Sesbania Fallows Research Articles

Effects of improved fallow with Sesbania sesban on maize productivity and Striga hermonthica infestation in Western Kenya

Striga hermonthica is a major constraint to smallholder subsistence agriculture production in the sub-Saharan African region. Low soil fertility and overall environmental degradation has contributed to the build-up of the parasitic weed infestation. Improved cropping systems have to be introduced to address the interrelated problems of S. hermonthica and soil fertility decline. Thus, the effects of improved fallow with leguminous shrub Sesbania sesban on maize yields and levels of S. hermonthica infestation on farm land in the bimodal highlands of western Kenya were investigated. The experimental treatments were arranged in a phased entry, and randomized complete block scheme were six months Sesbania fallow, 18 months Sesbania fallow, six months natural fallow consisting of regrowth of natural vegetation without cultivation, 18 months natural fallow, continuous maize cropping without fertilizer application, and continuous maize cropping with P and N fertilization. Results show that Sesbania fallows significantly (p<0.05) increase maize yield relative to continuous unfertilized maize. S. hermonthica plant populations decrease in continuous maize between the first season (mean = 428 000 ± 63 000 ha−1) and second season (mean=51 000 ± 15 000 ha−1), presumably in response to good weed management. S. hermonthica seed populations in the soil decrease throughout the duration of the experiment in the continuous maize treatments. Short-duration Sesbania fallows can provide modest yield improvements relative to continuous unfertilized maize, but short-duration weedy fallows are ineffective. Continuous maize cultivation with good weed control may provide more effective S. hermonthica control than fallowing.

AGRONOMIC AND FINANCIAL BENEFITS OF PHOSPHORUS AND NITROGEN SOURCES IN WESTERN KENYA

SUMMARYA better understanding of the agronomic and economic benefits of integrating organic and inorganic fertilizers is essential for their wide-scale dissemination and adoption in smallholder farming systems in developing countries. Field studies were conducted for 10 cropping seasons (1996–2000) on a Kandiudalfic Eutrudox soil to compare the effects of Minjingu phosphate rock (MPR) and triple superphosphate (TSP) on the yield of maize (Zea mays), in combination with three nitrogen (N) sources: urea, Tithonia diversifolia and Sesbania sesban leafy biomass. Urea and tithonia were applied to provide 60 kg N ha−1. Phosphorus (P) from either MPR or TSP was added either once at 250 kg P ha−1 at the beginning of the experiment or annually at 50 kg P ha−1 for five years, the two rates representing two P recapitalization strategies. From the second year, potassium (K) was added to half of each plot to correct for the deficiency that emerged. Over the 10 cropping seasons, the agronomic and economic benefits of the two P recapitalization strategies were similar and were not influenced by the P sources used. With N and K application, both P sources resulted in at least a two-fold maize grain yield increase over the control with no P application that averaged 1.1 tons ha−1. Comparing the N sources, although urea and tithonia had similar net benefits when P was applied, the total cost associated with tithonia was considerably higher. Without P application, sesbania fallow was the most financially attractive option. Sesbania fallows as an N source were also least sensitive to increases in the price of fertilizers and the cost of labour although the financial benefits of this system remained low in the absence of P and K application.

Tree species and pruning regime affect crop yield on bench terraces in SW Uganda

Integration of trees on farms may exert complementary or competitive effects on crop yield. This 4 year study examined novel systems in which Alnus acuminata (alnus), Calliandra calothyrsus (calliandra), Sesbania sesban (sesbania) or a mixture of all three were grown on the degraded upper part of bench terraces in Uganda; beans or maize were grown on the more fertile lower terrace during the short and long rains. Three pruning treatments (shoot, root or shoot + root pruning) were applied to the tree rows adjacent to the crops; shoot prunings were applied as green manure to the woodlot from which they came. Pruning increased survival in calliandra and reduced survival in sesbania; alnus was unaffected. Pruning reduced tree height and stem diameter in alnus, but did not affect calliandra or sesbania. Maize yield adjacent to unpruned calliandra, alnus and sesbania or a mixture of all three was reduced by 48, 17, 6 and 24% relative to sole maize. Shoot pruning initially sustained crop performance but shoot + root pruning became necessary when tree age exceeded 2 years; shoot + root pruning increased maize yield by 88, 40, 11 and 31% in the calliandra, alnus, sesbania and tree mixture systems relative to unpruned trees. Bean yield adjacent to unpruned calliandra, alnus, sesbania and the tree mixture was 44, 31, 33 and 22% lower than in sole crops and pruning had no significant effect on crop yield. The results suggest that sesbania fallows may be used on the upper terrace without reducing crop yield on the lower terrace, whereas pruning of alnus is needed to sustain yield. Calliandra woodlots appear to be unsuitable as crop yield was reduced even after pruning.

