all to preserve the humidity of the soil for the benefit of the seeds and this is more important than the fertilising effect of mulch on the soil. This explains why the straw is burnt two days after sowing on the so-called "cold" soils where water storage capacity is the highest (Lafay and Ranson, 1994, Hien et al 1997).
Sorghum is the crop most often associated with mulching whereas millet is considered to be better adapted to drought conditions. Maize is much more demanding of water and nutrients and, as far as this region is concerned, has become a marginal crop only planted in fields that receive more dung and household refuse.
Fields nearest to the house (champs de case) receive priority as far as household refuse (38 percent of production in Tagalla in 1994) and cow dung with or without mixed straw (31 percent) is concerned. The fields situated in the proximity of the village (champs de village) are the most mulched (50 percent of the household of Tagalla in 1994), followed by the fields nearest to the house. When cow dung is in short supply to fertilise these fields, straw and other inputs are used to complement it. The fields in the bush (champs de brousse) that are farthest away from the house receive the least input. In 1994, 53 percent of bush fields in Tagalla received no inputs at all.
Lafay and Ranson (op cit.) have observed that, in Tagalla, the popularity of stone lines as an anti- erosion measure have been accompanied by an increase in mulching. Barning and Dambré (1994) noted that in the province of Sanmatenga in general the practice of mulching decreases when the level of animal traction used by farmers increases. Ploughing before planting is never accompanied by mulching.
확대: 이유와 한계The analysis of the ecological and socio-economic conditions of agriculture in the Centre Nord reveals why the practice of mulching has spread. Faced with a pressing shortage of land and a growing population, even the poorest pieces of land have had to be brought into cultivation. This has meant that farmers have had to intensify their efforts. Mulching in this region, as well as the practice of Zaï in the northwest of Burkina, are ｅxpressions of this imperative.
According to Lafay and Ranson (op cit.), farmers in the central northern region attribute the increase in mulching to two essential factors. First, they have less fertile land to feed more people and second rainfall has decreased over the past 30 years. Rainfall has become more erratic and when it does fall it is often violent and stormy. In the province of Sanmatenga, annual rainfall decreased between 1960-1978 and 1979-1988, varying from 41 millimetres in the south of the province to 210 millimetres in the north (Hoek vd et al 1993). At the same time maximum intensity during a 30 minute interval can reach 108 millimetres per hour (Hien 1995). The number of days of rain has decreased and as a result the season is shorter.
Lafay and Ranson reported that from 1984, some agriculture extensionists began to advise mulching in the technical package presented to farmers. For this reason many farmers began mulching when they saw that their neighbours who practised this method were getting better results. These types of examples have played a very important role ensuring the spread of mulching.
There are two limitations to the development of mulching. First, the lack of straw and second the lack of means and time available to farmers. The straw of Loudetia togoensis is mainly harvested on more gravelly, shallow soils (Zagedega) that are generally considered impossible to cultivate. These non-cultivatable soils represent about 56.5 percent of the total area of Sanmatenga province, varying from 80 percent in the districts of the north to 26 percent in those in the extreme south (Hoek vd et al 1993). Moreover, much of this non-cultivatable land is extremely degraded. Like elsewhere in the Sahel, primary production on these soils is closely linked to rainfall patterns (Penning de Vries and Djitèye 1982; Breman and de Ridder 1991). At flowering the straw production varies from between 2500 and 4500 kilograms of dry material per hectare according to the season and the quality of the soils. An average of 6000 kilograms of straw has to be spread at the end of the dry season. The loss of biomass during this period is estimated to be about 25 percent of the plant at flowering stage. This means that the straw of two to four hectares of bush - according to the season - has to be harvested in order to meet the mulching needs of a one hectare field. This is why the bush has not been burnt for the last 10 years.
Transporting the straw is the second constraint facing the farmer. The straw is generally transported by foot from the bush to the fields and is mainly carried by men. The quantities transported vary from 10 to 15 kilograms per person and per trip (Lafay and Ranson op cit.). Depending on the distance from where the straw is gathered, to the field, and the number of active persons in the household and their carrying capacity, the farmer will need between 80 and 200 working hours to collect the amount of straw necessary to mulch one hectare of land. Thus, mulching takes the farmer about a month’s work before sowing begins. This is why bicycles can often be profitably used. However, the most practical means of transport is an animal-drawn cart but it is expensive. Since the devaluation of the local currency in 1994, transport costs are about F.CFA 200,000. This figure must be compared to F.CFA 55,000, the estimated annual average household income (Barning and Dambré 1994).
