Traditional fallows in Latin America

Traditional fallow systems of Latin America have not been extensively studied from either a socioeconomic or biophysical viewpoint. Only in the past decade have some of these systems — modified shifting cultivation in the Amazon, the bracatinga (Mimosa scabrella) fallow of southeastern Brazil, the babassu (Attalea spp.) system of the Amazon margins, and short bush fallows mostly for Phaseolus bean (the so-called frijol tapado) — received any attention. Over the past century, traditional cultivators have adopted several legumes such as Mucuna spp., Lathyrus nigrivalis, Canavalia spp., and Senna guatemalensis as green manures or managed fallows in food crop production systems, which have recently been the object of studies by social and biological scientists. Longer, monospecific fallows involving woody legumes have been studied to varying degrees. The legumes include Senna guatemalensis, Mimosa tenuiflora, and Gliricidia sepium. Systems in which crop production is alternated with animal grazing of secondary vegetation include the caatinga of Northeast Brazil, the espinales of Chile, the matorrales of northern Mexico, and the chaco of Argentina, Paraguay, and Bolivia. A classification is proposed depending on the nature of the species (woody or nonwoody) and their composition (mono- or multispecies) and land management (burned or not). Much work is still needed in more complete characterization, determination of economic importance and potential, understanding the relationships among components, studies of nutrient cycling, and verification of the effects of shorter fallow periods of these systems.This revised version was published online in November 2005 with corrections to the Cover Date.     

Nutrient stocks of short-term fallows on a high base status soil in the humid tropics of Papua New Guinea

In order to understand nutrient dynamics in tropical farming systems with fallows, it is necessary to assess changes in nutrient stocks in plants, litter and soils. Nutrient stocks (soil, above ground biomass, litter) were assessed of one-year old fallows with Piper aduncum, Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea. The experiment was conducted on a high base status soil (Typic Eutropepts), and in Papua New Guinea such soils are intensively used for agriculture. Soil samples were taken prior to fallow establisment and after one year when the fallows were slashed and above ground biomass and nutrients measured. The above ground and litter biomass of piper was 13.7 Mg dry matter ha^-1, compared to 23.3 Mg ha^-1 of gliricidia and 14.9 Mg ha^-1 of imperata. Gliricidia produced almost 7 Mg ha^-1 wood. Total above ground biomass returned to the soil when the fallows were slashed was the same for piper and gliricidia (8 Mg ha^-1). Gliricidia accumulated the largest amounts of all major nutrients except for K, which was highest in the above ground piper biomass. Imperata biomass contained the lowest amount of nutrients. The largest stocks of C, N, Ca and Mg were found in the soil, whereas the majority of P was found in the above ground biomass and litter. Almost half of the total K stock of piper and gliricidia was in the biomass. During the fallow period, soil organic C significantly increased under gliricidia fallow whereas no net changes occurred in piper and imperata fallows. The study has shown large differences in biomass and nutrient stocks between the two woody fallows (piper, gliricidia) and between the woody fallows and the non-woody fallow (imperata). Short-term woody fallows are to be preferred above grass (imperata) fallows in the humid lowlands of Papua New Guinea because of higher nutrient stocks.     

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.     

Spectral irradiance and stomatal conductance of enriched fallows with fast-growing trees in eastern Amazonia,Brazil

