Sesbania sesban improved fallows in eastern Zambia: Their inception, development and farmer enthusiasm

In eastern Zambia, nitrogen deficiency is a major limiting factor for increased food production. Soil fertility has been declining because of nearly continuous maize (Zea mays) cultivation with little or no nutrient inputs. The use of short-duration tree fallows was one of several agroforestry options hypothesized to restore soil fertility. Sesbania sesban, an indigenous N₂-fixing tree was the most promising among species tested in screening trials. Several studies since 1987 have demonstrated the dramatic potential of two- or three-year sesbania fallows in restoring soil fertility and increasing maize yields. Analyses showed that these improved fallow systems were feasible, profitable, and acceptable to farmers. Results suggest that high maize yields following fallows are primarily due to improved N input and availability by the fallows. The potential to increase maize production without applying mineral fertilizers has excited thousands of farmers who are enthusiastically participating in the evaluation of this technology. The number of farmers who are testing a range of improved fallow practices has increased from 200 in 1994 to over 3000 in 1997. Presently, a strong network of institutions comprising government, NGOs, development projects, and farmer organizations is facilitating the adaptive research and expansion of improved fallow technology in eastern Zambia. Key elements in the research process that contributed to the achievements are effective diagnosis of farmers' problems, building on farmers' indigenous knowledge, generating several different fallow options for farmers to test, ex-ante economic analysis, farmer participation in on-farm trials, and development of a network for adaptive research and dissemination.This revised version was published online in November 2005 with corrections to the Cover Date.     

Farmer assessment and economic evaluation of shrub fallows in the Humid Lowlands of Cameroon

Food crop production in highly populated areas around major cities of the humid lowlands of Cameroon is highly dependent on a fallow system (two–four years duration) mainly of Chromolaena odorata. Where such fallows have been in use for some time, problems of soil fertility with declining crop yields and higher incidence of weeds were reported. Although improved fallows have been widely adopted in sub-humid zones, there is no evidence of successful adoption of agroforestry-based technologies for soil fertility improvement in the humid forest areas. In response, ICRAF has developed a short fallow system with Cajanus cajan for soil fertility improvement in the humid lowlands of West Africa. Farmers' response to these cajanus fallows is positive. Benefits reported are higher crop yields after cajanus fallows compared to natural fallows, clearing of cajanus is easier and the shrubs shade out the weeds. Women particularly appreciate the technology for its low labour demand and for the fact that these shrubs can be planted on land with less secure tenure. Economic analysis of cajanus fallows compared to natural fallow over six years shows that cajanus fallows are profitable under most tested scenarios, both in terms of returns to land and to labour. It seems that improved fallows with Cajanus cajan are a good response to shortening natural fallows for households in the humid lowlands of Cameroon with land constraints. However, wider dissemination of the technology requires a targeted extension approach and adequate seed supply strategies, which should be based on joint efforts between farmers, extension services and research.This revised version was published online in November 2005 with corrections to the Cover Date.     

Food-crop-based production systems as sustainable alternatives for Imperata grasslands?

Purely annual crop-based production systems have limited scope to be sustainable under upland conditions prone to infestation by Imperata cylindrica if animal or mechanical tillage is not available. Farmers who must rely on manual cultivation of grassland soils can achieve some success in suppressing Imperata for a number of years using intensive relay and intercropping systems that maintain a dense soil cover throughout the year, especially where leguminous cover crops are included in the crop cycle. However, labour investment increases and returns to labour tend to decrease in successive years as weed pressure intensifies and soil quality declines.Continuous crop production has been sustained in many Imperata-infested areas where farmers have access to animal or tractor draft power. Imperata control is not a major problem in such situations. Draft power drastically reduces the labour requirements in weed control. Sustained crop production is then dependent more solely upon soil fertility management. Mixed farming systems that include cattle may also benefit from manure application to the cropped area, and the use of non-cropped fallow areas for grazing. In extensive systems where Imperata infestation is tolerated, cassava or sugarcane are often the crops with the longest period of viable production as the land degrades.On sloping Imperata lands, conservation farming practices are necessary to sustain annual cropping. Pruned tree hedgerows have often been recommended for these situations. On soils that are not strongly acidic they may consistently improve yields. But labour is the scarcest resource on small farms and tree-pruning is usually too labour-intensive to be practical. Buffer strip systems that provide excellent soil conservation but minimize labour have proven much more popular with farmers. Prominent among these are natural vegetative strips, or strips of introduced fodder grasses.The value of Imperata to restore soil fertility is low, particularly compared with woody secondary growth or Compositae species such as Chromolaena odorata or Tithonia diversifolia. Therefore, fallow-rotation systems where farmers can intervene to shift the fallow vegetation toward such naturally-occurring species, or can manage introduced cover crop species such as Mucuna utilis cv. cochinchinensis, enable substantial gains in yields and sustainability. Tree fallows are used successfully to achieve sustained cropping by some upland communities. A variation of this is rotational hedgerow intercropping, where a period of cropping is followed by one or more years of tree growth to generate nutrient-rich biomass, rehabilitate the soil, and suppress Imperata. These options, which suit farmers in quite resource-poor situations, should receive more attention.     

