Combating food insecurity on sandy soils in Zimbabwe: The legume challenge

The continued rise in mineral fertilizer costs has demanded cheaper alternative N sources for resource-constrained smallholder farmers, with N₂-fixing legumes presenting a viable option to maintain crop productivity. A study was conducted over two years on a coarse sandy soil (Lixisol with <80 g clay kg^?1 soil) to determine the productivity of (i) five grain legumes, (ii) a green manure legume, and (iii) maize on smallholder farmers’ fields, identified as SOFECSA Leaming Centres, in Chinyika, north-east Zimbabwe. The objective of the study was to promote appropriate targeting of soil fertility technologies to different farmer resource groups. Emphasis was put on establishing the scope for improving nutrient resource allocation efficiency and crop yields in relation to different management practices as dictated by resource endowment. Both biomass and grain yield results indicated a general conformity to farmer resource group as follows: Resource-endowed farmers (RG1) > Intermediate farmers (RG2) > Resource-constrained farmers (RG3). Although overall biomass productivity for the grain legumes was generally low, <2.8 Mg ha^?1 across all Learning Centres, soyabean grain yields increased by between 30% (RG1) and >500% (RG3) over the two seasons. However, there was a general preference for bambara nut by RG3 farmers who cited low cash demands in terms of seed and external inputs, and pest-resistance compared with other grain legumes. Increased maize grain yields following legumes, and which exceeded 7 Mg ha^?1 for RG1 under green-manure, was apparently due to an increase in soil available N. The results showed scope for enhancing the contribution of legumes to both soil fertility and household nutrition within smallholder farming systems if targeted according to farmers’ resource endowment. The challenge is availing the minimum level of external inputs to RG3 farmers to achieve significant yield benefits on poor soils. The paper presents three main scenarios constituting major challenges for integrating legumes into the current farming systems.     

Sustainable agriculture in the semi-arid tropics through biological nitrogen fixation in grain legumes

Sustainable agriculture relies greatly on renewable resources like biologically fixed nitrogen. Biological nitrogen fixation plays an important role in maintaining soil fertility. However, as BNF is dependent upon physical, environmental, nutritional and biological factors, mere inclusion of any N₂-fixing plant system does not guarantee increased contributions to the soil N pool. In the SAT where plant stover is also removed to feed animals, most legumes might be expected to deplete soil N. Yet beneficial legume effects in terms of increased yields in succeeding cereal crops have been reported. Such benefits are partly due to N contribution from legumes through BNF and soil N saving effect. In addition, other non-N rotational benefits, for example, improved nutrient availability, improved soil structure, reduced pests and diseases, hormonal effects are also responsible. In this paper we have reviewed the research on the contribution of grain legumes in cropping systems and the factors affecting BNF. Based on the information available, we have suggested ways for exploiting BNF for developing sustainable agriculture in the semi-arid tropics (SAT). A holistic approach involving host-plant, bacteria, environment and proper management practices including need based inoculation for enhancing BNF in the cropping systems in the SAT is suggested.     

The success of BNF in soybean in Brazil

Approximately forty years after commercial cropping of soybean in Brazil began, the total area under this crop has reached over 13 M ha with a mean productivity of 2400 kg ha^–1. Soybean varieties introduced from the USA and varieties rescued from early introductions in Brazilian territory were part of the Brazilian soybean-breeding programme which spread the crop from high to low latitudes. Disease-resistance, pest-resistance, tolerance to low fertility soils, as well as production of plants with pods sufficiently high above the ground for efficient mechanical harvesting, were all aims of the programme. Although BNF was not explicitly considered as a trait for selection in the breeding/selection programme, maximisation of biological nitrogen fixation (BNF) was favoured by conducting selection and breeding trials on soils low in N, in which the seeds were inoculated with efficient Bradyrhizobium inoculants but without N fertiliser application. Several efficient imported Bradyrhizobium strains were found to be unable to compete with native soil micro-flora and other previously-introduced Bradyrhizobium strains. Surprisingly, after being in the soil for many years one or two of these strains had become more competitive while maintaining their high BNF capacity. Today, these strains are included amongst the recommended Brazilian inoculants and have promoted significant improvements in grain yields. The breeding of soybeans in conditions that made grain yield highly dependent on BNF, and the continuous attention paid to the selection of Bradyrhizobium strains appropriate for the newly released varieties, have been the main contributors to today's high yields and their great benefit to the Brazilian economy. There seems to be no reason why this ongoing research programme should not serve as an appropriate model to improve BNF inputs to grain legumes in other countries of the world.     