Water balance and maize yield following improved sesbania fallow in eastern Zambia

Sesbania [Sesbania sesban (L.) Merr.] fallows are being promoted as a means for replenishing soil fertility in N-depleted soils of small-scale, resource-poor farmers in southern Africa. Knowledge of soil water distribution in the soil profile and water balance under proposed systems is important for knowing the long-term implications of the systems at plot, field and watershed levels. Soil water balance was quantified for maize (Zea mays L.) following 2-year sesbania fallow and in continuous maize with and without fertilizer during 1998–1999 and 1999–2000 at Chipata in eastern Zambia. Sesbania fallow increased grain yield and dry matter production of subsequent maize per unit amount of water used. Average maize grain yields following sesbania fallow, and in continuous maize with and without fertilizer were 3, 6 and 1 Mg ha−1 with corresponding water use efficiencies of 4.3, 8.8 and 1.7 kg mm−1 ha−1, respectively. Sesbania fallow increased the soil-water storage in the soil profile and drainage below the maximum crop root zone compared with the conventionally tilled non-fertilized maize. However, sesbania fallow did not significantly affect the seasonal crop water use, mainly because rainfall during both the years of the study was above the normal seasonal water requirements of maize (400 to 600 mm). Besides improving grain yields of maize in rotation, sesbania fallows have the potential to recharge the subsoil water through increased subsurface drainage and increase nitrate leaching below the crop root zone in excess rainfall seasons.

Effect of rotational fallows on abundance of soil insects and weeds in maize crops in eastern Zambia

Improved fallows or the rotation of fast growing nitrogen-fixing legume species with cereals have been shown to accumulate nitrogen and organic matter, recycle nutrients in the soil and improve soil physical and chemical properties, and increase crop yield compared to traditional fallows. However, the effect of soil nutrients added by fallow species on the incidence of pests, weeds and pathogens in the subsequent crop has not been assessed in southern Africa. In this study, we assessed the relationships between nutrients in the soil after fallows of crotalaria ( Crotalaria grahamiana), pigeon pea ( Cajanus cajan), sesbania ( Sesbania sesban), tephrosia ( Tephrosia vogelii), and their mixtures, and the incidence of soil insects, namely, snout beetles ( Diaecoderus sp.) and termites, and weeds in eastern Zambia. Rotational fallows of sesbania+crotalaria, sesbania+tephrosia, sesbania+pigeon pea and tephrosia+pigeon pea increased infestation of maize by snout beetles as compared to the natural fallow or unfertilised maize grown continuously in monoculture. The beetles showed aggregated spatial distribution, influenced mainly by the nitrate and total inorganic nitrogen content of the soil. Termite incidence was higher in maize after a natural fallow and pure crotalaria, which had 11 and 7 times as much damage as maize planted after pigeon pea+tephrosia mixture. Total weed biomass in maize grown after a natural fallow was six times higher than in maize planted after pure sesbania fallows. The weed biomass was correlated positively with the potassium content of the top 20 cm soil and negatively with plant litter on the soil surface. It is concluded that organic inputs from pure sesbania, sesbania+pigeon pea, sesbania+tephrosia and tephrosia+pigeon pea reduce infestation by termites and weeds, and give maize grain yield comparable with the recommended rates of inorganic fertilisers. However, these fallows have the potential to increase infestation by snout beetles. This is the first study on the snout beetle in Africa, and we recommend more systematic investigation on its ecology in agroforestry systems.