Green manures for maize—bean systems in eastern Uganda: Agronomic performance and farmers' perceptions

Researchers worked with farmers in eastern Uganda to develop alternatives for soil management using crotalaria (Crotalaria ochroleuca), mucuna (Mucuna pruriens var. utilis), lablab (Dolichos lablab), and canavalia (Canavalia ensiformis) as green manures in short-term fallows. The participatory research was part of a community-based approach for systems improvement. Grain yields of maize (Zea mays) and bean (Phaseolus vulgaris) following one season of crotalaria fallow were 41% and 43%, respectively, more than following a two-season weedy fallow. Grain yields of maize following a one-season fallow with mucuna and lablab were 60% and 50% higher, respectively, as compared with maize following maize. Maize and bean yield were more, although effects were small, during the second and third subsequent seasons, indicating probable residual effects of the green manures. Mucuna and lablab were successfully produced by intersowing into maize at three weeks after sowing maize, although the yields of the associated maize crop were reduced by 24% to 28%. Farmers estimated the labor requirements for mucuna and lablab to be less than for crotalaria. Farmers independently experimented on how these species can be integrated into banana (Musa spp.), coffee (Coffea robusta), sweet potato (Ipomoea batatas), and cassava (Manihot esculenta) production systems. Farmers reported that the beneficial effects of the green manures included higher food-crop yields; weed suppression; improved soil fertility, soil moisture, and soil tilth; and erosion control. Mucuna and lablab were preferred because of reduced labor requirements and increased net benefits compared with continuous cropping. Farmer participation in the green manure research resulted in efficient generation and adaptation of green manure technology now being promoted in eastern and central Uganda.This revised version was published online in November 2005 with corrections to the Cover Date.     

Adoption potential of selected organic resources for improving soil fertility in the central highlands of Kenya

Soil fertility decline is the major cause of declining crop yields in the central highlands of Kenya and elsewhere within the African continent. This paper reports a study conducted to assess adoption potential of two leguminous trees, two herbaceous legumes, cattle manure, and Tithonia diversifolia either solely applied or combined with inorganic fertilizer, for replenishing soil fertility in the central highlands of Kenya. The study examined biophysical performance, profitability, feasibility and acceptability, and farmers experiences in managing and testing the inputs. The study was based on a series of studies incorporating both sociological and experimental approaches for two and a half years. Results of on farm trials showed that manure + fertilizer and tithonia + fertilizer treatments increased yields by more than 100% above the control. These treatments were the most profitable having highest net benefits and benefit cost ratios. They were also the most commonly preferred by farmers who used them on larger plots compared to the other inputs. In conclusion, cattle manure and tithonia were found to be the organic materials with the highest adoption potential for soil fertility improvement in this area. Calliandra calothyrsus and Leucaena trichandra, on the other hand, have potential for use as animal fodder. The herbaceous legumes had the least adoption potential due to poor performance recorded on the farms that possibly led to low preference by the farmers. However, issues of sustainable seed production could have played a role. This study recommends some policy issues for enhancing adoption and research issues focusing on exploring strategies for increasing biomass production and use efficiency on farms. Andre Bationo was formerly working for Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT) as the African network Coordinator.     