Quantification of the contribution of biological nitrogen fixation to tropical green manure crops and the residual benefit to a subsequent maize crop using 15N-isotope techniques

In this study the contribution of biological N2 fixation (BNF) to leguminous green manures was quantified in the field at different sites with different 15N methodologies. In the first experiment, conducted on a Terra Roxa soil in Cuba, the BNF contribution to three legumes (Crotalaria juncea, Mucuna aterrima and Canavalia ensiformis) was quantified by applying 15N-labelled ammonium sulphate to the soil. The second experiment was planted in a very low fertility sandy soil near Rio de Janeiro, and the 15N natural abundance technique was applied to quantify BNF in C. juncea, M. niveum and soybean. In both studies the advantages of using several non-N2-fixing reference plants was apparent and despite the much greater accumulation of the C. juncea in the experiment performed on the fertile soil of Cuba, the above ground contributions of BNF at both sites were similar (40-80 kg N x ha(-1)) and greater than for the other legumes. In a further experiment the possible contribution of root-derived N to the soil/plant system of two of the legumes was quantified using a 15N-leaf-labelling technique performed in pots. The results of this study suggested that total below-ground N could constitute as much as 39 to 49% of the total N accumulated by the legume crops.     

Nostoc cyanobacterial inoculation in South African agricultural soils enhances soil structure, fertility, and maize growth

Many soils in South Africa have low nutrient supply, poor structural stability and are prone to soil erosion due to susceptibility to surface sealing and crusting. Two crusting soils from the Eastern Cape Province, South Africa were used to evaluate the effects of inoculation with a strain of Nostoc on soil structure, fertility and maize growth. The Nostoc suspension was uniformly applied over potted soils at a rate of 6g (dry weight) per square meter soon after maize germination. Nostoc inoculation increased soil N by 17% and 40% in Hertzog and Guquka soils, respectively. Soil C was also increased significantly and this increase was strongly associated with that of soil N (R ² = 0.838). The highest contents of soil C, soil N and mineral N, however, were found in non-cropped Nostoc inoculated soils. Nostoc inoculation increased maize dry matter yields by 49% and 40% in Hertzog and Guquka soils, respectively. Corresponding increases in maize tissue N were 23% and 14%, respectively. Scanning electron microscopy (SEM) revealed that soil particles and fragments of non-cropped inoculated soils had coatings of extracellular polymeric substances (EPS) with other particles enmeshed in networks of filaments, whilst by contrast little or no EPS and/or filaments were observed on cropped and/or non-inoculated soils. This was consistent with chemical analysis which showed that Nostoc caused significant increases in the EPS and soil C contents of non-cropped soils. The proportion of very stable aggregates was increased by inoculation with Nostoc possibly due to the greater quantities of soil C and EPS observed in inoculated soils. Inoculated soils cropped with maize had a lower proportion of stable aggregates presumably due to their low soil C and EPS contents compared to non-cropped soils. The results suggested that Nostoc could improve the fertility and structural stability of the studied degraded soils.     