DURATION OF SESBANIA FALLOW EFFECT FOR NITROGEN REQUIREMENT OF MAIZE IN PLANTED FALLOW–MAIZE ROTATION IN WESTERN KENYA

The duration of the residual effect of sesbania (Sesbania sesban) fallow on subsequent crops will determine the interval at which sesbania must be grown to replenish N in a planted fallow–crop rotation cycle. An experiment was conducted from 1995 to 1998 (seven cropping seasons) on two farms in western Kenya, an area subject to a bimodal annual rainfall pattern. The aim was to compare the effect of a single-season sesbania fallow with continuous annual cropping with and without phosphorus fertilizer, on a P-deficient soil. Phosphorus was applied at a rate of 500 kg ha−1 in a single application to meet the phosphorus needs of subsequent crops for the next five to ten years. Sesbania was established simultaneously with maize by direct seeding in the first rainy season of 1995 and allowed to grow as a pure fallow through the second rainy season. Following the harvest of this fallow crop, sole maize in the first post-fallow season and maize-bean intercrops in the subsequent four seasons were grown with and without nitrogen at a rate of 100 kg ha−1. Added phosphorus on average increased maize yields by 3.7 times over the control, indicating that phosphorus fertilizer is essential for good yields. The amount of phosphorus recycled by sesbania fallow was inadequate to meet the crop needs in P-deficient soils. While continuously cropped maize in the presence of phosphorus responded to nitrogen in all seasons, the crop following sesbania responded only from the third season. In the first post-fallow season, sesbania increased maize grain yields over continuous maize by 1.4 t ha−1 with phosphorus fertilizer and by 1.3 t ha−1 without phosphorus fertilizer. The residual effect of sesbania with phosphorus fertilizer lasted for two seasons, while without phosphorus it lasted for only one. In these Kenyan highlands, farmers who can afford fertilizer should buy phosphorus fertilizer and rely for nitrogen on planted fallow with species such as sesbania grown for one season every two years. For farmers who cannot afford fertilizer, one-season fallow every year may be more attractive because of labour savings and the firewood produced by sesbania.

Effects of planted tree fallows on soil nitrogen dynamics, above-ground and root biomass, N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N2-fixation was estimated by measuring both leaf 15N enrichment and whole-plant 15N enrichment by the 15N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha−1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha−1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha−1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N2-fixation. Estimates based on leaf 15N enrichment and whole-plant 15N enrichment were strongly correlated. The N added by N2-fixation amounted to 280–360 kg N ha−1 for Sesbania and 120–170 kg N ha−1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha−1 and 90–140 kg N ha−1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N2 from the atmosphere.

Survival, longevity and fecundity of overwintering Mesoplatys ochroptera Stål (Coleoptera: Chrysomelidae) defoliating Sesbania sesban (Leguminosae) and implications for its management in southern Africa

1 The defoliator beetle, Mesoplatys ochroptera Stal, is a serious pest of the legume tree sesbania (Sesbania sesban (L.) Merrill) in agroforestry systems in southern Africa. The survival of the overwintering stage of M. ochroptera and post- emergence longevity and fecundity of adults in sesbania fallows in eastern Zambia were quantified. 2 Only adult M. ochroptera survived during the winter (May–August) and the dry season (September–October) hidden under weeds, plant litter, in soil cracks and under rocks. During these periods, some beetles were occasionally found feeding on S. sesban, particularly in natural stands in humid areas. The adults were parasitized by the braconid Perilitus larvicida van Achterberg at a rate of 8.5–16.4%. The beetles stayed in the winter refuges for up to 210 days and emerged with the first heavy rains in October–November. The effective survival of beetles overwintering in an insectary was 18.1% in 1998 and 37.7% in 1999, and that of beetles overwintering in the field was estimated at 0.4 and 2.8% in 1998 and 1999, respectively. 3 The post-emergence longevity and oviposition period of females in the insectary was 14–31 days and 5–29 days, respectively. The post-emergence fecundity varied from 87 to 783. 4 It is concluded that the serious M. ochroptera infestation observed every year following the rains is due to synchronous emergence of overwintered resident adult populations and their high capacity for reproduction on many species of Sesbania. Recommendations for pest management in sesbania planted-fallows are given.

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 (r2 = 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.