Socioeconomic factors affecting the adoption potential of improved tree fallows in Africa

In many parts of Africa, farmers periodically fallow their land, which is allowing land to lie idle for one or more seasons primarily to restore its fertility. This paper assesses the feasibility, profitability, and acceptability of improved tree fallows, which are the deliberate planting of trees or shrubs in rotation with crops to improve soil fertility. Improved tree fallows are assessed at different stages of intensification, drawing on farmers' experiences in three different settings. In extensive systems where land is plentiful and existing fallows with natural regeneration of vegetation restore soil fertility (southern Cameroon), farmers have little incentive to invest labor in establishing improved fallows. Where population density is higher and fallow periods are decreasing and farmers perceive a decline in soil fertility (eastern Zambia), improved fallows have great potential. In intensive systems where land is unavailable and cropping is often continuous (western Kenya), many farmers find it difficult to fallow land. Even here, there is scope for introducing improved fallows, especially among farmers who have off-farm income. Labor constraints and institutional support were found to greatly influence the feasibility of improved fallows. In intensive systems, low returns to cropping, low base yields, and a high opportunity cost of labor increase the returns to improved fallows. Principal factors associated with acceptability include past perception of soil fertility problems, past use of measures for improving soil fertility, current fallowing, economic importance of annual cropping, and wealth level. Adoption potential may be increased by reducing fallow periods, intercropping trees and crops during the first season, reducing establishment costs, producing higher value by-products, and by encouraging farmers to test improved fallows on high-value crops.This revised version was published online in November 2005 with corrections to the Cover Date.     

Potential of improved fallows to increase household and regional fuelwood supply: evidence from western Kenya

Fuelwood is the main energy source for households in rural Africa, but its supply is rapidly declining especially in the densely populated areas. Short duration planted tree fallows, an agroforestry technology widely promoted in sub-Sahara Africa for soil fertility improvement may offer some remedy. Our objective was to determine the fuelwood production potential of 6, 12 and 18 months (the common fallow rotation periods) old Crotalaria grahamiana, Crotalaria paulina, Tephrosia vogelli and Tephrosia candida fallows under farmer-managed conditions in western Kenya. Based on plot-level yields, we estimated the extent to which these tree fallows would meet household and sub-national fuelwood needs if farmers planted at least 0.25 hectares, the proportion of land that is typically left under natural fallows by farmers in the region. Fuelwood yield was affected significantly (P < 0.05) by the interaction between species and fallow duration. Among the 6-month-old fallows, T. candida produced the highest fuelwood (8.9 t ha⁻¹), compared with the rest that produced between 5.6 and 6.2 t ha⁻¹. Twelve months old T. candida and C. paulina also produced significantly higher fuelwood yield (average, 9.6 t ha⁻¹) than T. vogelli and C. grahamiana of the same age. Between the fallow durations, the 18-month fallows produced the most fuelwood among the species evaluated, averaging 14.7 t ha⁻¹. This was 2–3 times higher than the average yields of 6 and 12-month-old fallows whose yields were not significantly different. The actual fuelwood harvested from the plots that were planted to improved fallows (which ranged from 0.01 to 0.08 ha) would last a typical household between 11.8 and 124.8 days depending on the species and fallow duration. This would increase to 268.5 (0.7 years) and 1173.7 days (0.7–3.2 years) if farmers were to increase area planted to 0.25 ha. Farmers typically planted the fallows at high stand densities (over 100,000 plants ha⁻¹ on average) in order to maximize their benefits of improving soil fertility and providing fuelwood at the same time. This potential could be increased if more land (which fortunately exists) was planted to the fallows within the farms in the region. The research and development needs for this to happen at the desired scale are highlighted in the paper.     

Cajanus cajan (L.) Millsp. as a potential agroforestry component in the Eastern Province of Zambia

Farmers in the Eastern Province of Zambia are faced with problems common to other parts of the tropics: increased pressure to expand food production leading to accelerated forest clearing, decrease in traditional fallow periods, increased soil erosion, and reductions in soil fertility. Of special concern are shortages of labor during their growing season, a shortage of staple foods during January through March, pest (termite) problems, and seasonal fires. Alleycropping appears able to solve some of the farmers' problems. Both on-farm and experiment station trials were initiated to screen potential agroforestry species. Perennial pigeonpea, Cajanus cajan (L.) Millsp., a species indigenous to the Province, showed particular promise. Cultivars grew over 3 m tall and produced up to 4.8 tons/ha dry matter (in 7 months after pruning) for green manure. Farmers reacted favorably to their experience with the on-farm trials. Ease of establishment and production of food (green pod and grain) make perennial pigeonpea a special agroforestry option in the Province, deserving additional research.     