Improving nitrogen efficiency: lessons from Malawi and Michigan

Two case studies are presented here of nitrogen (N) dynamics in potato/maize systems. Contrasting systems were investigated from (1) the highland tropics of Dedza, Malawi in southern Africa and (2) the northern temperate Great Lakes region of Michigan. Formal surveys were conducted to document grower perceptions and N management strategies. Survey data were linked with N budgets conducted by reviewing on-farm data from representative farms in the targeted agroecosystems and simulation modeling to estimate N losses. Potential N-loss junctures were identified. Interventions that farmers might accept are discussed. The Malawi system uses targeted application of very small amounts of fertilizer (average 18 kg N ha(-1)) to growing plants. This low rate is on the steep part of plant response to N curve and should serve to enhance efficiency; plant growth, however, is generally stunted in Malawi due to degraded soils and weed competition. Very limited crop yields reduce N efficiency from a simulated 60 kg grain per kg N to an actual of approximately 20 kg grain per kg N (at 40 kg N ha(-1) applied). Legume-intensified systems could improve growth potential and restore N use efficiency through amelioration of soil quality and transfer functions and from biological fixation N inputs. In the Michigan system, N efficiency is enhanced currently through multiple, split applications of N fertilizer tailored to plant growth rate and demand. Fertilizer N rates used by growers, however, averaged 32% higher than recommended rates and 40% higher than N removed in crop product. Application of 50 kg N ha(-1) to cover crops in the fall may contribute to the apparent high potential for N leaching losses. Careful consideration of N credits from legumes and residual soil N would improve N efficiency. Overall, N budgets indicated 0 to 20 kg N ha(-1) loss potential from the Malawi systems and tenfold higher loss potential from current practice in Michigan maize/potato rotations. Best management practices, with or without integration of legumes, could potentially reduce N losses in Michigan to a more acceptable level of about 40 kg N ha(-1).     

Maize productivity and nutrient dynamics in maize-fallow rotations in western Kenya

One-season fallows with legumes such as Crotalaria grahamiana Wight & Arn. and phosphorus (P) fertilization have been suggested to improve crop yields in sub-Saharan Africa. Assessing the sustainability of these measures requires a sound understanding of soil processes, especially transformations of P which is often the main limiting nutrient. We compared plant production, nitrogen (N) and P balances and selected soil properties during 5.5 years in a field experiment with three crop rotations (continuous maize, maize-crotalaria and maize-natural fallow rotation) at two levels of P fertilization (0 and 50 kg P ha^?1 yr^?1, applied as triple superphosphate) on a Kandiudalfic Eutrudox in western Kenya. The maize yield forgone during growth of the crotalaria fallow was compensated by higher post-fallow yields, but the cumulative total maize yield was not significantly different from continuous maize. In all crop rotations, P fertilization doubled total maize yields, increased N removal by maize and remained without effect on amounts of recycled biomass. Crotalaria growth decreased in the course of the experiment due to pest problems. The highest levels of soil organic and microbial C, N and P were found in the maize-crotalaria fallow rotation. The increase in organic P was not accompanied by a change in resin-extractable P, while H₂SO₄-extractable inorganic P was depleted by up to 38 kg P ha^?1 (1% of total P) in the 0–50 cm layer. Microbial P increased substantially when soil was supplied with C and N in a laboratory experiment, confirming field observations that the microbial biomass is limited by C and N rather than P availability. Maize-legume fallow rotations result in a shift towards organic and microbial nutrients and have to be complemented by balanced additions of inorganic fertilizers. Abbreviations: BNF – biological nitrogen fixation; COM – continuous maize; LR – long rainy season; MCF – maize-crotalaria fallow rotation; MNF – maize-natural fallow rotation; SR – short rainy season; TSP – triple superphosphate.     

The contribution of nitrogen by promiscuous soybeans to maize based cropping the moist savanna of Nigeria