덮기의 생태적 영향For the farmers of the Centre Nord, mulching increases the production of sorghum. The work of Lafay and Ranson (1994) and Slingerland and Masdewel (1996) sheds some light on the socio-economic aspects of the technique. Research conducted after 1996 has focused on the ecological performance of mulching practices (Hien et al 1997 and 1998). Experiments carried out at farm level in Tagalla village have made it possible to compare the effects of four treatments that combined mulching, stable dung and natural phosphate on the hydrodynamic parameters of the soil, the sorghum production, and nutrient flows and balances. The goal of these experiments was to come to a better understanding of the processes that determine increases in sorghum production and to make it possible to evaluate the sustainability of the system in terms of nutrients. During the cropping season of 1996 with 623 millimetres of rainfall - nearly a normal pattern - three treatments with six repetitions were compared using non-mulched plots as a control:
• simple mulching (6000 kilograms DM ha-1) with the dry leaves of Loudetia
togoensis: refered to here as Pa
• mulching (6000 kilograms DM ha-1) + 2000 kilograms. ha-1 of dung or PF
• mulching (6000 kilograms DM ha-1) + 200 kilograms. ha-1 of "Burkina Phosphate" (natural phosphate ) or PP
After the straw had been spread following normal local farming practice, evaporation was measured over a period of ten days from the time that a local variety of white sorghum was sown. Measuring rainfall and run-off made it possible to calculate the infiltration levels and the run-off thresholds (Hien 1995; Hien et al 1997). At the same time, the germination and seedling settlement of the plants were monitored and the yield levels of grain and straw were measured at harvest time. The total reserves of nitrogen (N), phosphorous (P) and potassium (K) in the first 0–30 centimetres of soil have also been assessed. The absorption of N, P, and K by the plants during the growing season was monitored every two weeks from the fortieth day after planting up to the flowering stage. Thus, it was possible to measure the changes of N (Nb), P (Pb) and K (Kb) in the sorghum biomass above the ground. At the same, the straw and dung have been analysed and finally, on the basis of data related to nutrient flows in the agricultural systems of Burkina’s central plateau (Piéri 1989; Lompo 1993) and of the Sahel in general (Penning de Vries and Djitèye 1982), an evaluation of the N and P balances was made. The results of this work can be summarised as follows:
• Mulching significantly improves the hydrodynamic conditions of
As far as water flows are concerned, mulching secured 64 percent reduction in water losses and, in comparison to the control plots, infiltration into the soils has improved 4 to 7 times. Evaporation was reduced by 30 percent on mulched plots.
• Mulching improves crop development conditions and crop yields.
Statistical analysis of the results shows that mulching significantly improves the period of germination. It reduces primary mortality (after germination) of sorghum and positively influences the production of grains and straw. For straw production, the difference was highly significant (p<1 percent) between the control T (480 kilogram DM ha-1) and the treatments with straw: Pa (2265 kilogram DM ha-1), PF (2729 kilogram DM ha-1) and on the hand was also significant (p<5 percent) between the control T and the treatment PP (1836 kilogram DM ha-1). The treatments with straw when compared to each other did not show any significant difference. As far as the grain yield is concerned, the analysis also showed that the treatment control T (140 kilogram ha-1 on average) revealed a highly significant difference when compared to the treatment Pa (774 kilogram ha-1), PF (1064 kilogram ha-1) and PP (687 kilogram ha-1). The treatments PF and PP showed a difference at the 5.7 percent level while there was no significant difference between PF and Pa.
• From the point of view of mineral balance mulching has some risks
The N and P balances have been calculated on the basis of the principal that, within the local agricultural system, all the crop residues at harvest are exported. This implies that the calculated immobility corresponds to the mineral exports. The balances of N and P at the end of the 1996 rainy season are given in the following table:
Despite the fact that important quantities of straw are involved, the mineral input linked to mulching with Loudetia togoensis hay is extremely weak. This is because of the bad quality of the straw at the end of the dry season. The level of N in the straw at that time is less than 0.23 percent. on the other hand, the organic input associated with this practice is very important (5600 kilogram ha-1 of organic matter). Simple mulching (Pa) revealed a relative phosphorous shortage in the soils. The input of dung or "Burkina Phosphate" has made it possible to mitigate this shortage which resulted into a better P/N balance (Van Duivenbooden 1996): the P/N ratio at flowering stage was 0.1. Indeed, the input of dung has considerably improved the availability of nitrogen. Equally, the input of 200 kilogram ha-1 of "Burkina Phosphate" stimulated the absorption of P. The greater availability of this element has improved the uptake of N with the result that the N balance has become negative.
In general, the calculated P balance confirms the observation (Lompo 1993) that the P input, whatever its source, improves the P balance. This is especially the case when the P source is less soluble (as with " Burkina Phosphate" in our case). These data also confirm the conclusion that mulching, applied alone, primarily aims to improve the bio-physical condition of the so-called ‘hot’ soils in order to improve germination and seed establishment (Lafay and Ranson 1995; Slingerland and Masdewel 1996; Hien et al 1997).