Predators of Mesoplatys ochroptera in sesbania planted-fallows in eastern Zambia

Mesoplatys ochroptera Stal (Coleoptera: Chrysomelidae) is a serious pest of the tropical legume sesbania (Sesbania sesban (L.) Merrill) widely used for soil fertility improvement in southern Africa. Surveys were conducted between October 1997 and June 1999 in order to identify the predators of M. ochroptera in sesbania fallows in eastern Zambia. The Heteroptera Afrius yolofa (Guerin-Meneville), Glypsus conspicuus (Westwood), Macrorhaphis acuta Dallas, Mecosoma mensor (Germar), Rhinocoris segmentarius (Germar), and Deraeocoris ostentans (Stal), the carabid beetle Cyaneodinodes fasciger (Chaudoir), the ants Tetramorium sericeiventre Emery and Pheidole sp., and the lacewing Mallada sp. were recorded as predators of M. ochroptera for the first time. The pentatomid bugs G. conspicuus, M. acuta and M. mensor were the most common predators in sesbania fallows. Adults and the different nymphal stages of the three species preyed on larvae, adults and occasionally on eggs of the beetle. In the insectary, the adults of G. conspicuus, M. acuta and M. mensor consumed a significantly higher number of larvae (p < 0.001) compared to adults of M. ochroptera under both free-choice and no-choice conditions. The voracity of the three species showed a positive response to prey density. The fifth instar nymphs showed variation in daily consumption of larvae, the highest number of prey being consumed two to three days after the fourth molt. These predators also attacked other pests associated with sesbania and crop plants. The potential role of the predators in the natural control of pests in agroforestry systems is discussed, with a review of their prey species.

Biological and social feasibility of Sesbania fallow practice in small holder agricultural farms in developing countries: a Zambian case study.

Many small holder farmers in developing countries face problems of declining soil fertility and crop yields and insufficient money to buy expensive inorganic fertilizers. The Sesbania fallow system, an agroforestry technology, seems to hold a key to these problems. Based on field studies in eastern Zambia, this paper reports that fallow system has the potential to improve and sustain soil productivity in the small holder farms. However, the paper also reports that the ability for subsistence farmers to adopt the Sesbania fallow system is affected by gender differences in resource allocation to productive resources and institutional, cultural, and social structural settings in which farmers exist and make decisions.

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1. After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1, soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Tree legumes in medium-term fallows: Nz fixation, nitrate recovery and effects on subsequent crops

Fallows improved with tree legumes have a potential role in soil fertility management in the sub-humid tropics. In addition to N2 fixation, deep rooting species can be used to redistribute leached nutrients to the topsoil while producing useful woody by-products. Such fallows are not very effective in improving phosphorus availability, however, and supplying P from other sources may be required to take full advantage of the improved fallows. Busumbu rock P is a potential source of P, but of low reactivity. Our objectives were to compare three tree or shrub legumes with Mucuna [Mucuna pruriens (L.) DC. var. utilis], to determine their rotation effects on subsequent crops and on P availability from Busumbu P rock. Legume effects on the fixation of atmospheric N and nitrate recovery were determined in a sub-humid, bi-modal rainfall system. Fallows improved with sesbania (Sesbania sesban) and tephrosia (Tephrosia vogellii) produced more biomass and fixed more N than those fallows improved with pigeonpea (Cajanus cajan) or mucuna. Pigeonpea derived only about 11% of plant N from the atmosphere compared to 49% for sesbania and tephrosia. The maize-bean [Zea mays (L.)]-[Phaseolus vulgaris (L.)] intercrop was most productive following sesbania and tephrosia fallow and least productive after the fallow improved with pigeonpea. All legumes and food crops failed to acquire significant amounts of P from the P rock. Sesbania and tephrosia were most efficient in recovery of nitrates from below 50 cm depth. The carbon:N ratios were low enough for the biomass of all fallows to allow early mineralisation of N, but the C:P ratios were within the critical level range for a net immobilisation of P following application. Generally, recovery by the subsequent maize-bean intercrops of N supplied in the biomass of tephrosia and sesbania was poor and the results indicate that much of the biomass N was lost. Recovery of N may be improved by transfer of a major part of the legume biomass to manure adjacent land which was continuously cropped. Key Words: Cajanus cajan, cover crops, fallow, green manure, Mucuna pruriens, Sesbania sesban, Tephrosia vogellii, Uganda RÉSUMÉ Les jachères améliorées avec des arbustives ont un rôle potentiel dans la gestion de la fertilité du sol dans la région sous humide tropicale. En plus de la fixation de l'azote , des espèces à racines profondes peuvent être utulisées pour recycler des élements lessivés de la partie supérieure du sol en même temps produisant du bois utile. De telles jachères ne sont pas effectives dans l'amélioration de la disponibilité du phosphore, cependant, en fournissant du P des autres sources peuvent être nécessaires pour donner avantages aux jachères améliorées. La roche à P Bususmbu est une source potentielle du P mais à faibe réaction. Nos objectifs étaient de comparer trois arbres ou arbustes avec Mucuna [Mucun a prureins (L)DC. Var. utilis], déterminer leurs effets de rotation sur les cultures subséquentes et sur la disponibilité du P de. (African Crop Science Journal 8(3) 2000: 263-272)