Agronomic results indicate that maize grain yields generally are higher when the crop is planted following soybean than in continuous maize cultivation in the moist savanna agroecological zones of West Africa. Many factors have been hypothesized to explain this phenomenon, including enhanced N availability and the so-called `rotational effect'. There is, however, hardly any quantitative information on the residual N benefits of promiscuous soybeans to subsequent cereal crops grown in rotation with soybean. Three IITA promiscuous soybean breeding lines and two Brazilian soybean lines were grown in 1994 and 1995 at Mokwa in the southern Guinea savanna, Nigeria, to quantify the nitrogen contribution by soybeans to a succeeding crop of maize grown in rotation with soybean for two consecutive years, 1996 and 1997 using two methods of introducing ¹⁵N into soil (fresh ¹⁵N labelling and its residual ¹⁵N) and three maize cultivars (including one cultivar with high N use efficiency) used as reference plants. The nodulating soybeans fixed between 44 and 103 kg N ha^–1 of their total N and had an estimated net N balance input from fixation following grain harvest ranging from –8 to 43 kg N ha^–1. Results in 1996 and in 1997 showed that maize growing after soybean had significantly higher grain yield (1.2 – 2.3-fold increase compared to maize control) except for maize cultivar Oba super 2 (8644-27) (a N-efficient hybrid). The ¹⁵N isotope dilution method was able to estimate N contribution by promiscuous soybeans to maize only in the first succeeding maize crop grown in 1996 but not in the second maize crop in 1997. The first crop of maize grown after soybean accumulated an average between 10 and 22 kg N ha^–1 from soybean residue, representing 17–33% of the soybean total N ha^–1. The percentage ¹⁵N derived from residue recovery in maize grown after maize was influenced by the maize cultivars. Maize crop grown after the N-efficient hybrid cultivar Oba Super 2 (844-27) had similar ¹⁵N values similar to maize grown after soybeans, confirming the ability of this cultivar to use N efficiently in low N soil due to an efficient N translocation ability. The maize crop in 1997 grown after maize had lower ¹⁵N enrichment than that grown in soybean plots, suggesting that soybean residues contributed a little to soil available N and to crop N uptake by the second maize crop. The differential mineralization and immobilization turnover of maize and soybean residues in these soils may be important and N contribution estimates in longer term rotation involving legumes and cereals may be difficult to quantify using the ¹⁵N labelling approaches. Therefore alternative methods are required to measure N release from organic residues in these cropping systems.     

Role of legumes in providing N for sustainable tropical pasture systems

Forage legumes have long been lauded for their ability to fix atmospheric nitrogen and contribute to the sustainability of agricultural production systems. However despite the benefits they bring in terms of increased herbage and animal production they are not widely used in temperate or tropical regions. In this review the amounts of biological nitrogen fixation (BNF) needed to sustain the soil-plant-animal system are discussed and related to the amounts fixed in tropical pastures. The data suggest that tropical forage legumes have the capacity to meet the requirements to balance the N cycle of grazed pastures. The actual amounts required will depend on the rate of pasture utilization and the efficiency of recycling via litter, excreta and internal remobilization. The efficiency of nitrogen fixation (% of legume N derived from fixation) is usually high in tropical pastures (>80%) and is unlikely to be affected by inorganic soil N in the absence of N fertilizer. Thus an estimate of the amoutns of N fixed could be obtained from simple estimates of legume biomass provided tissue levels of other nutrients such as phosphorus and potassium are adequate. Key factors for the achievement of sustainable grass/legume pastures include the selection of appropriate germplasm adapted to the particular environment and the judicious use of fertilizers such as phosphorus, potassium, calcium, magnesium and sulphur on acid infertile soils typical of the sub-humid and humid tropics. The main constraints to the widespread adoption of forage legumes include a lack of legume persistence, the presence of anti-quality factors such as tannins, variable Bradyrhizobium requirements and lack of acceptability by farmers. Strategies for the alleviation of these constrainst are discussed. Forage legumes can be used to recuperate degraded soils via their ability to improve the physical, chemical and biological properties of soils and these benefits could be of particular use for small-scale resource-poor farmers. The incorporation of forage legumes into agropastoral systems is discussed as an environmentally and economically attractive means to encourage the widespread adoption of legumes in the humid tropics.     