The biomass production of both indigenous and introduced plant species in tropical fallow systems depends on the degree to which these species can acclimate to the light and water environments. Results for light spectral composition monitoring within the canopy of enriched fallows and for the leaf stomatal conductance of fast-growing leguminous trees and indigenous fallow species are presented. All measurements were made in a smallholder farm in Igarapé-Açu, northeastern Pará State, Brazil. Light spectral composition (330 to 1100 nm) was monitored at two heights (ground level and 1 m) in a six-year-old natural fallow, 1.5-year-old natural fallow, and 1.5-year-old fallows enriched with Acacia angustissima, Acacia mangium, Clitoria racemosa, Inga edulis, Sclerolobium paniculatum, and a mixture of these trees. Light-quality parameters including photosynthetically active radiation, phytochrome active radiation, and blue active radiation changed most drastically in the stands enriched with A. mangium. Stomatal conductance was higher for A. mangium than the other trees and four common indigenous fallow vegetation species (Phenakospermum guyannense, Davilla rugosa, Lacistema pubescens, and Myrcia bracteata). Results suggest that the enrichment of fallows with A. mangium may promote changes in light and water vapor exchange regimes, with potential effects on species diversity in fallows.This revised version was published online in November 2005 with corrections to the Cover Date.     

Planted legume fallows reduce weeds and increase soil N and P contents but not upland rice yields

Shortened fallows have resulted in declining upland rice yields in slash-and-burn upland rice systems in northern Laos. We studied the benefit of planted legume fallows for rice productivity, weeds, and soil nitrogen and phosphorus availability. Four systems were evaluated over a 5-year period: 1-year fallow with native species, 1-year Cajanus cajan fallow, 1-year Leucaena leucocephala fallow, and continuous annual rice cropping. Rice was grown either once each year as continuous annual cropping or in alternate years of 2001, 2003, and 2005. C. cajan and L. leucocephala were sown with rice during the 2001 growing season. In subsequent years, L. leucocephala regenerated from root stock and did not have to be resown, whereas C. cajan was resown in 2003. Establishment of either C. cajan or L. leucocephala had no significant effect on rice yield in 2001, and rice yields ranged from 2.0 to 2.3 t/ha. Rice yields declined rapidly in succeeding years, and rice yields in the four systems ranged from 0.7 to 1.1 t/ha in 2003 and from 0.3 to 0.5 t/ha in 2005. Although two planted fallow systems increased nitrogen input because of greater biomass accumulation in 2003 and 2005 and soil phosphorus availability was higher following L. leucocephala fallow in 2005, there were no significant differences in rice yields among the four systems in either year. Weed biomass during the rice growing season increased each year in all systems and increased more rapidly for continuous annual rice cropping, in which the dominant weed species was Ageratum conyzoides L. Among the other three systems, there were no significant differences in the weed biomass in 2003 and 2005. We conclude that C. cajan and L. leucocephala as 1-year fallows do not offset the negative effects of increased cropping intensity on rice yield in this region.     

Essential oil production increases value of Psiadia altissima fallows in Madagascar’s eastern forests

Fallow with Psiadia altissima is one of the most common post-‘slash and burn’ vegetation successions described in the evergreen forests of eastern Madagascar. Some fallows consist of almost pure stands of this species, of which the leaves produce an essential oil offering international commercial interest. The present research aims to evaluate the production potential of essential oil derived from different fallows rich in P. altissima. The study has revealed that fallows aged 4 and 6 years since the last crop abandonment produce the most essential oil (around 20 l ha⁻¹), but relative to fallow duration, the youngest fallows (1- or 2-year-old) are the most productive, respectively producing 12 and 6 l ha⁻¹ year⁻¹. Additionally, the trees from the youngest fallows have a substantial capacity for regeneration from coppice shoots, on condition that the cut is performed well above the root collar. Although farmers earn five times less from harvesting leaves than from cultivating rice from tavy, the possibility is there for them to complement their income and diversify their production. The overall results show that sustainable exploitation of fallows of P. altissima is a conceivable option. However, this can only be achieved through an integrated approach that takes into account the environmental and social constraints associated with the development of this new activity.     

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.     