Effects of Phosphorus Recapitalisation and Agroforestry on Soil, Water and Nutrient Conservation in Phosphorus-Deficient Soils of Western Kenya

AbstractIn the highlands of western Kenya, poor crop growth due to severe nutrient depletion over the past two to three decades has exacerbated soil erosion. The potential benefits of phosphorus (P) recapitalisation with a large one-time application of 500 kg P ha−1 and agroforestry in terms of soil, water and nutrient conservation were investigated on P-deficient soils in western Kenya for 3.5-years. The agroforestry systems compared were one-season sesbania (Sesbania sesban) fallow followed by annual crops, and contour hedgerows of calliandra (Calliandra calothyrsus) plus Napier grass (Pennisetum purpureum). Continuous cropping without P fertiliser produced an average runoff of 367 mm (37% of rainfall) and soil displacement of 105 t ha−1 per season, with a loss of 159 kg of total N and 46 kg of total P ha−1. Phosphorus application reduced runoff by 31% and soil loss by 59% compared to non-fertiliser applied cropping. In P-replenished soils, neither one-season sesbania fallow nor contour hedgerows affect...

Soil Phosphorus Fractions in Unfertilized Fallow‐Maize Systems on Two Tropical Soils

AbstractThe rotation of crops with either legumes or natural regrowth of native vegetation is known to supply N to crops in tropical land‐use systems, but little is known about the effect of such rotations on P supply to crops. An experiment was conducted on two soils in Kenya, an Oxisol (very fine, isohyperthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic Paleustalf), to determine the effect of unfertilized, organic‐based land‐use systems on fractions of soil inorganic P (Pi) and organic P (Po). The land‐use systems involved growth of three maize (Zea mays L.) crops (18 mo total duration) after 17 mo of either (i) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (ii) natural regrowth of vegetation without cultivation (natural fallow), (iii) three crops of unfertilized maize (maize monoculture), or (iv) bare uncultivated soil (bare fallow). Soil (0–15 cm deep) was collected before post‐fallow maize and after the three post‐fallow maize crops. Sesbania and natural fallows increased the amount of P in light fraction soil organic matter (OM) (&gt;150 µm, &lt;1.13 Mg m‐3) and macroorganic matter (250–2000 µm) on each soil and increased chloroform extractable P on the Oxisol. Land‐use systems had no effect on extractable Pi (bicarbonate, Bray‐1, iron‐oxide impregnated paper), except for resin P on the Oxisol. Unfertilized sesbania and natural fallows cycled soil P through plant residues, resulting in increased P in soil microbial biomass immediately after the fallows and increased P in light fraction soil OM for three post‐fallow cropping seasons.

The root-knot nematode problem in sesbania fallows and scope for managing it in western Kenya