Improved fallows come of age in the tropics

Improved fallows are the deliberate planting of fast-growing species — usually legumes — for rapid replenishment of soil fertility. Improved fallows are rapidly spreading in several regions of the tropics as a sensible way for in situ accumulation of large quantities of N in vegetation and soil, as well as for providing sustainability enhancing services. Research on improved fallows increased after the mid 1980s with the development of what is known as the second soil fertility paradigm, which is based on sustainability considerations. Many lessons have emerged from short-term improved fallows (<5 years duration). These include the diversity of farm sizes where improved fallows are used, the advantage of sequential versus simultaneous systems, the utilization of dry seasons unfavorable for crop production, the comparative advantages of woody versus herbaceous leguminous fallows, the magnitude of N accumulation, the strategic use of N fertilizers, and the importance of P. Other key services provided by fallows include fuelwood production, recycling of nutrients besides N, provision of a C supply to soil microorganisms, weed suppression, Striga control, and improved soil water storage. Natural fallows of non-legume shrubs belonging to the Asteraceae family, collectively called daisy fallows, may provide lessons for the development of improved fallows. The maintenance of genetic diversity in fallows is paramount. The main limiting factor in Africa is clearly the supply of germplasm of improved fallow species. This must be overcome though large-scale seed orchards and nursery development before impact at the scale of millions of farmers can take place.This revised version was published online in November 2005 with corrections to the Cover Date.     

Potential for adoption of Sesbania sesban improved fallows in Zimbabwe: A linear programming-based case study of small-scale farmers

Farmers' adoption of improved technologies is the ultimate measure of the success of any agricultural innovation. In a joint project of the International Centre for Research in Agroforestry (ICRAF) and the Department of Research and Specialist Services of Zimbabwe, the potential for adoption of the improved planted fallow technology using Sesbania sesban was assessed in the Mangwende Communal Area. The study was based on experimental data of maize (Zea mays) yields following 1-, 2- and 3-year improved fallows at Domboshawa Training Center, northern Zimbabwe where the improved fallows were promising. The data indicated that maize yields were higher after S. sesban fallows than after Cajanus cajan and Acacia angustissima fallows. A five-year linear programming model sensitive to the diversity within households was developed to simulate the livelihood system of households in the Mangwende Communal Area. Improved fallows of S. sesban were incorporated into the model to determine the potential for their adoption. Model results indicated that there is potential for the technology to be adopted by 80% of the farmers. According to the model, the new technology on average occupies 60% of the area under maize. Nevertheless, households continue to use fertilizers and cattle manure. One-year improved fallows are planted every other year; three-year improved fallows are also planted. Farmers who adopt the fallow technologies realize an increase in the cash available for discretionary spending. Factors such as composition of the household in terms of fulltime workers, size of the arable land owned by the farmer, and whether the household differentiates activities by gender, determine the adoption of the improved-fallow technology. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dry-season sesbania fallows and their influence on nitrogen availability and maize yields in Malawi

Nitrogen deficiency is widespread in southern Africa, but inorganic fertilizers are often unaffordable for smallholder farmers. Short-duration leguminous fallows are one possible means of soil fertility restoration. We monitored preseason topsoil (0 to 20 cm) ammonium and nitrate, fallow biomass production and grain yields for three years in a relay cropping trial with sesbania [Sesbania sesban (L.) Merr.] and maize (Zea mays L.). Sesbania seedlings were interplanted with maize during maize sowing at 0, 7400 or 14,800 trees ha^–1, in factorial combination with inorganic N fertilizer at 0 or 48 kg N ha^–1 (half the recommended rate). After maize harvest, fallows were allowed to grow during the seven-month dry season, and were cleared before sowing the next maize crop. Both sesbania fallows and inorganic N fertilizer resulted in significantly greater (P < 0.01 to 0.05) preseason topsoil nitrate-N than following unfertilized sole maize. In plots receiving no fertilizer N, preseason topsoil inorganic N correlated with maize yield over all three seasons (r ² = 0.62, P < 0.001). Sesbania fallows gave significantly higher maize yields than unfertilized sole maize in two of three years (P < 0.01 to 0.05). Sesbania biomass yields were extremely variable, were not significantly related to sesbania planting density, and were inconsistently related to soil N fractions and maize yields. Short-duration fallows may offer modest yield increases under conditions where longer duration fallows are not possible. This gain must be considered against the loss of pigeonpea (Cajanus cajan L. Millsp) harvest in the similarly structured maize-pigeonpea intercrop common in the region.This revised version was published online in November 2005 with corrections to the Cover Date.     