Plant growth, biomass production and nutrient accumulation by slash/mulch agroforestry systems in tropical hillsides of Colombia

Planted fallow systems under ‘slash and mulch’ management were compared with natural fallow systems at two farms (BM1 and BM2) in the Colombian Andes. The BM1 site was relatively more fertile than the BM2 site. Planted fallow systems evaluated included Calliandra calothyrsus CIAT 20400 (CAL), Indigofera constricta (IND) or Tithonia diversifola (TTH). During each pruning event slashed biomass was weighed, surface-applied to the soil on the same plot and sub-samples taken for chemical analyses. While Indigofera trees consistently showed significantly greater (p < 0.05) plant height and collar diameter than Calliandra trees at both study sites, only collar diameter in Indigofera was significantly affected at all sampling times by differences between BM1 and BM2. After 27 months, TTH presented the greatest cumulative dry weight biomass (37 t ha^–1) and nutrient accumulation in biomass (417.5 kg N ha^–1, 85.3 kg P ha^–1, 928 kg K ha^–1, 299 kg Ca ha^–1 and 127.6 kg Mg ha^–1) among planted fallow systems studied at BM1. Leaf biomass was significantly greater (P < 0.05) for CAL than IND irrespective of site. However, CAL and IND biomass from other plant parts studied and nutrient accumulation were generally similar at BM1 and BM2. At both sites, NAT consistently presented the lowest biomass production and nutrient accumulation among fallow systems. Planted fallows using Calliandra and Indigofera trees had the additional benefit of producing considerable quantities of firewood for household use. This revised version was published online in June 2006 with corrections to the Cover Date.     

Shifting cultivation effects on Brazil nut (Bertholletia excelsa) regeneration

Brazil nut (Bertholletia excelsa), has emerged as the cornerstone of the extractive economy in much of the Amazon, but the debate continues as to whether or not current harvest levels have a detrimental effect on Brazil nut seedling recruitment. While regeneration studies to date have been conducted solely within mature forest, our study provides further insight into current Brazil nut regeneration dynamics, with a unique first look at regeneration in swidden fallows within two multiple-use areas in Acre, Brazil. Recruitment of individuals ≤10 cm diameter at breast height (dbh) was evaluated within three 9 ha mature forest plots and six fallows of 0.5–1.0 ha each. Higher mean densities of B. excelsa seedlings (individuals <1.5 m in height) were observed in fallow than in mature forest (12.7 and 5.3 individuals ha⁻¹, respectively). A generalized linear mixed model analysis revealed that in addition to forest type, proximity to seed sources also explained seedling densities. Height and diameter growth of young B. excelsa individuals was positively related to light availability. Photosynthetic photon flux density levels were over three times higher in fallows versus mature forest during peak sunlight hours. Results suggest that fallow environments may provide favorable conditions for B. excelsa regeneration. Not only can anthropogenic disturbance in the form of shifting cultivation play a positive role in B. excelsa regeneration, its historic presence on the landscape could explain current Brazil nut densities and distributions. Finally, swidden fallows could potentially be managed for enhanced Brazil nut densities, which may provide an opportunity for greater income for extractive families while contributing to the sustainability of Brazil nut extraction and forest conservation in the long term.     

Management of pigeon pea in short fallows for crop-livestock production systems in the Guinea savanna zone of northern Ghana

Crop and livestock production in the Guinea savanna zone of northern Ghana has been declining over the past years as a result of increasing pressure on land. To sustain soil productivity, pigeon pea(Cajanus cajan), a leguminous perennial crop was evaluated for its potential as a short duration fallow crop for fodder and grain, and maize (Zea mays)production. It involved comparing a natural fallow (i.e., control) and four improved fallows of pigeon pea pruned annually at 30 cm, 60 cm and 90 cm from the ground, and unpruned pigeon pea over a two-year period. After this time, the land was cleared manually and planted to maize. The highest mean annual biomass of pigeon pea over the two-year period of 6.1 t ha⁻¹ dry matter (DM) was obtained by pruning at 60 cm. The highest leaf litter production and pigeon pea seed yield was obtained from the no pruning treatment. The mean maize grain yield from the improved fallow (3.02 t ha⁻¹) in the first year after clearing was significantly (P < 0.05) greater than that of the natural fallow (1.54 t ha⁻¹). Considering the biomass of pigeon pea from pruning, pigeon pea seed yield and maize grain yield after the pigeon pea, pruning pigeon pea at 60 cm is the most promising regime for crop-livestock production systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Performance of an improved fallow system in the Peruvian Amazon—modelling approach