Sesbania (Sesbania sesban), which is promoted as a short-duration-fallow species in eastern and southern Africa, is an alternative host to root-knot nematode (Meloidogyne spp.). Therefore, it is important to know the extent of sesbania infection by the nematode on farms and its effects to crops grown in rotation with sesbania. A survey of 14 farms in western Kenya indicated that sesbania was infected by root-knot nematode in all the farms. The potential to build up the nematode population was much greater on farms where maize (Zea mays) was previously intercropped with a susceptible bean (Phaseolus vulgaris). Screening of 30 single plant accessions of four sesbania provenances (Kakamega, Siaya, Kisumu, and Kisii) from western Kenya in pots indicated limited scope for selecting material resistant to the nematode. All four provenances were infested by the nematode, and they increased its population in soil and root. A rangewide collection of sesbania germplasm needs to be screened to determine the scope of finding nematode-resistant material. Transplanted seedlings produced in heat-sterilized, nematode-free soil showed less nematode infection than direct-seeded plants. However, both direct seeding and transplanting seemed to increase nematode population similarly. A number of Crotalaria spp. that were found resistant to nematodes can be considered as alternatives to sesbania-planted fallows, but further field-scale testing is necessary to select appropriate species based on biomass production. Among other species suitable for short-duration fallows, only pigeonpea (Cajanus cajan) and senna (Senna siamea) showed promise. Species such as neem (Azadirachta indica), casuarina (Casuarina junghuhniana), and grevillea (Grevillea robusta) that are suitable for boundary plantings were free from the root-knot nematode.

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.

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 indige- nous 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.

Sesbania Tree Fallows on Phosphorus‐Deficient Sites: Maize Yield and Financial Benefit

AbstractRotation of Sesbania sesban (L.) Merr., a fast‐growing N2‐fixing tree, with maize (Zea mays L.) has potential for increasing fertility of tropical soils, where fertilizer use by resource‐poor farmers is limited. At two sites in Kenya (Ochinga, with a Kandiudalfic Eutrudox soil, and Muange, with a Kandic Paleustalf), we compared maize yields and financial returns for (i) sesbania grown for three or four seasons followed by three maize crops (sesbania fallow), (ii) one maize crop followed by natural regrowth of vegetation for three seasons and then three maize crops (natural fallow), and (iii) maize monoculture for seven seasons. After the fallows, plots were split with and without added P. Maize responded to P at both sites. Cumulative grain yields for seven seasons of maize monoculture were 8.4 Mg ha−1 at Ochinga and 5.6 Mg ha−1 at Muange. They were comparable to cumulative maize yields for sesbania fallow (Ochinga, 10.6 Mg ha−1 Muange, 4.5 Mg ha−1) and natural fallow (Ochinga, 7.7 Mg ha−1; Muange, 4.2 Mg ha−1), even though maize was grown for only three or four seasons in the fallow treatments. Sesbania fallow was financially attractive at Ochinga (≥500 mm rain in each season) but not at Muange, where low rainfall (&lt;300 mm in each postfallow season) limited maize yield. Phosphorus fertilization of maize at Ochinga increased (P &lt; 0.2) net benefit for sesbania fallow. Improved fallows have potential to supply nutrients to crops, but they are unlikely to eliminate the need for P fertilizers on P‐deficient soils.

Soil nitrogen availability as affected by fallow-maize systems on two soils in Kenya

Simple methods for the measurement of nitrogen (N) availability are needed to assess the effect of low-input, organically based land management systems on the N supply of tropical soils. Our objectives were to determine the effect of contrasting land-use systems (LUS) on soil N availability and to identify measures of N availability that correlated with maize (Zea mays L.) grain yield. The LUS at the two sites in Kenya involved growth of a maize crop following 17 months of either: (1) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (2) natural regrowth of vegetation without cultivation (natural fallow), (3) three crops of unfertilized maize (maize monoculture), or (4) bare uncultivated soil (bare fallow). Soil was collected before the post-fallow maize crop was sown. The LUS had no effect on total soil N or amount of N in the heavy fraction soil organic matter (SOM) (>150 μm, >1.37 Mg m–3). Sesbania and natural fallows, as compared to maize monoculture, increased the N in light fraction SOM (>150 μm, 150 μm, 1.13 to 1.37 Mg m–3), ammonium-N and aerobic N mineralization at a depth of 0–15 cm. Maize yields were highest following the sesbania fallow. Nitrate-N, inorganic-N (ammonium plus nitrate) and anaerobic N mineralization correlated with maize grain yield at both sites. The relationship between maize yield and pre-season nitrate-N improved when the depth of soil sampling was increased to 1 m at one site (an Alfisol), but did not improve at the site with anion adsorption in the subsoil (an Oxisol). The sesbania fallow was more effective than the natural fallow in increasing available soil N. Maize yield was better related to pre-season nitrate than N in size-density fractions of SOM.