What do farmers think? Farmer evaluations of hedgerow intercropping under semi-arid conditions

Results from farmer evaluations of exploratory on-farm trials with hedgerow inter-cropping in a semi-arid region of Kenya (the Yatta Plateau of Machakos District) are presented. Hedgerows ofCassia siamea, Gliricidia sepium andLeucaena leucocephala, which were managed by eight farmers to enhance soil fertility, were established in the mid-1980s. Although trial farmers appear convinced of the ability of the technology to increase crop yields, reduce erosion and provide firewood, most did not feel that the short-term risks (i.e. reduction in crop productivity) and the extra labour required were worth the long-term benefits. Hence, few of the eight farmers were willing to extend their hedges. The discussion suggests that in the risky production environment of the Yatta Plateau, where crop yields fluctuate constantly with the amount of rainfall, soil fertility is not of urgent concern to farmers. A participatory case study approach, which emphasizes farmers' opinions, preferences and ideas elicited through regular farmer evaluation exercises, as distinct from on-farm technology testing for validation, is recommended for on-farm research on new and complex agroforestry technologies. This approach permits between understanding of some of the complex socio-economic factors affecting the adoption of hedgerow intercropping and is the most cost-effective way to gather in-depth and contextual information on farmer decision-making and management of externally introduced agroforestry technologies.     

Cost-benefit analysis of alternative forms of hedgerow intercropping in the Philippine uplands

Considerable resources have been expended promoting hedgerow intercropping with shrub legumes to farmers in the Philippine uplands. Despite the resources committed to research and extension, persistent adoption by farmers has been limited to low cost versions of the technology including natural vegetation and grass strips. In this paper, cost-benefit analysis is used to compare the economic returns from traditional open-field maize farming with returns from intercropping maize between leguminous shrub hedgerows, natural vegetation strips and grass strips. An erosion/productivity model, Soil Changes Under Agroforestry, was used to predict the effect of erosion on maize yields. Key informant surveys with experienced maize farmers were used to derive production budgets for the alternative farming methods. The economic incentives revealed by the cost-benefit analysis help to explain the adoption of maize farming methods in the Philippine uplands. Open-field farming without hedgerows has been by far the most popular method of maize production, often with two or more fields cropped in rotation. There is little persistent adoption of hedgerow intercropping with shrub legumes because sustained maize yields are not realised rapidly enough to compensate farmers for establishment and maintenance costs. Natural vegetation and grass strips are more attractive to farmers because of lower establishment costs, and provide intermediate steps to adoption. Rural finance, commodity pricing and agrarian reform policies influence the incentives for maize farmers in the Philippine uplands to adopt and maintain hedgerow intercropping. This revised version was published online in August 2006 with corrections to the Cover Date.     

Abundance of insect pests and their effects on biomass yields of single vs. multi-species planted fallows

Indigenous and exotic leguminous shrubs that are promising for planted fallow for soil fertility replenishment in east and southern Africa have been found to harbour many herbivorous insects, giving suspicion that widespread adoption of fallow systems may aggravate insect pests. Studies were conducted on farms in western Kenya from 1999 to 2001 to monitor the abundance of herbivorous insects and assess their effects on biomass yields of pure and mixed fallows. The treatments tested were single and two-species mixtures of Tephrosia vogelii, Sesbania sesban and Crotalaria grahamiana and a natural fallow in a split plot design, with the fallow systems in the main plots and protection vs. no protection against insects in sub-plots spread over six farms. Eighteen insect species belonging to seven orders and 14 families were identified as pests of␣the fallows with varied abundance and infestation level across the sites. While Hilda patruelis and Amphicallia pactolicus were most damaging to C. grahamiana, Mesoplatys ochroptera was detrimental to S.␣sesban. T. vogelii hosted fewer insects than others. Nevertheless the pest infestation did not cause significant biomass yield reduction during the study period. Pest attack was generally greater in villages that had been testing the planted fallows for some years compared with villages that took up the fallows recently. This indicates the potential for increased pest infestation with increased adoption of the system by farmers. Multi-species fallows did not indicate any advantage over single species fallows in terms of either reduced pest incidence or increased biomass production.     