Inoculation with indigenous <Emphasis Type="Italic">rhizobium</Emphasis> strains increases yields of common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) in northern Spain,although its efficiency is affected by the tillage system

Common bean (Phaseolus vulgaris L.) crops hold the potential to obtain higher yields by enhancing their biological nitrogen fixation (BNF) with Rhizobium. However in contrast to other legumes, common bean has shown a lack of positive response to inoculation with Rhizobium in many cases. This has led to a limited use of rhizobial inoculants in this crop, especially in Europe. The adaptation of bacterial strains to the rhizosphere is a key factor in the success of any inoculant, especially in a promiscuous legume such as common bean. This research aimed at increasing common bean yields via inoculation with effective indigenous Rhizobium leguminosarum strains. Three highly effective strains (LCS0306, LBM1123 and ZBM1008) which were selected according to their effectiveness at BNF in hydroponic conditions were separately inoculated onto common bean in a field experiment. The experiment was carried out under three environments and three tillage systems: conventional-tillage (CONVT), no-tillage (NT) and a cover-crop (CC). The grain yield observed with seed inoculation was significantly higher than the yield obtained with uninoculated seed under CONVT and CC. However, under NT inoculation had no effect. Furthermore, under CONVT and CC, inoculation with R. leguminosarum LCS0306 produced even higher yields than those obtained in nitrogen-fertilised or control plots. This is the first attempt to explain the inoculation performance of common bean under different tillage systems in Europe. A gene–based hypothesis has been used to explain the effectiveness of indigenous common bean rhizobia as nitrogen fixers in this crop.     

Symbiotic performance of grain and wild herbaceous legumes in the Okavango Delta and Tswapong region of Botswana

The low inherent soil fertility, especially nitrogen (N) constrains arable agriculture in Botswana. Nitrogen is usually added to soil through inorganic fertilizer application. In this study, biological nitrogen fixation by legumes is explored as an alternative source of N. The objectives of this study were to measure levels of N₂ fixation by grain legumes such as cowpea, Bambara groundnut and groundnut in farmers’ fields as well as to estimated N₂ fixation by indigenous herbaceous legumes growing in the Okavango Delta. Four flowering plants per species were sampled from the panhandle part of the Okavango Delta and Tswapong area. Nitrogen fixation was measured using the ¹⁵N stable isotope natural abundance technique. The δ¹⁵N values of indigenous herbaceous legumes indicated that they fixed N₂ (?1.88 to +1.35 ‰) with the lowest value measured in Chamaecrista absus growing in Ngarange (Okavango Delta). The δ¹⁵N values of grain legumes growing on farmers’ fields ranging from ?1.2 ‰ to +3.3 ‰ indicated that they were fixing N₂. For grain legumes growing at most farms, %Ndfa were above 50% indicating that they largely depended on symbiotic fixation for their N nutrition. With optimal planting density, Bambara groundnuts on farmers’ fields could potentially fix over 90 kg N/ha in some parts of Tswapong area and about 60 kg N/ha in areas around the Okavango Delta. Results from this study have shown that herbaceous indigenous legumes and cultivated legumes play an important role in the cycling of N in the soil. It has also been shown that biological N₂ on farmer’s field could potentially supply the much needed N for the legumes and the subsequent cereal crops if plant densities are optimized with the potential to increase food security and mitigate climate change.     

Managing native and legume-fixed nitrogen in lowland rice-based cropping systems

Lowlands comprise 87% of the 145 M ha of world rice area. Lowland rice-based cropping systems are characterized by soil flooding during most of the rice growing season. Rainfall distribution, availability of irrigation water and prevailing temperatures determine when rice or other crops are grown. Nitrogen is the most required nutrient in lowland rice-based cropping systems. Reducing fertilizer N use in these cropping systems, while maintaining or enhancing crop output, is desirable from both environmental and economic perspectives. This may be possible by producing N on the land through legume biological nitrogen fixation (BNF), minimizing soil N losses, and by improved recycling of N through plant residues. At the end of a flooded rice crop, organic- and NH₄-N dominate in the soil, with negligible amounts of NO₃. Subsequent drying of the soil favors aerobic N transformations. Organic N mineralizes to NH₄, which is rapidly nitrified into NO₃. As a result, NO₃ accumulates in soil during the aerobic phase. Recent evidence indicates that large amounts of accumulated soil NO₃ may be lost from rice lowlands upon the flooding of aerobic soil for rice production. Plant uptake during the aerobic phase can conserve soil NO₃ from potential loss. Legumes grown during the aerobic phase additionally capture atmospheric N through BNF. The length of the nonflooded season, water availability, soil properties, and prevailing temperatures determine when and where legumes are, or can be, grown. The amount of N derived by legumes through BNF depends on the interaction of microbial, plant, and environmental determinants. Suitable legumes for lowland rice soils are those that can deplete soil NO₃ while deriving large amounts of N through BNF. Reducing soil N supply to the legume by suitable soil and crop management can increase BNF. Much of the N in legume biomass might be removed from the land in an economic crop produce. As biomass is removed, the likelihood of obtaining a positive soil N balance diminishes. Nonetheless, use of legumes rather than non-legumes is likely to contribute higher quantities of N to a subsequent rice crop. A whole-system approach to N management will be necessary to capture and effectively use soil and atmospheric sources of N in the lowland rice ecosystem.IRRI-NifTAL-IFDC joint contribution.     