As traditional slash-and-burn systems with prolonged fallow periods are no longer feasible in most parts of the tropics, improved agroforestry systems have high potential to increase the productivity of farming systems and sustain continuous crop production. Our objective was to assess biophysical and economic performance of planted leguminous tree fallow (using Inga edulis) compared to the traditional slash-and-burn farming system, practiced by farmers on fields infested with noxious weedy grass Imperata brasiliensis around the city of Pucallpa, Peru. An existing agroforestry model SCUAF was used to predict biophysical factors, such as changes in soil characteristics and farm outputs (crop and tree yield). While a cost–benefit analysis spreadsheet, which uses the output from SCUAF and economic data on input/output levels and prices, calculates economic performance of the systems. The Inga fallow system can provide improvements to a range of soil biophysical measures (C, N, P content). This enables higher levels of farm outputs to be achieved (higher cassava yields). However, for smallholders the improved system must be more economically profitable than the existing one. At prices currently encountered, the Inga fallow system is more profitable than the Imperata fallow system only in the long-term. In adopting the Inga fallow system, smallholders will incur lower profits in the first years, and it will take approximately 10 years for smallholders to begin making a profit above that achievable with the Imperata fallow system. Unless smallholders are capable of accepting the lower profitability in first years, they are less likely to adopt the new system.     

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.     

Weed biomass dynamics in planted fallow systems in the humid forest zone of southern Cameroon

Interest in planted fallow systems has focused on soil fertility improvement, neglecting other potential benefits of such systems. It is important to quantify other processes responsible for crop yield increases under planted fallows, such as weed control. The suppressive potential on weeds of Flemingia macrophylla [(Willd.) Merrill] and Pueraria phaseoloides (Roxb.) Benth, planted fallows was evaluated in field trials in three villages in southern Cameroon. In each village, experiments were set up in 4–5 year-old bush fallow dominated by Chromolaena odorata (L.) R. M. King & H. Rob. and 20 year-old secondary forest. Total aboveground biomass production of P. phaseoloides was 7.45 Mg ha⁻¹, 4.2 times higher than F. macrophylla (1.78 Mg ha⁻¹ ; P < 0.05). The high biomass of P. phaseoloides resulted in a significantly greater reduction in total weed biomass compared to Flemingia macrophylla in both wet and dry seasons. In the wet season (11 and 18 MAP), there were significant fallow system × land use and fallow system × village interactions for total weeds and broadleaf weeds. P. phaseoloides in bush (0.55 Mg ha⁻¹), and P. phaseoloides at Ngoumou (0.09 Mg ha⁻¹) had the lowest total weeds in the wet seasons. After the dry season, the lowest total weed mass was consistently recorded in P. phaseoloides while the highest was in the natural regrowth. The population of grasses was always higher in the F. macrophylla system than in P. phaseoloides system throughout the wet and dry seasons. Grass biomass in the P. phaseoloides-forest LUS was the least (0.01 Mg ha⁻¹), 58 times lower than in F. macrophylla-bush (0.58 Mg ha⁻¹). Biomass production of P. phaseoloides was highly significantly correlated with total weed biomass (r = −0.64; P = 0.004) while no relationship was found between biomass production of F. macrophylla and total weed biomass (r = −0.08, P = 0.747). It was concluded that P. phaseoloides was a suitable leguminous species for weed control. But for F. macrophylla, its low biomass production coupled with a compact plant architecture compromised it as an appropriate species for weed control in a planted fallow system.     

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.     

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.     