As far as the mineral balance is concerned, two treatments would, in the long term, imply a risk of soil mining. Simple mulching (Pa) that only improves the availability of water would mine the soils of N and P, and the treatment straw and natural phosphate (PP) might mine nitrogen reserves more rapidly because of an increased input of phosphorous. The combination of mulching and dung (PF) show the best sustainability characteristics at plot level.
The weak yield levels observed in the control plots, as well as the higher levels of nitrogen and phosphorus in the sorghum biomass at the flowering stage indicate quite clearly that, without mulching, production is primarily limited by water. Thus, it is logical that the simultaneous improvement of soil water conditions and N and P availability (treatment PF) has given the best results.
덮기의 사회경제적 이익과 한계
Mulching appears to be a common practice amongst Mossi farmers (Barning and Dambré, 1994; Lafay and Ranson, 1995), because there is little dung available within their households. Peuhl herders who have access to cattle and dung, only mulch very rarely. In order to get an adequate supply of cow dung, Mossi farmers sometimes enter into "cow dung contracts " with the Peuhls. The Peuhl are paid in kind or in natura and they leave their cattle overnight on Mossi fields for part of the dry season.
Compared to the average yield levels in the region which are about 450 kilogram ha-1 on top soils (DEP-MAE, 1988 and 1989 cited by Hoek vd et al 1993), mulching alone would, in a normal rainfall year, allow an increase in sorghum yield of about 50 to 75 percent on this type of soil. When this is also combined with dung (2000 kilogram ha-1) the yield levels realised in Tagalla are at least two times better than the regional average. As far as the financial balance of the households is concerned, evaluation shows that even with a doubling of yield levels, the time dedicated to the transport of straw (about 180 hours of work per hectare) does not constitute an improvement in financial terms. only the transport of straw by animal-drawn carts would result in a ten-fold reduction in the number of hours worked. This would contribute to an improvement or maybe even bring the financial balance of the households into equilibrium.
결론과 논의Farmers quite clearly see the agro-ecological advantages and limitations of the mulching systems as they confirmed that mulching by itself is the most effective when rainfall is inadequate (Hien et al 1998). In conditions of drought dung when used alone would cause growing difficulties because of water stress and would result in lower biomass and grain production. By contrast, when there is good or normal rainfall, mulching alone would result in an important growth of the biomass but grain yield levels are lower. In other words as Barning and Dambré (1994) and Lafay and Ranson (1995) have shown, farmers prefer dung to simple mulching in normal rainfall conditions. They know that the straw does not add significantly to soil nutrients but does improve humidity. Mulching appears to be the only solution when it comes to improving sorghum yields in situations where dung is not available in sufficient quantity and where chemical fertilisers are too expensive.
The work of Hien et al (1997 and 1998) show that for farm households it is at least possible to achieve a positive mineral balance by combining mulching and dung application. The N and P balances obtained by combining 6000 kilogram ha-1 of straw and 2000 kilogram ha-1 of dung show that it is possible, at least at a certain level, to achieve sustainable agriculture based on low levels of external inputs. Dung in this context is the most limiting input. Simulation shows that, by combining it with mulching, it becomes possible to reduce the input of dung by half to 1000 kilogram ha-1 without compromising the N and P balances and still increase the content of organic matter in the soil.
At the village level, however, improving the sustainability of the system necessarily involves a combination of various measures that would allow an increase in the availability of both straw and dung. Hoek vd et al (1993) has shown that the need for organic matter or compost to maintain the fertility of cultivated soils in Sanmatenga is 2.7 tons per hectare per year. Even if there were a complete, 100 percent, stablisation of cattle and other livestock in the province, this would only provide enough dung for 25 percent of the land being cultivated. At the same time Hoek vd et al (1993) have observed that, in financial terms, the expenditure required for composting would largely exceed the income it produced. By reducing dung input by half in a system that combines mulching and dung, the availability of dung could be increased. However, it is likely that the absence of investments aiming at the conservation and rehabilitation of sylvo-pastoral lands will contribute to a decrease of fallow land, as compared to total cultivated land needed to maintain the organic and mineral equilibrium of the production system. This lack of investment favours the accelerated degradation of sylvo-pastoral land (Hien 1995) and reduces the level of straw production. Finally, the stabling of cattle and other livestock - a pratice that has been pursued for the last ten years in a number of villages in Sanmatenga - assumes that the production of dung at household level can be doubled and that, at the same time, the regeneration of non-cultivated land is helped by reducing animal pressure on it. There are not many choices available in situations like those found in the Centre Nord.
Translated from the French by Bert Lof and Carmen Rodriguez.
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