Effect of VAM inoculum carry-over on the successive cropping of maize and mungbean

A field study to determine the endomycorrhizal inoculum carry-over effect of the first crop [maize inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe] on the succeeding crop (mungbean) was carried out in fumigated and nonfumigated acidic soil (pH 5.3) with moderate extractable P (Olsen 23 ppm). G. mosseae inoculation increased maize dry matter and grain yield over the uninoculated control in the nonfumigated soil. The maize inoculation failed to carry the effective inoculum over to the mungbean crop planted immediately after maize harvest and thus did not increase root colonization and grain yield of the succeeding crop. Fresh inoculation of the mungbean with G. mosseae increased grain yield over the uninoculated control.     

Niche-based assessment of contributions of legumes to the nitrogen economy of Western Kenya smallholder farms

Nitrogen (N) deficiency is a major constraint to the productivity of the African smallholder farming systems. Grain, green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N₂-fixation. The N₂-fixation of bean (Phaseolus vulgaris), soyabean (Glycine max), groundnut (Arachis hypogaea), Lima bean (Phaseolus lunatus), lablab (Lablab purpureus), velvet bean (Mucuna pruriens), crotalaria (Crotalaria ochroleuca), jackbean (Canavalia ensiformis), desmodium (Desmodium uncinatum), stylo (Stylosanthes guianensis) and siratro (Macroptilium atropurpureum) was assessed using the ¹⁵N natural abundance method. The experiments were conducted at three sites in western Kenya, selected on an agro-ecological zone (AEZ) gradient defined by rainfall. On a relative scale, Museno represents high potential AEZ 1, Majengo medium potential AEZ 2 and Ndori low potential AEZ 3. Rainfall in the year of experimentation was highest in AEZ 2, followed by AEZ 1 and AEZ 3. Experimental fields were classified into high, medium and low fertility classes, to assess the influence of soil fertility on N₂-fixation performance. The legumes were planted with triple super phosphate (TSP) at 30 kg P ha^?1, with an extra soyabean plot planted without TSP (soyabean-P), to assess response to P, and no artificial inoculation was done. Legume grain yield, shoot N accumulation, %N derived from N₂-fixation, N₂-fixation and net N inputs differed significantly (P<0.01) with rainfall and soil fertility. Mean grain yield ranged from 0.86 Mg ha^?1, in AEZ 2, to 0.30 Mg ha^?1, in AEZ 3, and from 0.78 Mg ha^?1, in the high fertility field, to 0.48 Mg ha^?1, in the low fertility field. Shoot N accumulation ranged from a maximum of 486 kg N ha^?1 in AEZ 2, to a minimum of 10 kg N ha^?1 in AEZ 3. Based on shoot biomass estimates, the species fixed 25–90% of their N requirements in AEZ 2, 23–90% in AEZ 1, and 7–77% in AEZ 3. Mean N₂-fixation by green manure legumes ranged from 319 kg ha^?1 (velvet bean) in AEZ 2 to 29 kg ha^?1 (jackbean) in AEZ 3. For the forage legumes, mean N₂-fixation ranged from 97 kg N ha^?1 for desmodium in AEZ 2 to 39 kg N ha^?1 for siratro in AEZ 3, while for the grain legumes, the range was from 172 kg N ha^?1 for lablab in AEZ 1 to 3 kg N ha^?1 for soyabean-P in AEZ 3. Lablab and groundnut showed consistently greater N₂-fixation and net N inputs across agro-ecological and soil fertility gradients. The use of maize as reference crop resulted in lower N₂-fixation values than when broad-leaved weed plants were used. The results demonstrate differential contributions of the green manure, forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers’ needs.     