Weed seedbank characteristics of arable fields under different fallow management systems in the humid tropical zone of southeastern Nigeria

The composition and pattern of weed flora in arable fields are determined by their seedbank structure; but the influence of fallow management practices on weed seedbank structure is presently unknown. The objective of this study was to investigate weed seedbank characteristics and weed population dynamics in arable fields in natural and planted-fallow systems. The study plots were at Mbaise, a densely populated area of southeastern Nigeria, where farmers regenerate their exhausted soils by maintaining planted fallows of Dactyladenia barteri (Hook. F. ex Oliv.) Prance & F. White, and at Umuahia, a less-densely populated area in the same region, where farmers depend on natural bush fallow for soil regeneration. The effect of three years of fallow on the weed flora of arable fields in the two fallow management systems differed remarkably. The first flush of weeds on fields that were cultivated after three years of planted D. barteri fallow (Mbaise) consisted of 80% broadleaf weeds, 7% grass weeds and 13% sedges. On the other hand, the first flush of weeds on the natural bush fallow fields (Umuahia) of the same fallow duration as the D. barteri fallow system consisted of 17% broadleaf weeds, 70% grasses and 13% sedges. Three years of planted fallow caused 36% decrease in weed seedbank at Mbaise relative to the cropped field while the same duration of natural bush fallow caused a 31% increase in weed seedbank at Umuahia. These results show that the planted D. barteri fallow system has a higher potential to reduce weed pressure in smallholder agriculture than the natural bush fallow system and may explain in part why farmers in this humid forest zone have adopted the practice. This revised version was published online in June 2006 with corrections to the Cover Date.     

Incidence of Mesoplatys ochroptera Stål (Coleoptera: Chrysomelidae) on Sesbania sesban in pure and mixed species fallows in eastern Zambia

The sesbania beetle, Mesoplatys ochroptera Stål (Chrysomelidae: Coleoptera), is a serious defoliator of Sesbania sesban in eastern and southern Africa. Developing integrated pest management practices against the beetle is an important aspect of the adoption of S. sesban as an improved fallow species. Field studies were conducted in eastern Zambia with the objective of determining the incidence of M. ochroptera on S. sesban in pure and mixed species fallows. M. ochroptera did not successfully feed or breed on species other than S. sesban either in the pure or mixed species fallows. The density of adult and immature stages of M. ochroptera was higher in mixtures of S. sesban with Mucuna pruriens, Macrotyloma axillare, Macroptilium atropurpureum or Crotalaria grahamiana compared to a pure S. sesban fallow. Although mixed fallows of S. sesban with M. axillare and M. pruriens may produce fodder, increase the amount and quality of organic inputs and improve nutrient cycling in the soil, they appear to be incompatible with management of M. ochroptera. Mixing S. sesban with G. sepium neither increased larval and adult populations of M. ochroptera feeding on S. sesban nor damage to it. While this mixture appeared to favour development of pupae to the adult stage, it increased mortality of emerging adults. Therefore, it is concluded that mixing S. sesban with G. sepium is more robust in nutrient cycling, improving resource utilisation and management of M. ochroptera on S. sesban.This revised version was published online in November 2005 with corrections to the Cover Date.     

Fallow enrichment of a traditional slash/mulch system in southern Costa Rica: comparisons of biomass production and crop yield

The slash and mulch system of frijol tapado or covered bean is a pre-hispanic system of bean production found throughout much of Central America. However, land use pressures have forced farmers to shorten the traditional fallow period, thus resulting in decreasing productivity. A potential solution is to enrich the fallow by using leguminous nitrogen-fixing trees. The enriched fallow systems evaluated in this study include both single and mixed species treatments: 1) Erythrina poeppigiana; 2) Calliandra colothrysus; 3) Gliricidia sepium; 4) Inga edulis; 5) Inga edulis and Erythrina poeppigiana; 6) Inga edulis and Calliandra calothrysus; and 7) Inga edulis and Gliricidia sepium. Biomass production of the fallow vegetation is shown to be of greater quality and quantity in all fallow enrichment treatments. However, bean yields did not show a significant response to the fallow enrichment treatments.     

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.