Alley cropping Sesbania sesban (L) Merill with food crops in the highland region of Rwanda

Food production in the densely populated Rwandan highlands is impeded by soil erosion and loss in fertility. Alley cropping leguminous shrubs with food crops on contours is purported to minimize the problem and to provide wood and forage. This study reports the effect of Sesbania prunings plus moderate levels of N and P on bean (Phaseolus sp) and maize (Zea mays) yields in alley cropping. Experimental design was a randomized complete block with split-split plots. Main plots were alley width: 2, 4, 6 and 8 m. Phosphorus (P) at 0, 30 and 60 kg P₂O₅/ha occupied the subplot and nitrogen (N) at 0, 30 and 60 kg/ha were assigned at the sub-sub plot level. No P was applied to maize during the second cropping season. Crop yield in kg/ha included the land space taken by hedgerows. Bean yield in 6 m alleys (1100 kg/ha) was about twice that in 2 m alleys (500 kg/ha). Bean responded to N and P. Optimum alley width and N for bean yield were 6 m and 30 kg/ha, respectively. Cuttings from alley hedgerows provided stakes for climbing beans. Maize responded to N but not to residual P. The highest maize yield came from 8 m alleys with 40 kg/ha, but yields from 8 and 6 m alleys with the same N treatment were not significantly different. Maize plants in middle rows were significantly taller than plants in rows adjacent to hedgerows. Maize rust development showed significant alley width and row position effect. There were significantly fewer uredinia in the Sebania alleys relative to the control plots without shrub hedgerows. Rust development on maize in middle rows was significantly greater than development in border rows.     

Priority setting in research for sustainable land use: the case of the Adja Plateau,Benin

TheProjet Recherche Appliquée en Milieu Réel is situated on the Adja Plateau, Southwest Benin. The project is responsible for a FSR&D programme that aims at sustainable use of the so-calledTerres de barre. Technologies tested in this context include seasonal fallowing, usingMucuna pruriens cv. group Utilis (weed control, green manure), planted fallows using fast-growing species such asAcacia auriculiformis and hedgerow intercropping systems (various woody auxiliary species). In this paper the local farming system is analyzed to identify key parameters that determine the possibilities for technology adoption. Parameters include: function and history of fields; tenureship; the field's position with regard to fertility flows in the farming system.The paper also aims at understanding the reasons behind low crop yields, in this case maize. An analysis of the soil's chemical fertility, using a computer-based fertility assessment model and data from literature, shows some anomalies with respect to the availability of nutrients from organic matter. Data presented on the use of green manures and oil palm fallow compared to cultivated fields make it likely that organic matter management is a crucial factor in sustainable use of these soils (maintenance of soil structure and soil life). Given key parameters, a matrix is used to determine potential niches in the farming system for the various biomass production technologies mentioned previously, including the locally developed planted fallow based on oil palm.     

Soil fertility changes and response of maize and beans to green manures of leucaena,sesbania and pigeonpea

Three multipurpose tree species (MPTS)-leucaena (Leucaena leucocephala), sesbania (Sesbania sesban var. nubica) and pigeonpea (Cajanus cajan) were pruned at a height of 60 cm above the ground every two months, and resulting plant biomass was incorporated into the soil as green manure. For comparison, maize (Zea mays) stover was also incorporated into some plots, while some other plots were left fallow. Varying quantities of plant biomass which were incorporated into the soil over a period of 12 months caused large changes in major soil plant nutrients, and it substantially improved soil fertility. To test for improved soil fertility, test crops of maize and beans (Phaseolus vulgaris) were grown on the plots after six biomass incorporations of 4806, 13603, 16659 and 7793 kg. ha^–1yr^–1 for pigeonpea, sesbania leucaena and maize, respectively. Responses of the test crops indicated that sesbania and leucaena green manures improved maize stover, cobs and grain yields; and bean haulms and grain yields by 77.6% when compared to fallow plots. Residual effects of green manures still resulted in significant (P < 0.05) yield differences in the test cropin the third testing season. Economic significance of green manures in increasing food crop yields to small scale farmers is discussed.     