Influence of maize/lablab intercropping on lepidopterous stem borer infestation in maize

Lepidopterous stem borers seriously affect production of maize, Zea mays L., in sub-Saharan Africa. Intercropping maize with legumes such as lablab, Lablab purpurens (L.), is one of the effective systems to control stem borers. Sole culture maize and maize/lablab intercrop system of different lablab densities were planted at two locations to investigate the effects of intercrop system on incidence and severity of stem borers with particular reference to Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae). Stem borer infestation was found to be more severe in sole culture maize than maize in maize/lablab intercrop. There was a significantly negative relationship between lablab densities and maize grain yields, suggesting a possible competition for resources between the two crops. It was concluded that density of lablab and date of planting of lablab in maize/lablab intercropping have significant affects on stem borer populations and maize grain yields.     

Meta-analysis of maize yield response to woody and herbaceous legumes in sub-Saharan Africa

A number of studies have tested the effect of woody and herbaceous legumes on soil fertility and maize yields in sub-Saharan Africa. However, their effects on maize productivity are much debated because results have been variable. A meta-analysis was conducted with the aim of evaluating the evidence in support of yield benefits from woody and herbaceous green manure legumes. A total of 94 peer-reviewed publications from West, East and southern Africa qualified for inclusion in the analysis. Maize yield from herbaceous green manure legumes (54 publications), non-coppicing legumes (48 publications), coppicing woody legumes (10 publications), natural fallows (29 publications), and fully fertilized monoculture maize (52 publications) were compared. Mixed linear modelling using yield differences (D) and response ratios (RR) indicated that the response to legumes is positive. The mean yield increase (D) over unfertilized maize was highest (2.3 t ha^?1) and least variable (CV?=?70%) in fully fertilized maize, while it was lowest (0.3 t ha^?1) and most variable (CV?=?229%) in natural fallows. The increase in yield over unfertilized maize was 1.6 t ha^?1 with coppicing woody legumes, 1.3 t ha^?1 with non-coppicing woody legumes and 0.8 t ha^-1 with herbaceous green manure legumes. Doubling and tripling of yields relative to the control (RR > 2) was recorded in coppicing species (67% of the cases), non-coppicing legumes (45% of the cases), herbaceous green manure legumes (16% of the cases) and natural fallows (19% of the cases). However, doubling or tripling of yields occurred only in low and medium potential sites. Amending post-fallow plots with 50% of the recommended fertilizer dose further increased yields by over 25% indicating that legume rotations may play an important role in reducing fertilizer requirements. Except with the natural fallow, the 95% confidence intervals of D and RR were higher than 1 and 0, respectively indicating significant and positive response to treatments. Therefore, it is concluded that the global maize yield response to legumes is significantly positive and higher than unfertilized maize and natural vegetation fallows.     

The Effects of Crop Rotation and Nitrogen Fertilization on Soil Chemical and Microbial Properties in a Guinea Savanna Alfisol of Nigeria

The impacts of crop rotation and inorganic nitrogen fertilization on soil microbial biomass C (SMBC) and N (SMBN) and water-soluble organic C (WSOC) were studied in a Guinea savanna Alfisol of Nigeria. In 2001, fields of grain legumes (soybean and cowpea), herbaceous legume (Centrosema pascuorum) and a natural fallow were established. In 2002, maize was planted with N fertilizer rates of 0, 20, 40 and 60 kg N ha⁻¹ in a split-plot arrangement fitted to a randomized complete block design with legumes and fallow as main plots and N fertilizer levels as subplots. Surface soil samples were taken at 4 weeks after planting and tasselling stage of the maize. Inorganic N fertilization had no significant (P>0.05) effect on SMBC, SMBN and WSOC, while crop rotation significantly (P<0.0001) affected both SMBC and WSOC. These results demonstrate that crop rotation do not necessarily influence the gross soil microbial biomass, but may affect physiologically distinct subcomponent of the microbial biomass. The soils under the various rotations had a predominance of fungi community as indicated by their wide biomass C/N ratio ranging from 9.2 to 20.9 suggesting fungi to be mainly responsible for decomposition in these soils. Soil microbial biomass and WSOC showed significant (P<0.05) correlation with both soil pH and organic carbon but no relationship with total N. Based on these results, it appears that the soil pH and organic carbon determined the flux of the soil microbial biomass and amount of WSOC in these soils.     

Biological nitrogen fixation for sustainable agriculture: A perspective

The economic and environmental costs of the heavy use of chemical N fertilizers in agriculture are a global concern. Sustainability considerations mandate that alternatives to N fertilizers must be urgently sought. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound means of reducing external inputs and improving internal resources. Symbiotic systems such as that of legumes and Rhizobium can be a major source of N in most cropping systems and that of Azolla and Anabaena can be of particular value to flooded rice crop. Nitrogen fixation by associative and free-living microorganisms can also be important. However, scientific and socio-cultural constraints limit the utilization of BNF systems in agriculture. While several environmental factors that affect BNF have been studied, uncertainties still remain on how organisms respond to a given situation. In the case of legumes, ecological models that predict the likelihood and the magnitude of response to rhizobial inoculation are now becoming available. Molecular biology has made it possible to introduce choice attributes into nitrogen-fixing organisms but limited knowledge on how they interact with the environment makes it difficult to tailor organisms to order. The difficulty in detecting introduced organisms in the field is still a major obstacle to assessing the success or failure of inoculation. Production-level problems and socio-cultural factors also limit the integration of BNF systems into actual farming situations. Maximum benefit can be realized only through analysis and resolution of major constraints to BNF performance in the field and adoption and use of the technology by farmers.     

Yields and accumulations of N and P in farmer-managed intercrops of maize–pigeonpea in semi-arid Africa

Maize (Zea mays L.) is a major staple food in Sub-Saharan Africa but low soil fertility, limited resources and droughts keep yields low. Cultivation of maize intercropped with pigeonpea (Cajanus cajan L. Millsp.) is common in some areas of eastern and southern Africa. The objectives of this study were (1) to investigate dry matter, nitrogen (N) and phosphorus (P) accumulation in different plant components of maize–pigeonpea intercropping systems and (2) to report the effects of the intercrops on soil fertility. Maize–pigeonpea intercrops were compared to sole maize grown using farmersȁ9 practices. Intercropping maize and pigeonpea increased (P < 0.05) total system yield compared to sole maize in terms of biomass, N and P accumulation. Pigeonpea planted in maize did not reduce (P < 0.05) the accumulation of dry matter, N nor P in the maize grain. The harvest indices of maize, calculated on basis dry matter, N or P did not differ either (P < 0.05). Total soil C and N contents and inorganic N content, nitrate and ammonium, were not affected by two seasons of maize–pigeonpea intercropping compared to sole maize (P > 0.11). Nitrate and ammonium levels in soil were still not affected by the treatments after the soils were incubated in anaerobic conditions for 8 days at 37°C (P > 0.11). However, pigeonpea added up to 60 kg of N ha⁻¹ to the system and accumulated up to 6 kg of P ha⁻¹ and only 25% of this N and P were exported in the grain. In conclusion, beside the added grain yield of pigeonpea in the intercropped systems, pigeonpea increased the recirculation of dry matter, N and P, which may have a long-term effect on soil fertility. Furthermore, the stems from pigeonpea contributed to household fuel wood consumption. The intercropped system thus had multiple benefits that gave significant increase in combined yield per unit area without additional labour requirements. The main requirement in order to up-scale the maize–pigeonpea intercropping approach is sufficient supply of high-quality pigeonpea seeds.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.