The persistence of secondary forests on colonist farms in the BrazilianAmazon

While slash-and burn farmers convert forest to agriculture, they also regenerate significant areas of secondary fallow forests on their farms. Under what conditions does secondary forest cover persist on slash-and-burn farms? Survey data from Pará, Brazil show that secondary forests occupy 20% of farm area even after a century of settlement. In addition to restoring soil fertility, secondary forests contribute over 20% of farmers' income through products such as charcoal, fruit, game animals and firewood for on-farm processing. Econometric analysis shows that slow rates of population growth and increases in agricultural incomes through on-farm processing of agricultural products enable farmers to maintain long fallows and result in diversified systems compatible with secondary forest cover in the study area. On the other hand, declining agricultural productivity, subsidized credit, declines in the growth rate of secondary forests and policies favoring speculative land acquisition threaten secondary forest persistence. In older settlement areas, secondary forests are often the only forest resource available to the rural poor. Fallow areas should therefore be managed not only for agricultural productivity, but also for conserving forest resources.     

Biomass production of tree fallows and their residual effect on maize in Zimbabwe

The rotation of maize (Zea mays) with fast-growing, N₂-fixing trees (improved fallows) can increase soil fertility and crop yields on N-deficient soils. There is little predictive understanding on the magnitude and duration of residual effects of improved fallows on maize yield. Our objectives were to determine the effect of fallow species and duration on biomass production and to relate biomass produced during the fallow to residual effects on maize. The study was conducted on an N-deficient, sandy loam (Alfisol) under unimodal rainfall conditions in Zimbabwe. Three fallow species — Acacia angustissima, pigeonpea (Cajanus cajan), and Sesbania sesban — of one-, two-, and three-year duration were followed by three seasons of maize. Pigeonpea and acacia produced more fallow biomass than sesbania. The regrowth of acacia during post-fallow maize cropping provided an annual input of biomass to maize. Grain yields for the first unfertilized maize crop after the fallows were higher following sesbania (mean = 4.2 Mg ha^–1) than acacia (mean = 2.6 Mg ha^–1). The increased yield of the first maize crop following sesbania was directly related to leaf biomass of sesbania at the end of the fallow. Nitrogen fertilizer did not increase yield of the first maize crop following one- and two-year sesbania fallows, but it increased yield following acacia fallows. Nitrogen fertilizer supplementation was not required for the first maize crop after sesbania, which produced high-quality biomass. For acacia, which produced low-quality biomass and regrew after cutting, N fertilizer increased yield of the first post-fallow maize crop, but it had little benefit on yield of the third post-fallow maize crop.This revised version was published online in November 2005 with corrections to the Cover Date.     

Short fallows of Tithonia diversifolia and Crotalaria grahamiana for soil fertility improvement in western Kenya

Managed short-duration fallows may have the potential to replace longer fallows in regions where population density no longer permits slow natural fallow successions. The purpose of fallows is not only to improve subsequent crop performance but also to restore soil fertility and organic matter content for the long term. We therefore evaluated the soil organic matter and nutrient flows and fractions in a short fallow experiment managed in the western Kenya highlands, and also compared the experimental area with a 9–12-yr-oldadjacent natural bush fallow. The factorial agroforestry field experiment with four land-use and two P fertilizer treatments on a Kandiudalfic Eutrudox showed that 31-wk managed fallows with Tithonia diversifolia(Hemsley) A. Gray and Crotalaria grahamiana Wight &Arn. improved soil fertility and organic matter content above those of a natural weed fallow and continuous maize (Zea mays L.). Post-fallow maize yields were also improved, although cumulative three-season increases in yield were small (0–1.2 Mg ha⁻¹) when the yield foregone during the fallow season was accounted for. Improvements in yield and soil quality could be traced to quantity or quality of biomass recycled by the managed fallows. The non-woody recycled biomass produced by the continuous maize, weed fallow, and tithonia treatments was near 2Mg ha⁻¹, whereas crotalaria produced three times more recyclable biomass and associated N and P. Increases in topsoil N due to the fallows may have been attributable in part to deep acquisition and recycling of N by the fallows. Particulate macro-organic matter produced by the fallows contained sufficient N(30–50 kg ha⁻¹) to contribute substantially to maize production. Organic Paccumulation (29 kg ha⁻¹) similarly may play a significant role in crop nutrition upon subsequent mineralization. The effect of the P fertilizer application on soil properties and maize yield was constant for all land-use systems (i.e., no land-use system × P fertilizer interactions occurred). There was an indication that tithonia may have stimulated infestation of Striga hermonthica (Del.) Benth., and care must be taken to evaluate the full effects of managed fallows over several seasons. This revised version was published online in June 2006 with corrections to the Cover Date.